Heatshit
Intro
With the combined power for a $15 000 solar system and a $15 000 heat pump system, you can save literal tens of dollars a year on heating!
In similar vein to Solarshit, this page aims to collect information about heating and cooling systems (HVAC), with a primary focus on air-source heat pumps. You can find answers to pressing matters, such as
how do I into heating?
how do I into cooling?
wtf how can heat pump be 400% efficient it brakes the laws of thermodynamics!!!1!1!
and
I got a heat pump and it's a piece of shit fuck you, went back to gas boiler
Sister guides:
https://rentry.co/solarshit
https://rentry.co/insulationshit - building insulation, heat loss
Terminology
W - basic unit of power. Heating system components can be defined in many other metrics, but W is by far the most universal and easiest to do apples-to-apples comparisons with, hence you should probably be using W (and Wh) instead of BTUs and cubic meters of gas and cords of wood.
Wh, kWh, MWh - One device using 1 000W consumes 1 000Wh in one hour. A 1 000W resistive heater (ie. electric radiator, heating fan) produces 1 000Wh of heat in one hour at 100% efficiency. Gas can also be measured in Wh (or more commonly, MWh), see conversions below.
COP - Coefficient of Performance - how many kW of heating you can get out of 1 kW of input in a given scenario or moment.
COP = (heat output / electricity input). I.e. 8 000W heat out/2 000W electricity in = 4 COP. Can be interchangeably used with W or Wh depending on scenario, since it's always just about the proportion of input and output.
A resistive heater has a COP of 1. Typical heat pumps can reach up to 8 in ideal conditions.
SCOP - Seasonal Coefficient of Performance - the COP of the whole heating season (or whole year). This is the more practically important metric, that ultimately influences how much you're paying in running costs. A bad heat pump install will be around 2-2.5. A good heat pump install will be around 4-4.5.
UFH - underfloor heating
DHW - domestic hot water
Total heat output will generally refer to the practical heat energy output of the heat pump, in W or kW.
Calculations-conversion reference
Skip this segment if you are just starting out
Gas cubic-meter to MWh, BTU: Factor of 0.011 (1 cubic meter = 10.55kWh = 0.011MWh = 36 600BTU)
MWh to gas cubic-meter, BTU: Factor of 0.095 (1MWh = ~95 cubic meters = ~3500 MBTU)
BTU to MWh: Factor of 0.293 (1MBTU = 0.293MWh)
HSPF to SCOP: SCOP = HSPF x 0.293 (SCOP 1 = 3.41HSPF, SCOP 4 = 13.65 HSPF)
Wood burning: Modern stoves are about 70% efficient, older stoves under 50%. Burning wood with low moisture content produces about 4-4.5kWh per kg, after losses about 3kWh of useful heat per kg.
Key calculations (explained lower in the guide):
Mass flow rate:
(Flow rate) = (Power in kW) / ((deltaT in °C) * (water heat capacity))
Pipe diameter for power:
(Pipe diameter in mm) = (2000 * (square root( (Power in kW) / ( (deltaT in °C) * 1000 * 4.2 * 3.14 * (Flow Velocity m/s) ) ) ) / (Pipe factor))
Celsius and Kelvin can be used interchangeably in any deltaT calculations, you might encounter other reading material where there's a °K instead of °C.
There is typically a price difference factor of approximately 3 between gas and electricity (i.e. cost of 1MWh of electricity = 3MWh of gas) around Europe. Outliers exist (ie. Romania 6, Sweden 0.8).
ECelebs and other fags
https://www.youtube.com/@HeatGeek - your main source of information on how to not fuck your heat pump install. Newer videos are more prone to being clickbait, but nevertheless many very useful guides.
https://www.youtube.com/@UrbanPlumbers - secondary source of information on how to not fuck your heat pump install, caveat: sometimes gets minor system design details wrong
https://www.youtube.com/@HVACEducationHub - Croatian with an accent teaches you grade school math you should already know, but don't
https://www.youtube.com/@RenewableHeatingHub - mostly people that don't know what they're doing, but some useful information here and there
Useful links
https://heatpumpmonitor.org/ - mostly UK real-time system performance data. Extremely detailed performance information. Some installs have a YouTube link on the side - you can see exactly how the system is laid out.
https://www.heatgeek.com/
https://mcscertified.com/product-directory/ - UK directory of standardized, independently tested SCOP for many heat pump models. Can be useful to compare specific models.
https://heatpunk.co.uk/ - heat loss calculator/heating system designer
https://build-calc.com/calculation-of-water-underfloo-heating - UFH calculator, best used together with the above heat loss calculator
Heat pump choice
A heat pump is any device that moves heat energy from once place to another - in this case, taking energy out of the cold outdoor air to push it into the warm indoors, or vice versa.
Air to air is the most common type of heat pump - your regular air conditioner. Most heat pumps in the US are air-to-air even in central heating systems, using ducts to distribute warm or cold air. Broadly speaking, they are less efficient (3-3.5 estimated SCOP) than a good air to water install. However, for well insulated properties, the low price of the unit and lack of a need for radiators and piping can make them a better investment.
Air-to-air is inferior to air-to-water in two key areas, putting COP aside:
- being purely convective in heating (meaning the indoor unit is heating the air directly, and not radiating any heat) - this, combined with the fact that it's constantly blowing air in the room, can make rooms feel colder, requiring you to keep them at higher temperatures by 1-2 degrees.
- noise - the indoor unit will always have a fan running.
But, again, it is a decent budget option for well insulated homes.
Air to water heat pumps are more common for central heating in Europe. The unit cools the outdoor air, heats up water, said water is distributed into radiators or UFH.
Ground to water heat pumps typically have long water pipes laid ~1m underground over a large area, and exchange heat with the soil. Alternative configurations are, for example, well exchangers, lake exchangers, open loop well exchangers. None of these are very relevant in (current year) due to usually being very expensive to install and offering very little performance advantage. They can have a unique advantage in that, if you are using your heat pump for both cooling in summer and heating in the winter, the soil/water can be used as a heat battery - the soil retains some of the heat deposited in the summer, thus improving your COP in the winter, and vice versa. Even with this in mind, they are usually not worth the cost - air source heat pumps can get very close in COP. A rough estimate is that a very good air-to-water heat pump can get to ~4.5 SCOP, a very good ground-to-water heat pump can get to ~5.5 SCOP. Installation costs are somewhere in the range of 2-10x higher than air-source.
This guide will mostly focus on air-to-water heat pumps, to be used with UFH and radiators.
Brands
Ching Chang - you get what you pay for, including no real warranty and/or spare parts. Still can be viable at low prices.
LG, Samsung - budget choice, may have questionable manufacturer support (depending on your location), may be a little noisier, but aside from that, should be okay
Vaillant - the go-to premium brand. +0.2-0.5 SCOP over the budget stuff.
Mitsubishi, Ideal, Nordic, Ferroli, et cetera - other theoretically premium brands
Life expectancy
Any (non-Chang) heat pump should last at least 10-15 years provided reasonable maintenance and not bad installation. 20 years is the ideal lifetime of a high quality unit.
Warranty
Not uncommon is that manufacturers tie any product warranty (or extended warranty) to getting the system initialized by one of their own technicians (other than all the other warranty conditions - filters, install location, install conditions, et cetera). This means that whoever fits the thing isn't necessarily relevant (and that is the bulk of the cost), but if you want good warranty coverage, you have to pay for some guy to come and press buttons on your heat pump, at least once.
In the same vein, some manufacturers also require the install itself to be done by their own certified installers, in this case, you're fucked and won't have warranty coverage if you /diy/. However, you have to consider that a /diy/ install might cost $4000, a certified install with 7 years extended warranty might cost $10000 for the same unit. Is the warranty worth 1.5 times the price of the unit and materials? No. But does your local code allow you to /diy/ at all, or will it get you fined?
It's also typical to tie extended warranty coverage to annual, or bi-annual maintenance by a maintenance technician.
This is all heavily subject to local regulations and manufacturer quirks, consult an authorized dealer of the given brand before buying the heat pump.
Refrigerant
R32 - Bog standard current year choice, a relatively environmentally friendly HFC. Most cheaper units will be on R32. Up to about 55C flow temperature.
R290 - Propane. More environmentally friendly than R32. Looking at various heat pumps, you may notice the label high temperature. Heat pumps with R290 can indeed reach higher water temperatures, up to 65-70C. They tend to be marginally (+0.1-0.2 COP) more efficient than R32, more owing to typically being more modern designs than any inherent advantage of R290.
At what cost? The efficiency (when making such hot water) is dogshit, the unit costs at least 50% more than the equivalent R32, some countries will hit you with extra regulations for installation due to propane being flammable. TL;DR - it's a meme, but a meme we will just have to eventually learn to take up the ass.
If you live in a gommie country (ie. anywhere in the EU), do NOT buy anything that is not R32 or R290, as many countries are working hard to make other heat pumps illegal to even just sell spare parts for, to phase out the less environmentally friendly refrigerants
Sizing, oversizing
I'll just install the biggest heat pump, better have more power than not enough!
Oversizing a heat pump relates to cycling and the unit's ability to modulate. When you chuck a big fucking heat pump onto a system that only needs 50% of its rated power at peak, it will hurt your COP all the other times, when the system would only need 5-10% of its rated power, and results in some excessive cycling - heat pumps can typically modulate down to about 20% of their rated power.
In general, it's better and, long term, cheaper to install a heat pump that's about big enough to cover your heat loss, and add a resistive electric backup heater. In other words, it's better to waste a few kWh more on extra cold days than to be wasting power on reduced COP for 4-5 months a year. Oversizing is less of an issue on well insulated but large houses - as the large system volume (in relation to the heat pump's power) allows the heat pump to avoid excessive cycling.
Cycling can be addressed by settings (hysteresis) and increasing system volume, so should not necessarily be a death sentence, but in principle it's best to avoid it. See also - system design/open system design.
However, to make life harder, many different manufacturers use different "naming schemes" to advertise their models. When you have a property with 4.5kW heat loss and you buy a 5kW heat pump for it, it may not be sufficient, even though logically, it should be.
So where's the catch? In the following table:
Now, LG, assuming these values are real, has a sensible naming scheme - as you can see, down to -7C outdoor temperature, the TC maintains 9kW up to 45C LWT; this is about how it should be. Vaillant has random performance and naming scheme, where they usually perform above their rating (e.g. a 7kW unit produces 8.5kW at 35C LWT/+7C OT), though there are some reports of lower than rated real-world performance.
In any case, takeaway is to carefully read the specs of whatever heat pump you're installing, ideally try to find existing installs to verify their low temp performance.
Ok so I have a 30kW boiler now, I need a 30kW heat pump to replace it?
Lolno. Gas boilers are typically massively oversized. Sizing your heat pump accurately requires knowing your heat loss. Accurate heat loss calculation is a large topic of its own, but there are some approximations you can make.
Heat loss
The rough guesswork method is https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/
The more educated guesswork method is looking at your gas bills, calculating your approximate heat production, and then dividing the total heat in kW by the Heating Degree Days metric. You can find the approximate HDD for your location on google.
I.e. in central Europe - let's say you paid for 25MWh worth of gas - at ~90% efficiency, you produced about 22MWh of heat. The HDD for your location is 3000. 22 000/3000 = 7.3 > you probably need a 7-8kW heat pump.
Same principle applies to any other method of heating, you need to know amount of fuel and burn efficiency
The proper method is going over your house's exact outdoor measurements, all windows and doors, figuring out what materials and insulation were used for each wall, plugging the values into a designer such as the heatpunk.co.uk one, or doing the math manually. This is covered over at https://rentry.co/insulationshit#heat-loss
The very proper method is hiring a HVAC engineer, architect, HERS rater, energy auditor, whatever else, to calculate your exact heat loss.
AI/LLM can also help you with heat loss calculation, but you have to double check their U-values, as sometimes they get excessively optimistic or pessimistic with the numbers they use.
Monobloc vs. split
Monobloc, don't even think about it. If you get extreme cold winters, in the range of -20 - -30C, you might consider split, since the lines don't have to be frost protected in any way. But such extreme winters, maybe a heat pump isn't the best choice for heating, split or not.
Gas boilers
A word about the token alternative. Much cheaper to buy, much cheaper to use in any given system - just burn gas -> make hot water. Easy.
Efficiency
Regular boilers run at around 70-80% efficiency. Burn gas, make hot water.
Condensing boilers are known to be able to reach up to 98% efficiency! But in real life, they are usually around 85% due to bad configuration and poor system design. In general, the same principles apply as with heat pumps, but to a lesser extent - running them colder is better. 30C IN - 50C OUT is about 94% efficient, 50C IN - 70C OUT is about 86% efficient. Frequent cycling (due to oversizing) further ruins the efficiency.
tl;dr - run them on low flow temp and weather compensation.
More info: https://www.youtube.com/watch?v=wFOpHNz7Fdc
System design principles
Heat Pump Efficiency
This non-specific chart is the core of designing for a heat pump.
More lower water output temperature = more better.
Heating a home is purely about delivering enough kWh in heat to offset the losses of the building. 10KWh of heat delivered in 70C water is exactly as good as 10KWh of heat delivered in 35C water. But the heat has to be delivered.
For heat pump systems, there are 3 key points of this heat transfer, each with their unique limitations:
- Heat Pump
A limitation of performance that you must account for is the lowest temperature at which it can deliver the design water temperature at design heating power. If you calculate that your radiators must run at 40C when it's -5C outside, and you need 7kW delivered, you have to make sure the heat pump is actually capable of that. You will have to extrapolate the number, as you will typically find only specific points on the curve mentioned, namely LWT (Leaving Water Temperature) 35C at 7C OAT (Outdoor Air Temperature), 55C LWT at 7C OAT, sometimes 35C LWT at 2C OAT or -2C, -7C OAT. This is a very highly variable thing, you need to know what water flow temperature you need at design outdoor temperature.
For an example of a bad scenario: you need 7kW of heating power at 50C flow temp when it's -5 outside, but the heat pump can only do 7kW at 45C flow temp when it's -5 outside, so your radiators can't remove 7kW worth of heat from the water, so the total heat delivery drops to i.e. 5kW, and your indoor temperature drops. The heat pump could do 7kW worth of heat, but the radiators can't dissipate it, therefore the system is failing. Easy fix - upgrade radiators (or insulate)(or buy a bigger heat pump and lose COP).
Note: This is speccing for design temperature. If you're not talking about the design temp, but considering oddities - temperature extremes. It is usually better to fit a backup resistive heater for the rare extreme cold spell than to cheese your heat pump for a rare very cold -15C night. - Piping
The pipes need to be able to deliver the rated amount of heat at the rated delta-T (temperature differential) between the flow (output) and return (input) of the heat pump. For this calculation, the actual temperature is irrelevant, only the differential matters. Heat pumps generally like to work with a deltaT of 5C, at most 7C. With all this, you get the following equation for mass flow rate:(Flow rate) = (Power in kW) / ( (deltaT in C) * (water heat capacity) )
I.e.: Flow rate = 7kW / (5C * 4.2kJ/kgK) = 0.33L/sec = 20L/min = 1 200L/h
To know what size pipe you need to deliver this amount of water, there is also the flow speed variable, how fast water is moving through pipes. This is generally advised to be 0.9m/s or less to reduce turbulence, and thus noise and wear on components. An additional pipe factor exists to compensate for losses in less efficient piping (PEX-AL composite: 0.8-0.9, copper: 1.0). Thus the pipe sizing equation is:
(Pipe diameter in mm) = (2000 * (square root( (Power in kW) / ( (deltaT in C) * 1000 * 4.2 * Pi * (Flow Velocity m/s) ) ) ) / (Pipe factor) )
I.e. Pipe diameter in mm = (2000 * (sqrt(7kW / (5C * 1000 * 4.2 * 3.14 * 0.9m/s)))) / 0.9 = 24.1mm. Note that this is calculating the nominal OD/outside diameter, NOT the ID/inside diameter! Plumbers are apparently not very smart and have decided to call a 32mm pipe 32mm pipe regardless of whether the wall thickness is 1mm or 3mm. This is the reason you have to add the pipe factor - copper wall thickness is 1mm, PEX-Al is 3mm (plus PEX-Al fittings are even more narrow inside, while copper fittings are external).
Whenever your pipe splits, the following pipe should correspond to the heat capacity of the respective set of heating elements it feeds, respective to the (same) deltaT it's handling. Best is to throw the equation into excel and run the numbers whenever you're not sure.
I.e. 24mm input pipe handling 7kW at deltaT 5C split: Pipe 1 handles 5kW - needs to be at least 20mm OD. Pipe 2 handles 2kW - 13mm OD.
- Radiators, UFH, fan coils
The final element delivering heat. Fan coils have their own spec sheet for how much heat they can deliver. UFH is complicated, UFH design and calculations are outside the scope of the guide. As a rule of thumb, UFH is usually designed to deliver about 100-150W/sqm at 45C.
Radiators are generally labeled as 11, 21, 22, 33 - based on how many sets of panels and convection fins they have. Radiator specs will also usually list their dissipation capacity at a given deltaT, you can compare it to the chart below to know what to expect. Note that radiator output power is specced on ROOM VS. AVERAGE RADIATOR SURFACE TEMPERATURE deltaT, not the water input/output deltaT!
The square meter in the chart refers to the bounding box size of the radiator, NOT the total surface area. E.g. a 80cmx50cm radiator is 0.4sqm.
The output power has to be then compared to the rated heat loss of the room or building. Heat loss calculation is its own separate topic - see https://rentry.co/insulationshit
I.e.: Room with heat loss of 1500W, target room temperature 20C. It has two 80x50cm radiators of Type 22. What is the lowest flow temperature to achieve equilibrium?
2 x 0.8m x 0.5m = 0.8sqm radiator surface area
Divide target power by surface area to get reference power - 1500W/0.8 = 1875W
Look for 1875W in the Type 22 column of the chart - it sits at air-DeltaT 35C. Therefore the average radiator temperature would have to be (target room temperature + air-deltaT) 55C (at flow-deltaT of 5C, 57.5C input and 52.5C return).
Now you might find 57.5C flow temp quite high for a heat pump, and your efficiency would be pretty shit, so how can this be improved?
- install Type 33 radiators > 1800W delivered at air-deltaT 25C > average flow temp drops to 45C, acceptable
- install larger Type 33 radiators > i.e. +0.4sqm > reference power 1250W (1.2sqm total area) > average flow temp drops to 35C > good for heat pump
- improve room insulation > heat loss reduced to (arbitrary number) 750W > reference power 937W > air-deltaT of ~18C, average flow temp drops to 38C > okay for heat pump
The cheapest and easiest option is likely going to be installing large Type 33 radiators, provided you have large enough pipes to be able to carry 1500W of heat at 5C deltaT (which is about 12mm). If your pipework too small for the required power:
- if it's marginally smaller, you can increase deltaT 1-2C
- if it's much smaller, you have to either redo the segment of the pipe that can't handle the load, or insulate
- if you want to #YOLO it, put a pump on the other end so it squeezes a ton of water through at high flow rate
- insulation effectively reduces the required pipe diameter, since the required power is reduced
Bonus trick:
- increase heating in adjacent rooms if the heating system of adjacent rooms is better. I.e. if you have UFH on ground floor and a cold room on the 1st floor, setting UFH room to 23C will allow a lot of heat to diffuse to your colder room sitting at 19-20°C - somewhere in the range of 5-30W/sqm (depending on construction materials). If we assume the contact surface is pretty large, you can shave off many hundred watts of your heating requirement, allowing you to achieve low flow temperatures without any physical changes in the system. Will reduce comfort though. More on this in Open System Design section.
Retard's rule of thumb for reference flow temperature: Find the coldest room in the house on one of the coldest days of the year. Measure the temperature on the input and the output (maybe consider logging it with some cheapo smart WiFi/ZigBee temperature sensor, they can take 55°C). From the average flow temperature, you can extrapolate the average flow temperature you will need on a heat pump system. I.e.:
Outdoor temp average -3, radiator input temp average 45°C, radiator return temp average 30°C. DeltaT = 15°C. Temp midpoint = 37.5°C.
Thus at deltaT of 5C, you will need average input temp of 40°C and average return temp 35°C (as well as higher flow to maintain the deltaT).
This assumes a reasonably well working existing system! If your shit's fucked, and it's way colder in the room than you'd like, obviously this rule of thumb doesn't apply!
Open System Design - anti-zoning
More colder water output temperature = more better.
Required viewing: https://www.youtube.com/watch?v=zpTVIeUh04E
The video covers the entirety of the topic, so I will jump to the conclusions:
- reduce zoning as much as possible
- keep thermostats open, radiators running everywhere where it's possible to improve the total heat delivery surface for the system - this reduces flow temperature, which increases efficiency, which saves money, even if the heat loss of the property is larger
- exceptions exist:
- a large property with drastic differences (one old, poorly insulated building that you only need to heat during the weekend, and one new, insulated building that is used every day on the same heat pump - obviously should be zoned),
- one very poorly insulated room that has minimal contact with the rest of the building (i.e. a garage that sticks out with only a single contact wall)
- one room that gets a whole lot of door/window opening and thus has much larger heat loss than the rest of the property (i.e. a common hallway between multiple apartments/stores)
- in a typical house, you might still want to drop temperature in bedrooms for comfort reasons, or disable certain radiators in rooms that get a lot of solar heat gain on sunny days, but only consider comfort reasons, not saving money! It's important to only regulate radiators that really need the regulation.
The main takeaway is that rooms in the same building, with large contact area between rooms, are better off kept on the same proportional heating power. It's not even a matter of actual temperature, but rather that the maximum reasonable heat is being delivered - in simpler words, the radiators are open as much as reasonably possible. This is basically a repeat of the concept of the "bonus trick" from the above section.
Caveat: if you have many thermostats that you want to use despite everything, or you are installing for someone else who is retarded and will close thermostats even if you tell them not to, you need to verify that the heat pump can handle increased system resistance/subsequent reduced flow. At the very least, it shouldn't throw an error due to low flow. If you can't work around it with user education, and you're getting errors, either install a low loss header, or a bypass valve on the furthest radiator. Or just remove some of the thermostats and force some of the radiators to be always open.
Remember, the basic heat regulation for the vast majority of the house should be a function of setting up the heat pump controller well, not a function of opening and closing radiators, or even UFH.
The other aspect is base system volume - your shit can run poorly or fail, if somehow all radiators are closed and there isn't enough volume left to run defrost. See also: volumizer.
Power consumption
Total/seasonal power consumption is simply a function of COP, per given amount of heat delivered (or it'd be more accurate to say, COP is a function of power consumption and heat delivered).
If you have a house that requires 20MWh of heat (i.e. your gas bill from last year is for 22-25MWh) and you have a well set up heat pump that gets 4 SCOP (seasonal COP), it means you're paying for 5MWh of electricity. If you have a poorly set up heat pump and you get a 2 SCOP, you're paying for 10MWh of electricity.
In terms of real-time power consumption: using a 7kW heat pump as an example, it will virtually never consume anywhere near to 7kW of electricity. Usually they are in the range of 1.5-2kW rated power, 3kW peak power, and run on a 20A (dedicated) circuit breaker.
Backup resistive heaters, if equipped, can boost this peak power, typically by another ~3kW (single phase 220V) or ~9kW (3 phase 400V). Most heat pumps do not come with a resistive backup heater installed by default. The datasheet or install manual of the unit will definitely list the electrical specifications and whether it has a backup heater.
Hydraulic separation
Hydraulic separation refers to any kind of system or device that allows different flow rates in different parts of the circuit (term used loosely in HVAC). In a stricter definition, it would refer to a system that fully separates the fluids of each circuit while allowing heat transfer.
Common types of hydraulic separation are close-coupled tees (cheapest), low loss headers, and heat exchangers (either plate or buffer tank).
TL;DR of this section is: buffers and heat exchangers are the antichrist, do not use them with a heat pump.
When do you want hydraulic separation in a heating system?
Never.
When do you need hydraulic separation in a heating system?
- If there is a gross (potential) mismatch between the needs of the circuit(s) connected to the heat pump compared to the capability of the heat pump.
Consider that the heat pump always wants to work with deltaT of 5-ish, running at, let's say, 1500L/hour. But you have radiators piped with thin fucking pipes that cannot possibly handle 1500L/hour, maybe they can do 750L/hour. You can't fix the piping for whatever reason.
- The hydraulic separator lets the heat pump do 40C flow, 35C return water at 1500L/h on the primary, while a pump does 750L/h on the secondary with 40C flow, 30C return.
- The 750L/h of 30C return mixes into the remaining 750L/h of the primary flow, producing 35C return at 1500L/h. Heat pump is happy, heating works.
- Full hydraulic separation when there is some reason you can't mix the fluids on the two sides of the system
- I.e. rusted out shit radiators on one side that you don't want to pipe into the heat pump/boiler on the primary side
- High antifreeze content on the primary, with relatively narrow pipes on the secondary side - you don't want to reduce the heat capacity for the secondary side by running antifreeze mix (though the true solution is usually to just not use antifreeze fluid)
- if you're insane enough to do heated driveway, pool, or any other outdoor circuits connected to your heat pump - these must have antifreeze, and you have to isolate them to keep your main circuit normal water
Close coupled tees
The cheapest solution for hydraulic separation. The two Tees have to be very close together, at most 4 pipe diameter. This allows easy variability of flow across the 2 inputs and 2 outputs.
Explanation of close coupled T, other types of hydraulic separators: https://www.youtube.com/watch?v=A7V8P_ELKN8
Note: the layouts used in this video are for high temperature boilers, you shouldn't take design inspirations from this video for heat pump systems! It's purely for explanation of the hydraulics involved.
Low loss header
If it seems like it's just a small water tank with two inputs and two outputs, that's because it is. This device is a "get out of jail free"-card for heating systems. As long as you don't majorly fuck up the system design, and match the flow rates on the two sides, it will work decently well. It's said that it causes about 10-20% efficiency drop for a heat pump (assuming everything is set up correctly!).
Buffer tank, buffer tank with coil / accumulator tank, heat exchangers
Buffer tanks are a big water tank with 2+ inputs and 2+ outputs, essentially a big version of the low loss header. Heat exchanger tanks (buffer tank with coil) provide complete fluid separation between the heating unit and the secondary radiator/UFH circuits.
They are expensive to buy, mildly complicated to install, take up a lot of space, and they generally provide nothing of value compared to other, cheaper options.
So why does anyone install them at all? Hell, a lot of people install them.
a) there is an argument from buffer tank sellers (which includes heat pump manufacturers) that it allows your heat pump to cycle less, thus improving efficiency. This is bullshit, since you can, for one, usually set your heat pump to not cycle a lot, for two, it already comes from the factory pre-set to not cycle a lot. More about this in the respective section.
b) someone that cannot or doesn't want to do math might con you into thinking that you can store heat, for example, from cheap electricity (solar or low tariff) and thus gain further savings from the heat pump.
For argument a), if you really have to, you can just use a much cheaper volumizer to roughly the same effect. The volumizer will be at somewhat lower temperature, but you only need so much heat for defrost either way. Better yet, just keep an open system with an appropriate volume.
For argument b), this is the relevant math:
A 200L coil buffer tank with a big enough heat exchanger (it has to be big to achieve "good" efficiency of heat transfer) costs about $1500.
200L of water can store about 230Wh for each degree C you heat it. Assuming you want to utilize the tank as much as reasonably possible, let's say you heat it up to 55C (from 35C, which would be a generous normal assumed water temperature you need for your radiators). Thus the stored energy is about 6kWh. This is 6kWh of heat, not 6kWh of electricity! The equivalent electricity saved is a function of COP!
How much electricity did it take to generate the 6kWh of heat? There is a default loss of about 0.6-1.5COP just by having a heat exchanger in the system. You are heating into a very high water temperature, with very high deltaT against outdoor air, at average water temperature of 45C (35C to 55C). The COP would be about 3.5 at outdoor temp 7C, minus the heat exchanger loss of 0.6, so let's say, generously, 2.9 COP. Thus you used about 2.06kWH to generate 6kWH of heat.
Let's assume this is all free electricity, and your electricity is otherwise very expensive, $0.4/kWh. You've saved $0.8 per cycle. You will surely not have more than one cycle per day, therefore ROI is about 1800 days (not counting install costs). In average climate, there are about 150-200 days of heating season, so your ROI is about 10 years.
HOWEVER, part 1
You can usually just deliver the 6kWh of heat into the house through low flow temperature heating at better efficiency, or conversely get more heat out of the 2kWh input! The house stores it with about the same efficiency as the heat tank! In this case, the buffer tank's ROI is infinite - you've saved diddly fucking squat per day. To illustrate how little the buffer's stored energy helps, 6kWh of heat in a 100 square meter house raises average air temperature by about 0.5C, this 0.5C (temporary) increase raises your rated heat loss probably by about 2% - far less than the losses on the buffer tank.
HOWEVER, part 2
Let's say your house is small and 6kWh of unnecessary heat would reduce comfort somehow, or you just can't do that for whatever other reason. You're still competing with the heat pump's ability to deliver the heat at 4+ COP at low flow temperature, which would consume only 1.5kWh, not 2kWh, so ROI has to be calculated from this number, i.e. $0.6/day assuming $0.4/kWh. Thus ROI is 15 years at best.
At average $0.20/kWh priced electricity, the ROI is 30 years.
If you use realistic solar conditions for cold days, the ROI is maybe 60-150 years.
If you don't have solar, but use low tariff to accumulate heat, ROI is of course a function of the price difference between high and low tariff, presumably in the 30-60 year range. Note that the tank life expectancy is 20 years.
HOWEVER, part 3
Remember that 0.6 loss of COP from the calculation? That 0.6 loss of COP applies to your ENTIRE HEATING SYSTEM, ALL THE TIME. Even if you save $0.6 per day on cheap electricity, you lose $2-3 per day on overall system efficiency.
You could circumvent this by using automatic valves to switch between the tank and direct feed (which no installer does for you, they always install separate circuits), but for what? The automation system will cost you at least another $1k, probably more, if you don't /diy/. And you're still barely saving money.
Note that the 0.6 COP loss figure used is extremely generous, in reality you'd be more likely looking at 1 to 1.5 loss, and a potential loss of heat pump total heat output in cold weather (due to needing high flow temperature and/or having poorer heat delivery).
Consider that 2kWh of LFP battery storage is, at the absolute worst, priced around $1k ($400 is a more typical price, <$200 if you /diy/), takes up way less space and has way better lifetime. Just store electricity in a battery if you need to.
what if I have/want to buy solar water heaters?
You're still saving only 1.5kWh worth of electricity per day (with the 200L buffer, the solar heater part can do slightly more). If you have an existing system, the best use with the lowest installation cost is piping it into domestic hot water. If you want to buy solar water heaters - don't. Currently PV solar panels have way, way better ROI.
The one marginal exception is if you are very limited on space: the solar water heater can deliver about 2-3 times more power per square meter than a PV panel. But then, the heat pump can deliver 4 times more power from the PV electricity, so you are once again at worst at net zero, and the PV is cheaper to buy and cheaper to install.
what if I have blocking from the power company?
Use a volumizer or semi-buffer (see video on semi-buffer setup at the bottom) and recirculate the water if you really have to. You'll probably be fine even without that. Note that high/low tariff may not result in any significant money savings, considering you have to heat the house harder (aka higher flow temperature) in non-blocking hours (obviously depends on the price difference between high and low tariff).
(What is blocking?: grid operators have a system for delivering control signals to large consumers, such as heat pumps or car chargers, to disable them during regularly scheduled times, to reduce consumption during high demand hours. This is typically accompanied by a high/low tariff power scheme, where you pay less for electricity during non-blocking hours. Blocking hours are typically spread over 2-3 morning and 2-3 evening hours, separated with gaps).
Math on blocking:
- assume 100kWh per day heat loss, 7C outdoor air temp
- you can heat your house 24 hours a day at $0.20/kWh, or 20 hours per day at $0.16/kWh
- you need to increase your radiator air-deltaT ~20% to deliver the extra heat to offset the ~20% downtime
- increasing air-deltaT from 15 (35C) to 18 (38C), flow temp from 37.5C to 40.5C, reduces COP by about 0.2
- 100kWh heat at 4 COP is 25kW electricity, 100kWh heat at 3.8 COP is 26.3kWh = $5 at average tariff, and $4.2 at low tariff, 20% savings
But on a less ideal heat pump install:
- air-deltaT from 25C(45C average flowT) to 30C (50C average flowT, 52.5C flow), reduces COP by about 0.5
- 100kWh heat at 3.2 COP is 31kWh, at 2.7 COP 37kWh = $6.2 average tariff, $5.92 low tariff, only 5% savings
Note: high-low tariff schemes can be worth it more if you have no solar, and have high power consumption in general, heat pump aside. Your savings on the low tariff can be much more significant than the savings on heating.
Reading on hydraulic separation - efficiency:
https://protonsforbreakfast.wordpress.com/2024/06/17/heat-pump-experiment/
Absolute fuck you experiment on buffer tanks:
https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently
Plate heat exchangers do not store, they just introduce a permanent - 0.2 to - 0.5 COP to your system. Enjoy paying more for electricity and gaining nothing. They are sometimes sensible when retrofitting to extremely old heating systems, where replacing all the old rusted out radiators and pipes is not feasible, to prevent the dirty water from entering the modern heat pumps/boilers with their tiny impellers.
State change heat accumulators (PCM) - they are typically much smaller compared to accumulator/buffer tanks, and can store comparatively a lot of energy. The material inside melts, and this state change stores a lot of energy (think how it takes a lot of energy to melt 0C ice into 0C water).
The major issue is that these aren't any cheaper than buffer or accumulator tanks, therefore the economics aren't any better. Current year commercial products are also not exactly reliable. The principle of just buy lithium batteries applies to solar electric storage.
Volumizer vs. buffers
The only major difference between a volumizer and a buffer is how you connect them. They're both just a big water tank, but the buffer usually has 2+ inputs, 2+ outputs, the volumizer only 1 input and 1 output. The buffer is connected across the flow and return, the volumizer is connected inline, usually on the return side.
When do I need a volumizer?
Mostly when you have zoning and lots of thermostats that can shut off and reduce system volume significantly (in some countries, zoning might even be mandatory). The low water volume might mess up your defrost cycle, causing what's known as a defrost death spiral - when the unit can't deposit enough heat into the available water volume to conduct a proper defrost, effectively wasting electricity and doing 0 heating.
The low water volume can also cause frequent cycling, ruining your efficiency.
See also - Reduced output section.
However, the fundamental solution is to just not zone if at all possible, or force zoning valves open and set thermostats open.
Heat pump principles of operation
Total output power
Heat pumps are generally labeled based on their nominal output power - i.e. a 7kW heat pump can output about 7kW of heat in typical conditions. Typical conditions are usually defined as 7C outdoor temp, 35C flow temp, whatever rated flow in l/hour. When these conditions change, the output power changes. The flow should not arbitrarily change if you didn't fuck up the circuit, but the outdoor temperature will change, and the flow temperature can change depending on your heating system. 35C flow might be enough at 5C outdoor temperature, but may not be enough at -2C outdoor temperature.
Manufacturers vary wildly in how optimistic they are in labeling their products in this regard
A 7kW Vaillant Arotherm Plus is said to deliver 7kW of heat at -5 outdoor temperature even at 55C flow temperature.
A 7kW LG Therma V M lists a nominal 5.5kW of heat at 7C outdoor temperature at 55C flow temperature. But in the same brochure, it lists that it can do 55C flow at -7C while delivering 7kW, including defrost cycling. Anyone's guess what to make of LG's numbers, however, the principle of "nobody has magical tech" probably applies and both heat pumps probably have similar low temp performance.
Some manufacturers will artificially inflate this number by including the performance of a backup (resistive heater) into the number; however, those units usually come with such heater, so in the end, at least they still don't leave you cold. Vaillant also downrates their units, a 7kW Vaillant is more equivalent to a 9kW from other manufacturers.
Reduced output - modulation
Modern heat pumps are generally inverter-driven, meaning they can run (approximately) from 30% to 100% of their rated output, and adjust as needed.
In the above (non-specific) chart you can see that heat pumps deliver best efficiency at around 50%-ish of their rated power. Running at 30% or 100% can still be assumed to be more efficient than frequent cycling. This leads to certain takeaways:
- the majority of the heating season, you want your heat pump to be in the 40-70% range
- you will get comparatively poor COP when the weather is very mild and you're only heating a little (but you still pay only so much for electricity)
- running at 100% is not especially good, if you can help it, you should distribute the heating load better
See also: https://www.youtube.com/watch?v=tpaPbbtwU3w
https://www.youtube.com/watch?v=XdgIoLXrvPU
Cycling
Note how COP dips a bit at startup. It's not a major effect, many modern heat pumps can be manually configured to cycle less.
Following up the above concept of modulation,
- it's better to have your heat pump running for 50 minutes at 30% power than to have it start up 3 times for periods of 5 minutes at 100% power
This is where the buffer tank salesman interjects - if you have a small system, just add a buffer tank! But this can usually be adjusted in settings. Allow your property to drop to a lower temperature before heating starts up again (or set maximum cycling frequency - this has the same effect).
Defrost
Since the heat pump is (when heating your house) cooling down the outside, the heat exchanger will invariably be very cold. This will have two effects:
- Condensation will appear on the fins
- The condensation will then proceed to freeze
This only happens in a specific temperature range, partially dependent on humidity, usually around 7-0C ambient. Above 7C, the fan can keep the water from freezing. Below 0C, there's much less moisture in the air, so the condensate doesn't form as quickly.
The mechanism of defrost is simply reverse operation, the heat pump switches into cooling mode. It uses the heat from the water in the pipes and the system, potentially also pulling some heat from indoors. However, in a well designed system, the energy efficiency of the heating is still very good, even when defrost is happening! The defrost energy loss is typically calculated into all relevant COP and total heat output ratings that you find in manuals, or better said, the heat pump is already designed to work well even with defrost cycles.
Failure to defrost happens when system volume is too low - the unit cannot complete a full defrost cycle, freezes over quickly when it starts heating again, tries to defrost again, once again there isn't enough water to finish defrost - and stays in this cycle until either the weather gets hotter or someone manually unfucks things.
As mentioned in the system design section, you need to have a certain water volume available for well-functioning defrost - generally speaking, the more, the better. Heat pump spec sheets will sometimes specify the minimum volume, somewhere in the range of 20-30L for average-sized heat pumps. Bigger heat pumps with larger fins need more water for defrost.
COP caveats
One major caveat of aiming for high COP comes with well insulated properties - if your home is extremely well insulated, you can't achieve an especially high COP, because you need so little added heat that even the smallest heat pump you can buy (around 4kW) has to cycle a lot. So you might end up with a perfectly designed heat pump system that only does 3-3.5 SCOP - but your heating bill is still extremely low, since total electricity used is low.
DHW will usually fuck your SCOP at least a little. Buying the most expensivest DHW tank with the biggest fanciest heat exchanger can help.
Some heat pump manufacturers do not include any practical COP monitoring in the heat pump's control unit, even though the heat pump itself monitors both power use and power delivered! This is a great trick for helping installers get away with crappy installs, since aftermarket measurement devices easily add a couple hundred dollars, and the average person won't even remotely get to the point of thinking about buying them. Even if they call in another installer, they still can't find out the unit's performance. If it's fucking impossible to find out your COP, you aren't bothered by how bad it is - at most you can compare your power bills before and after.
how fix
Buy ESP32 for $3, buy 2 temperature sensors for ~$5, buy 1 flow sensor for $15, maybe a 1/3 phase power use monitor for $40, hook them up, ???, smart COP/SCOP monitor.
OR
Buy heating power monitor unit for $200 (which includes all the above except power use monitor), buy 1/3 phase power use monitor for $40, hook them up, do some math, ??? manual SCOP monitor.
OR
Buy this thing https://shop.openenergymonitor.com/ that basically has all the shit you need, with minimal setup/install time - but it's expensive.
OR
Before buying a heat pump, check the install manual for all the available readouts on the control unit. Some manufacturers (i.e. LG) give you some of the data through their phone app. Some manufacturers (i.e. Immergas) tell you to go fuck yourself.
OR
Check if you can find a MODBUS map, this allows you to hook into the unit's internal sensor readouts and lets you calculate accurate COP.
COP isn't the endgame, your total cost of heating the property is. Briefly mentioned above, a very well insulated property will likely have a not too impressive COP, because the heat pump just doesn't have to run that much. But the electricity bills will be low.
The lowest running cost of a system is when the heat pump is off.
Consider the following example: a commercial building needs 20C indoors for 8 hours a day, for 16 hours a day nobody is using the building. During the weekends, nobody is using the building. The nominal heat loss of the property is 7kW. Is it better to install a 7kW heat pump that runs with a SCOP of 4.5, keeping the building constantly heated to 20C (since it couldn't easily catch up with large temperature swings (at design outdoor temp) if you let it drop to, say, 12-14C when the building isn't utilized)? Or is it better to have a 12kW heat pump that runs with a SCOP of 3.5, that allows the property to drop down to lower temperatures when not in use, and has the capacity to heat it back up to 20C relatively quickly even when it's fairly cold outside?
Considering that the building is only used 25% of the week, you can save quite a lot by reducing your heat loss ~40-50% for 75% of the week, so it's likely worth taking a 20-30% hit on the SCOP (and in reality, your SCOP probably wouldn't take such a major hit, maybe closer to 10-20%). But in this case, you have to oversize the heat pump at least a little, and then set your setback/heating such that it has enough time to catch up with the target temperature.
This is all of course assuming that you have an otherwise well designed system.
System components
Filters
Both magnetic and regular strainer filters are required for any heat pump install. You must absolutely make sure that they are rated for the flow you need! Especially strainer filters can easily drop your flow to half of what you need if they are not suitable for high flow use (think dense mesh).
butt why buy more shit
Particles clog up your heat exchanger and pump, more importantly the heat exchanger. It can fuck your expensive heat pump. Any cleaning, if at all possible, will cost more than the filters.
Filters need to be cleaned regularly - the definition of "regular" depends on how messy and old your heating system is.
If your filters clog up very quickly, it's probably a sign that you need to get your system power flushed, as it's full of garbage. It's /diy/-able if you are so inclined, see https://www.youtube.com/watch?v=8EjHvMQa0fg
Antifreeze valves/glycol
Split heat pumps do not need antifreeze protection as such, since the refrigerant in the outdoor lines is much more frost-tolerant. This is for monobloc units. The unit will not freeze as long as it has power - they have low temp protection programmed in. Frost protection is only about protecting the unit and pipes during a power outage.
Do not use glycol if you can help it. It reduces the heat capacity of the refrigerant (water-glycol mix), requiring you to use larger bore pipes. Someone may con you into installing a full hydraulic separation heat exchanger, so the glycol isn't mixed into the secondary heating circuits. Just don't.
Antifreeze valves are mechanical temperature-controlled valves that you install next to your monobloc unit (outdoors). The valve is kept warm by the flowing water (and enveloped in insulation to remain warm). When there is a power outage, the valve will first pull warmer water from the inside to keep the lines warm, closing as it is warmed up by the water, then eventually it drains the whole system, thus preventing frost damage to the lines. Obviously you need to top the system up manually after such an event.
Some manufacturers recommend installing 2 antifreeze valves, one on each line, but 1 should be enough. It should be placed at a low point on the outdoor line, with decent drainage in the area (so you don't flood your basement with the water from your heating system).
Optionally, you can install NC (normally closed) solenoid valves indoors. The NC valve closes as soon as a power outage occurs, then if the antifreeze valve opens, it only drains a couple liters of water from the outdoor lines. The downside is that 1) the solenoids constantly drain some 10-20W of power, non-stop, 2) if your entire house were to freeze, your indoor piping would die. Of course, it would take very extreme conditions for an entire house to freeze over.
Expansion vessel
Expansion vessels are pressure tanks with a diaphragm inside. One side of the diaphragm is pressurized with air to a nominal pressure (usually 1.2-1.5 bar for central heating, for DHW the same pressure as your water input, usually 3-4 bar). When water is heated, it expands in volume slightly, the expansion vessel takes up this expansion. Without a vessel, the expanding water would either discharge through a safety valve or destroy your pipes and leak out.
All modern combi boilers and heat pumps have an expansion vessel inside. You have to check the spec sheet for how much system water they can handle (counting the maximum amount of water in the heating system). If you're unsure/have a large house, expansion vessels are cheap, just put in an additional external one.
Expansion vessels can fail, as well as lose pressure in mundane ways (poorly fitted cap, bad valve), when this happens, you might notice your system pressure dropping (due to the expanding water being discharged through the safety valves), possibly also water puddles appearing near your boiler/heat pump.
Piping
TL;DR: use crimped PEX-AL. Copper if you have money to burn.
- Copper:
Pros: durable, somewhat shapeable (with the right tool), can be crimped, soldered fittings are cheaper than PEX-AL crimped - can be overall cheaper than PEX-AL for central distribution areas with lots of corners and threaded connections. Much larger ID for the same OD compared to plastic piping. All fittings are external - they don't interfere with flow in the pipe. Looks cool.
Cons: expensive, can corrode, soldered connections require some skill (+ open flame), crimped fittings are very expensive - PEX-AL composite: plastic-aluminium-plastic pipe, the aluminium provides an oxygen barrier layer and some extra strength and shapeability to the pipe
Pros: much cheaper than copper, much easier to shape/turn/bend than copper - easier installation in tight spaces. Very good durability/longevity. Very resistant to corrosion. Can buy pre-insulated.
Cons: fittings are expensive, less temperature tolerance than copper, much smaller ID for a given OD (32mm has 3mm wall), fittings plug into the pipe - further increasing resistance and causing turbulence - may cause noise and flow issues if undersized. Generally requires stepping one size up for the same throughput as copper. Doesn't look very cool - you will never fully straighten the pipe.
Note: always buy PEX-AL pipes with welded alu layer, twisted alu layer is shit. You can see that it's a welded layer when it has a small overlap of the Al layer on one side - Stainless steel:
Pros: it's pretty good
Cons: it's pretty fucking expensive - Steel (welded)
Pros: extreme longevity and durability, pretty cheap to buy
Cons: corrosion - possibly requiring water treatment and more serious filtration. Installation cost can be way higher than most alternatives, since it's way more a hassle to weld pipes than crimp some fittings - PPR: usually gray, thick walled plastic pipes
Pros: Very cheap - cheapest out of all viable materials, easy to install - can either be glued or welded, the welding tool is cheap as well
Cons: Has been found to leach lead, copper and other fun things into the water. This is not a health issue with central heating, but some of the materials may accelerate corrosion. Not very UV stable - not a good choice outdoors. Typically does not have an oxygen barrier and leeches some oxygen. - HDPE: Isn't really suited to high temperatures, great for ground loops in ground-source shit
- PEX (non-composite): Has no oxygen barrier. Do not use in central heating, ever
- CPVC: leaches all kinds of good stuff, can become brittle from age or heat, outlawed in some countries
Copper fittings typically use U-type crimping dies, PEX-AL uses TH or U-type , stainless and other types of crimp fittings can typically work with U or M. You can either rent a battery powered crimping tool, or buy a manual hydraulic crimping tool from Chang for about $100.
why do I care about oxygen barrier lol just install automatic air vent
The presence of oxygen accelerates corrosion (your radiators are probably still steel, even if your pipes aren't) and, at extremes (especially in lower temp systems), enables bacterial growth/overgrowth, which can turn your heating fluid into sludge.
Circulation pumps
They're small AC pumps with tiny impellers pushing water around.
They are usually very low power, in the range of 20-100W. However, since they run a lot, they may still contribute significantly to energy bills. I.e. 50W * 24 (h) * 200 (days of heating) = 240 kWh. This is one of the indirect ways hydraulic separation is hurting your bottom line - hydraulic separation adds at least 1, likely unnecessary, pump, and $50-$100 down the drain yearly.
what do the numbers mean
First number is usually rated flow rate in L/min. Second is rated head (how much pressure they can create) in meters. Sometimes a third number denotes size or power.
how do I know what pump I need
You need to know the expected flow rate for whatever system you want to pump into. Next, you need to know the head (pressure requirement) of the system. Then you look at the spec sheet and see if the pump can deliver enough flow at a given head.
but I don't know any of that
Flow rate is calculated from the heat you need the system to deliver and the deltaT (see above - mass flow rate). The heat you need is from a heat loss calculation, or observing your existing system.
There's no easy way to calculate head on a complicated system. If you're doing UFH, you can calculate pressure loss (same thing as head) from the length of the pipe, diameter, and amount of pipes.
As a rule of thumb, just try to run the system without any additional pumps (adjusting pressure by gate or flow valves). If it doesn't deliver enough flow (and you've ruled out any alternate causes), you add a pump to the most restricted part of the system, or one big fucking pump to run everything.
Note: if you have an existing gas boiler, you can check what pump it has installed and what power it's working at, optionally measure flow rate from the pump (or calculate it from heat delivered - inverting the mass flow rate calculation)(or maybe your boiler will display it), and you can roughly work out your system head.
Big fucking pump added inline (in series) with the original pump works more or less in addition. Several pumps in parallel can cause all kinds of fuckery, stealing water from each other, especially if you have hydraulic isolation. If you have a gigantic house or extremely narrow pipework where you simply somehow need multiple pumps in parallel, you have to very carefully design the feeding pipes such that the pumps can't easily steal water from each other and the flowrates are well matched. See also close-coupled tees/low loss header.
Typically, in a regular sized home, if you don't have flow going to one or several radiators, it's not an issue of not having enough pump power, and it's not an issue to be fixed by adding more pumps. It's usually an issue of bad system design, poorly laid out existing pumps/valves, possibly unregulated radiators with excessive flow stealing flow from other radiators.
More viewing material on pumps:
https://www.youtube.com/watch?v=AiqERhITSSE
https://www.youtube.com/watch?v=Gk4Mzvcaen0
common failures
The tiny impeller likes getting clogged by all kinds of shit imaginable, if your not so new boiler/heat pump is somehow not heating as much as it used to, this may be one of the prime suspects. If you hear a high pitched sound from the pump, once again, it's probably debris in the impeller scratching the frame.
If you hear low frequency vibration in your radiators or near the pump, the pump bearing is dying. Reduce pump speed and the vibration should temporarily go away. Plan to replace the whole thing sooner rather than later.
Rarely, air can get trapped in the pump - the big screw in the middle is for bleeding air, carefully release it until some water drips out, then shut it again.
Valves
The ideal way to adjust power delivery to different parts of your system are valves, valves that you should not be adjusting often past the initial setup.
The ideal ideal way is to put a good lockshield valve on each radiator, which is adjusted to the flow rate you need for the given radiator. Larger groups in the system can be managed centrally either by flow adjustment valves or gate valves.
Gate valve/globe valve - control both flow and pressure, good for fine adjustment. Globe are the more advanced version with better reliability and finer adjustment, but also present a considerably larger obstruction in flow than gate valves (this obstruction is semi-intentional here - you want the valve to have more authority - the authority gives you more precise control).
Flow control valve/balance valve - more complicated and expensive valves that adjust for pressure drops to ensure constant flow. They should be wholly unnecessary in an open system, they're more useful if you have variable loads (zoning) that can cause pressure, and thus flow, spikes.
Ball valve - NOT AT ALL THE SAME AS GLOBE VALVES, used for shutoff, not for regulation. Make sure they are full bore valves for heat pumps - they have no flow obstruction when open.
Lockshield valve - typically found on radiators (on the return side, often covered by a cap - hence lockshield), for finetuning radiator flow. When balancing your system, these valves are the ones you should be turning down if any one radiator gets too much flow (rather than fiddling with the input valves).
More on lockshield valves: https://www.youtube.com/watch?v=zxZE6b3-hEs
Check valve - one way valve, when you don't want water to potentially go backwards. Can be useful if you are connecting two heat sources (ie. boiler + heat pump) or have hydraulic separation in certain areas (ie. UFH). For high flow pipes, always use swing type check valves - they do not cause flow obstruction.
Automatic air vent
There's a float inside, shutting off the outlet when it fills with water. When the whole thing fills with air, float drops down, outlet opens, releases air. When there's considerably more water coming out than air, or after several years you see lots of corrosion/water stains on the side, it's probably fucked, and you might want to replace it. The outlet should generally be left open, so that it's doing its air removal thing automatically. There is most likely one already installed in the boiler/heat pump, but adding more doesn't hurt. You should mostly be putting them on high spots and areas with slower flow (at the very least, never mount them upside down or on the bottom of a kink in the pipe).
There is an advanced principle that, since auto air vents can, and do, fail, you will have the very best system if you have zero auto air vents, and you only ever manually purge the air. Some manufacturers design their shit in such a way that you connect them to mains tap water and flush the system with mains pressure, purging air into the drain. I personally would rather put in auto air vents and reduce the risk of airlock.
DHW / Domestic Hot Water
Now with DHW, you don't have a choice on hydraulic separation. For DHW, there has to be a tank with a heat exchanger (ideally a very big heat exchanger). The heat pump can control a three-way electric valve to automatically switch to DHW preparation as needed, based on a temperature sensor in the DHW tank (which is also wired to the heat pump). Since people usually like to have pretty hot DHW, this can also be an area of major loss of efficiency - if, for example, you want 50-55C water. Further, if you want to have 60-65C water or do Legionella cycles, you either need an R290 heat pump, or for R32 heat pumps, a resistive immersion heater that finishes the heating from 55C to 65C. The R32 pumps are also equipped with the control logic to manage such an immersion heater.
However, Legionella is kind of a meme, and so is the 65C target temperature. For one, you can get Legionella just about anywhere, it probably grows in your shower regardless of what cycles you do in the hot water tank. For two, you can eradicate Legionella even at 55C.
In summary - do the Legionella cycle to 55C, not 65C, otherwise keep DHW at 40-45C for better efficiency, if possible. The SCOP for DHW will always be pretty bad, at best you can see numbers around 3.5.
If you have solar, and you have waste electricity that you're not using for anything, solar DHW immersion heaters can have alright ROI. It's still not a great deal, especially if you're paying a commercial installer to set it up.
Key heat pump (+combi boiler) settings
Weather compensation - the pump monitors the changes in the outdoor temperature, and (with a lag factor) changes water flow temperature to react to said change - thus delivering the heat equivalent to what was assumed to be lost in the temperature change. The indoor unit can potentially also add a small additional boost/deboost based on the temperature it reads inside - if the room is very cold, it heats more than it usually would, and vice versa. The specific characteristics of the weather comp curve will be described in the install manual of the heat pump.
Getting it right for your house is trial and error (and a function of all the stuff described in the "System design principles" section).
Building type or some equivalent setting - adjusts the above mentioned "lag factor" - with a "light" building, meaning it has a low thermal mass, the heating will respond quickly to weather changes, with a "heavy" building, one with large thermal mass, it will respond to weather changes more slowly.
DHW priority - usually goes along with weather compensation. DHW is heated when there is demand, plus maintained at some set temperature as needed. You'd normally want this enabled.
Constant temperature mode is the devil, do not use it. It makes the heat pump try to deliver a constant high temperature in the primary circuit. This is usually coupled with some kind of hydraulic separation - often a buffer/accumulation tank.
If you have a poorly designed system, turning it off (in favor of weather comp) can sometimes make things worse, so only fiddle with this once you at least vaguely understand the rest of the shit in the guide.
Note: for cooling, you will probably have to use constant temperature or some variation of temperature limiting due to condensation - the above only applies to heating!
Setback modes - "Nobody's home during the day, so I'll set daytime target temperature to 15C, then to 24C only in the evening when we are home, I'll save money!" is a quick and easy way to make your heat pump system fail. Using some setback is okay, but it's generally bad to introduce large temperature swings, especially when the outdoor temperatures are low.
There are two aspects to this:
- the more important one - when the outdoor temp is low, your (not oversized) heat pump is already having to deliver near-full power to maintain your target temperature. Meaning that it will take a very long time to deliver excess power to not just replace your heat loss, but increase indoor temperature. This obviously isn't an issue when outdoor temp is not low, but on cold days, it can cause the heat pump to fail to rewarm your house from the setback.
- when it's not so cold outside and the heat pump is suddenly told to make it 24C from a ~9 degree setback, it's going to aim for making very hot water to rapidly reheat the house, leading to losses in COP
This doesn't mean that you should never use setback - when used correctly, it will save you money. But you have to manage your settings (and expectations) accordingly. Give the heat pump enough time to reach target temperature after a setback, especially in winter months, and don't introduce enormous temperature swings. Alternatively, if you really want to play with large heat swings, an oversized heat pump can do it.
Cycling frequency/Hysteresis
Specific manufacturers may let you change one or the other, or neither. A cycling frequency setting forces a limit on amount of starts per a given time period.
Hysteresis inverts the concept, by letting you set how far you're willing to allow indoor temp (or DHW temp) to drop before a new heating cycle starts. The more hysteresis you allow, the less cycling you will see in transition months.
Note: some of the controls might be specific for the indoor control unit of the heat pump. In other words, you might not find the description in the heat pump install manual, but in the control unit's own install manual
Cooling with an air-water heat pump
Almost all air-water heat pumps can do central cooling, at no extra cost! How cool is that? So cool that your feet will freeze!
There are two major caveats for cooling, when we assume no system modification. (No modification meaning: existing radiators and UFH > pipe in cold-ish water from the heat pump > ??? > cooling!)
Caveat #1 is the dew point. If you hit the dew point (which is a function of humidity and temperature in the room) on any part of the system, anywhere in the building - pipes, radiator, including pipes in the walls - condensate will start to form. It can be a quite significant amount of condensate, so it's quite important to avoid this very much. The way to stay safe is to simply not set the heat pump too low - maybe set it to 18-20C flow temp if you have mid-range humidity and 25-28C indoors. The dew point should be around 10-18C (always depends on humidity).
Temp | Humidity | Dew point |
---|---|---|
24°C | 40% | 10.5°C |
24°C | 50% | 13.0°C |
24°C | 60% | 15.3°C |
26°C | 40% | 12.6°C |
26°C | 50% | 15.1°C |
26°C | 60% | 17.5°C |
28°C | 40% | 14.7°C |
28°C | 50% | 17.2°C |
28°C | 60% | 19.7°C |
Caveat #2 is the location of cooling. In "cooling mode", your radiators will be pulling in hot air from the top, and delivering it cold at the bottom. Then the cold air is perfectly content staying at the bottom 20cm of the room. Your feet will be in 20C, your torso and head in 25-28C.
how fix
a) The True Final Solution are radiant ceiling panels. They are basically underfloor heating, but in your ceiling. They deliver the cold from above, thus resulting in good mixing, plus they absorb some radiated heat. They can also be used for heating. Cons: it's very expensive to buy and install, you need to run pipes.
b) The meme solution are fancoils. It might well be cheaper to install a separate AC unit than to buy a fancoil and run separate piping from your heat pump (if we're talking about a retrofit. If you are planning a new build, it's not a bad idea). All the noise and discomfort of a split AC unit, now with potentially lower efficiency and cooling power. To be fair, there is a large market of various fancoils, and they are not a terrible option - fancoils generally also have the added bonus of removing moisture from the air, which reduces perceived temperature (but you also have to pipe that condensate water outside somehow).
c) The el cheapo solution are fans. Point fans at your radiators, mount small fans on the bottom of the radiator to push air upwards, introduce any means of mixing the air.
UFH is also pretty okay at cooling (at least better than radiators), owing to the large surface area absorbing a lot of radiant heat. Comfort is obviously going to be less good than the radiant ceiling panels, but UFH is still much cheaper even in a new build.
Considering that the radiators will be operating at air-deltaT of only 5-10C, the cooling power will be much lower than the usual heating power. I've yet to see concrete numbers, but you can probably expect about 1-2kW cooling power for each 10kW nominal heating power in an ill-prepared system (without any major modifications). All in all, in such a situation it's a free bonus function, so being better than nothing is good enough.
More material on cooling:
https://www.youtube.com/watch?v=GKDucs4zl7Y
https://www.youtube.com/watch?v=dn4Ja_q5M9g
Note: some manufacturers (Vaillant) require you to install what is basically a license chip for a light one-time payment of ~$300 to "unlock" the cooling ability on the heat pump. Enjoy current year.
Recuperation, ventilation
When you have a heat pump running at 4.0+ SCOP, it's difficult to justify the economics of significant expenses involved with various recuperation schemes; especially compared to how much better ROI you can get from installing/expanding a solar system to accommodate heat losses.
Local recuperation vents are a tube, a two-way fan, and a big ceramic element with lots of holes in them. Tube goes in the wall, fan pushes air out, heats up ceramic, fan switches to pulling air in, drains heat from ceramic. While manufacturers will grossly lie about these units potentially achieving 90%+ heat loss reduction, in reality it's more like 20-50%. The amount of heating power this saves you is borderline nothing, especially considering the $200+ price tag on these units.
Even then, if you want automatic humidity management and automatic airing, it's not the worst thing - all your alternatives are more expensive.
The worst part is probably the sound insulation loss. Now you have a hole in your wall that transmits noise from outside with great efficiency.
Local systems are ideally placed to multiple points of the house, and synced, such that one pushes air out while other pulls air in - thus much more effectively venting the entire house, rather than just one room. This synchronization is generally an advanced/"smart" feature you have to fork out extra for.
Local recuperation can be /diy/'d at fairly low cost, and you can set up synchronization with a couple ESP32's, so if you are willing to do the work and can find good spots for the vents where the potential noise isn't going to cause issues, it can be somewhat viable.
Central heat recovery ventilation (HRV), central dehumidifiers mitigate pretty much all the issues of the local recuperation, only that now you're paying several thousands for the unit and the ductwork. But, here as well, you're not looking at 90% reduction of heat loss, more around 50%, and a ROI of never. Again, it's more about automating air changes and ensuring constant humidity than any significant money saving.
TL;DR - central heat recovery can be good for comfort and automatic air management, at a fairly large initial cost and ROI of never. Local is not very good. Buy more solar panels.
Examples of heat pump systems
/diy/ budget 5kW Samsung heat pump, uninsulated property, 4.3 SCOP: https://www.youtube.com/watch?v=Hyv_vQEvHgo&list=PLfU4EBfUMOvnVTB0WhyqQ23pvt7hnrc54
Microbore (10mm) pipe not well insulated house, 4+ SCOP: https://www.youtube.com/watch?v=kxKEx1z00-4
Semi-buffer (3-port buffer) setup (timestamp 14:30): https://www.youtube.com/watch?v=u8ip-bUooJ0
The buffer tank is hooked up only at 3 points - heat pump flow, radiator flow, common return. This reduces mixing inside the tank and can improve efficiency. There is still not especially much reason to do this over a direct open system, but if you really want to store some energy for varied tariff billing, it's an option. Matching secondary pump flow to primary (heat pump) flow is important.
Heat pump gross sizing failure: https://www.youtube.com/watch?v=NPTvBBWLHxs
SkillBuilder/HeatGeek heat pump saga:
Lightly insulated, large house gets oversized, extremely poorly designed heat pump system - doesn't work for shit, terrible SCOP, massive bills. HeatGeek eventually overhauls it and achieves 4.5+ SCOP.
https://www.youtube.com/watch?v=Zar7RTQodnw - prequel, not much of importance
https://www.youtube.com/watch?v=1rKNT7-42J0 - everything wrong
https://www.youtube.com/watch?v=OzSEQ77t20I - heat loss calculation
https://www.youtube.com/watch?v=BesfqnHPxLU - fixing it
https://www.youtube.com/watch?v=XCMaup9Eosw - results
~5 SCOP professional install with UFH and fair amount of optimization of the building: https://www.youtube.com/watch?v=WrnnuPEcW6I
Heat pump integration with Home Assistant via MODBUS https://www.youtube.com/watch?v=Xuj2YFZ5zME
Technical overview of the internal components of a heat pump https://www.youtube.com/watch?v=Xjly2LSHUhM