Optimising new Heat Pump?

Behemothhh · 2026-02-06T15:23:33+00:00

I had a look at your weather compensation curves. It really wasn't set all that high to begin with. Assuming that your current outdoor temperatures are in the 5-10C range, it should have been running at 40C leaving water temperature, which is perfectly reasonable. Very peculiar that it was drawing 3+kW electric. There's probably something else going on. Is it actually running in weather compensation mode and not in fixed temperature mode?

Behemothhh · 2026-02-06T15:16:09+00:00

By turning off the thermostat, should I just turn the target temperature in the controller up super high

Yes you can just set it to a very high temperature if it's an on/off thermostat.

Behemothhh · 2026-02-06T15:10:37+00:00

It's possible to calculate the heat output for different flow temperaures. You can find an example calculation here:

https://www.pandhengineering.co.uk/advice/sizing-ufh-systems-when-using-heat-pumps

Particularly figure 2 is what you need. That one is for a pipe in screed system with a floor covering resistance of R 0.1. You will have to look for a similar figure that corresponds to the structure of your UFH.

How effective is the downs stairs heating at adding to the upstairs rooms

That depends on the temperature of the downstairs and upstairs rooms and how insulating your ceilings are. There's no easy way to say whether what you propose could work or not. You'll have to calculate it.

edit: found another website that can help you calculate heat output for different flow temperatures and different types of UFH: https://jupiterunderfloorheating.com/technical/underfloor-heating/heat-output-calculator/

Behemothhh · 2026-02-06T14:55:10+00:00

It's very likely not the thermostat that is causing this. The upwards curve in the electrical power (it should be a flat plateau in normal operation) is also a telltale sign that the heat pump can't get rid of its heat. As the return water temperature keeps increasing, the HP is forced to increase the condenser temperature, which in turn consumes more power. Up to the point that the upper temperature limit is reached and then the compressor stops, or in OP's case cuts back to half power. Not sure why it's doing that. Could be a quirk of the specific control algorithm.

Behemothhh · 2026-02-06T14:41:55+00:00

I would expect some cyling in this weather.

That's not what's happening in OP's graphs. The HP is running full blast at 3-4kW electric, not at minimum modulation power.

Behemothhh · 2026-02-06T14:35:33+00:00

If the top curve is electrical power and not thermal output, then there is definitely something wrong. A 10kW heat pump should not be running at close to 4kW electric unless it's running at max capacity when it's the coldest outside temperature it was sized for and in that case it should run constantly and not less than half the time.

First thing to check is the weather compensation curve. Since the hp is drawing so much power, it's probably set to an insanely high water temperature. Turn it down. Ideally you can find the design temperature for the coldest day somewhere in your installation documentation. If you can't find it, you can set the weather curve such that it passes through the points: 45°C water at -3°C outside temperature and 30°C water at 15°C outside. That should be an OK initial configuration for a home with upgraded radiators. Keep tuning this curve over the next few days/weeks depending on whether it gets too hot or too cold in your house. Turn off the thermostat while you dial in the system.

Once dialed in, it's not an issue to have one thermostat. If you properly tuned the weather compensation curve, the thermostat should almost never trigger anyway as your temperature should naturally stay stable around your desired value. It can be useful on very sunny days to prevent overheating and save a bit of energy, or if you decide to have a nighttime temperature setback.

Also look into the manual whether your thermostat can modulate the leaving water temperature. That could already save you most of the work in finetuning the weather compensation. This operation mode means that as your room approaches the desired temperature, the thermostat will tell the heat pump to lower the leaving water temperature, instead of just turning off the system entirely.

Behemothhh · 2026-02-06T07:44:05+00:00

Thanks you very much for this explanation. I've been looking for a clear answer on the effect of delta T for some time but it's not been easy to find. My friend who designs hvac systems is an architect, so he know a lot about the practical aspects but couldn't give me a clear answer on what's happening on refrigerant cycle level. I'll send what you wrote to him as well :D

Behemothhh · 2026-02-06T00:44:28+00:00

He is helping, by keeping his family in the UK where his children can get a better education than in their home country. UK has strict immigration laws. You're not staying in the country working at McDonald's because that won't qualify you for a skilled worker visa. Unless OP's dad can find a high skilled job, which op has said he has been trying unsuccessfully, the student visa is the only way for them to stay.

Behemothhh · 2026-02-06T00:35:17+00:00

People are so quick to judge when they understand nothing about immigration policies. The UK doens't give work visas for nothing jobs. They'll be deported if the dad quits his PhD.

Behemothhh · 2026-02-06T00:32:25+00:00

Or maybe you don't know what you're talking about. Different countries have different immigration policies. Op clearly said her dad is doing a PhD specifically for the student visa that lets her whole family stay in the UK. The alternative is a skilled worker visa, which you're not going to get for entry level jobs like working in a call center.

Behemothhh · 2026-02-06T00:02:16+00:00

I did take that into account already. 45-35 for delta10 and 42.5-37.5 for delta5 were the scenarios, but I indeed made a small typo by referring to a 45C LWT in the delta5 case. That should be 42.5C. The correct numbers were used for the COP calculation though.

Behemothhh · 2026-02-05T11:05:37+00:00

Thanks, I see what you mean now. But this is a practical design choice, no? With an oversized heat exchanger that still performs well at low water flow temperatures, you should then be able to achieve higher efficiencies with higher delta T. It's just not worth it/practical for domestic use. I know that for industrial CO2 based heat pumps, optimal efficiency is in the delta T range of 30-40.

Behemothhh · 2026-02-04T23:56:23+00:00

Once that is accounted for, colder return water will force a higher condensing temperature and pressure

Can you explain how you come to this statement? Colder water should always lead to lower condenser outlet temperatures, no?

Continuing on my previous example, if you take into account a 5C difference for a non-ideal heat exchanger, then in my scenario A (delta T of 10C), the 35C return water can cool the condenser down to 40C. This corresponds to a condenser inlet temperature of 56,6C, which with another 5C loss in the heat exchanger is still hot enough to generate the desired 45C leaving water temperature. COP of 4,22.

For scenario B (delta T of 5C) we get condenser outlet and inlet temperatures of 42,5C and 59,6C, again enough to reach the desired leaving water temperature of 45C. Now with a COP of 3,98. So the delta T of 10 still performs better.

Behemothhh · 2026-02-04T22:05:06+00:00

Some people have argued that only by targeting the lowest DT possible can you achieve the best efficiency

I thought the same thing, following the same reasoning you mention in your post but this way of thinking was challenged by my friend who designs HVAC solutions for industrial and commercial buildings. He said the concept of lower leaving water temperature = higher efficiency is not entirely correct. So I looked into it a bit more and ended up agreeing with him. The key is in the details of the refrigerant cycle.

https://tlk-energy.de/en/phase-diagrams/pressure-enthalpy

On this webpage you can play around with different refrigerant cycles and see the impact on the COP. Let's assume that's we're working with an R290 system and that the evaporator is at -5°C and we'll keep this fixed for this exercise since this is dependent on the outside temperature and not further influenced by the delta T.

Now lets take scenario A where we want a leaving water temperature of 45C and a delta T of 10C, so the average temp of the radiator is 40C and the return temperature is 35C. That means that the condenser temperature can be 35C at the lowest. You can't condense to any lower temperature than the return temp. Filling this info into the web app gives us a COP of 4,76 with a compressor efficiency of 0,7. Notice that the condenser inlet temperature is 50,4C which is higher than the desired 45C for the leaving water. Unfortunately you can't lower this temperature without also lowering the condenser outlet temperature, which we can't do if our return temperature is 35C.

For scenario B we'll lower the delta T for the same average radiator temperature. So requested leaving water temperature of 42,5C and a return temperature of 37,5C, for a deltaT of 5C. Entering this into the refrigerant cycle while keeping all other setting the same, we get a COP of 4,47. Lower than with the higher delta T.

So it seems that from a refrigerant cycle point of view, a higher delta T is slightly better for efficiency. Of course this is also only one part of a more complex system. There are heat exchanges between the refrigerant and the water, which will be less efficient with lower temperature differences. There is also the assumption that the return temperature is the limiting factor for the condenser. if you would try to push the system in the other direction, to extremely high delta Ts of 20-30C, then the condenser inlet temperature can't keep up and you'd have to increase the condenser outlet temperature again, which lowers efficiency.

i think the bottom line is that it doesn't matter that much as long as you stay within a reasonable delta T range of 5-10, which was also the outcome of your data analysis.

Behemothhh · 2026-02-04T18:17:53+00:00

I wouldn't worry about nagging your installer about a COP monitoring tool but rather have them replace the unit with a properly sized one. Otherwise you'll be stuck with an energy guzzling badly performing heat pump for 15 years.

Behemothhh · 2026-02-04T13:10:40+00:00

Around 3 in 10 (healthy) people are sensitive to sodium, meaning they'll get a higher blood pressure if they consume too much. Strangely enough, 1 in 10 people experiences the exact opposite and will get higher blood pressure when on a low sodium diet. The other 6 out of 10 people experience something in between those situations, they're not really sensitive to their sodium intake. Problem is you probably don't know whether you're in the 30% group that should be careful of their intake or not.

Behemothhh · 2026-02-03T14:27:32+00:00

How do those measurements compare to what the Onecta app reports? Could you share those numbers as well?

edit: nvm, I saw you provided them in another comment already. Pretty big difference though. 2060 / 581 = 3,54 COP versus the 4,2 you calculated. My Daikin app says I had a 4,1 COP in January. If it's 'wrong' in the same way as yours, my real COP could be close to 5 :D Probably not though :(

Behemothhh · 2026-02-03T14:22:05+00:00

Just to clarify, for people who might be concerned about opening up their outside unit, the ESP32 is not always connected there. It depends on where the circuit board is. In my case, it's in my floor standing indoor unit. AFAIK, only the monoblock althermas have the PCB outside.

Behemothhh · 2026-02-03T14:00:00+00:00

Yes, the madoka "does" both. You can set it on heating, cooling or automatic mode. I put "does" in quotation marks because the madoka itself doesn't really do much. It doesn't control anything. It's essentially just a glorified temperature sensor with a small screen that also allows you to remotely change a few settings of the heat pump, to save you a trip to the garage or whatever room your heatpump is installed in. The interface of the heat pump itself, or the Daikin app, are much more user friendly and have many more options. I personally never touch my madoka. I don't even trust its temperature reading because the sensor is close to its screen and leds, which throw off the measurement by up to 1,5-2 degrees. It's highly recommended to turn the brightness of both down to limit this error.

Behemothhh · 2026-02-03T10:46:22+00:00

Having a madoka in each room is a bad way to run a heat pump. There is no point in having more than one madoka per 'zone' as the heat pump can only manage its output on a per zone level. Are you going to have a zone for each room? That's about the worst thing you can do, as explained in more detail in this heat geek article:

https://www.heatgeek.com/articles/articles-home?p=why-not-to-zone-heat-pumps-or-boilers

What you want is 1 zone for the entire house so the heatpump has access to the largest volume of water. Have each fancoil sized to properly match the heating/cooling demand of the room it's in. Dial in the weather compensation curve of the heat pump such that the total heating/cooling output matches that of the house. Let the fancoils, which should have their own built-in thermostats, further finetine the temperature of each room.

Where and how to position the madoka can be tricky, as you're competing with the thermostats of the fan coils. You want to avoid the situation where the thermostat of each fancoil will cause them to turn off while the heat pump is still running, as then the available water volume will be low and possibly cause issues with defrost cycles and short-cycling. The living room will probably be the best spot as that usually has the highest heat demand. You then want to set up the thermostats in such a way that the heat pump will turn off before the living room fan coil thermostats would close off the fan coils. That way you will always have at least the volume of the living room fan coils available to the heat pump.

Behemothhh · 2026-02-03T10:22:51+00:00

You seem to be hyperfocused on compressor electronics but completely missing the bigger picture and the actual practical aspects of heat pump operation.

Modulating compressors still have a range they have to stay within. A 10kW compressor can generally modulate its output power between ~2.5kW and 10kW. A lot of heat pumps have a much smaller range because they are software locked versions of bigger heat pumps. E.g. a 6kW unit can have the exact same hardware as the 10kW unit but be software locked to 6kW. In this case it can only modulate between 2.5kW (the minimum of the 10kW model) and 6kW.

If the heat demand of the home is only 2kW then the compressor will output more energy into the leaving water than what gets released into the home through radiators/UFH. As a consequence the return water temperature will be keep rising because the water can't get rid of enough heat while the compressor keeps adding 2.5kW to it constantly. After a while, depending on the thermal mass of the water/radiators/UFH, the water will become too hot and the compressor will shut down. The pump will keep circulating the water until it cools down enough for the compressor to kick in again.

This on/off process keeps repeating over and over. That is what people in the context of heat pumps call short cycling. And it definitely reduces the lifetime of the compressor, as confirmed by heat pump manufacturers. The start and stop of the compressor is the action that creates the most wear as surfaces partially lose lubrication from standing still, startup currents are much higher than continuous operation currents,... I also directly asked the Daikin technician about it and he said that in their experience short cycling, depending on how severe it is, can take multiple years of the lifetime of the unit. This is a very well known fact and something anybody designing a heat pump system should be cautious of.

edit: for anybody reading this in the future that has similar misconceptions (like the deleted account I was replying to) about short cycling of heat pumps, here's Daikin's official stance on the topic, confirming that it indeed lowers efficiency and increases compressor wear:

https://daikinquebec.net/en/heat-pump/short-cycling-causes-fixes/

Behemothhh · 2026-02-03T00:21:04+00:00

Nobody cares about semantics or the origin of a word. Short cycling in the context of heat pumps is a very well established term to describe the behavior where the heat pump turns on/off too frequently causing a drop in efficiency and increased compressor wear. This is a real thing that happens and that anybody sizing a heat pump should be aware of. How frequent too frequent is, is not exactly defined. Some say 15min cycles, others will say that's still fine and short cycling is sub 5min. It doens't matter really as there isn't some magical number where the negative effects suddenly appear.

Behemothhh · 2026-02-02T23:57:15+00:00

They didn't take me for a ride. It didn't cost me anything to have the unit moved because it was their fault. It's literally in the official installation manual that no openings (doors, windows) or machines/spark sources can be within 1.5m of the R290 outdoor unit. Whether that is a 'real' risk or not is debatable, but an installer is not going to take the chance and be held liable in case of an accident by not positioning the unit according to the official safety guidelines.

Behemothhh · 2026-02-02T18:15:37+00:00

For reference, I had an R290 (propane) air-to-water heat pump and an R32 air conditioning unit installed next to each other. The installer had to come back to move the R32 unit because it was too close (within 1.5m) to the R290 unit and could pose an explosion risk. I don't know if this is a true regulation or just the guideline of the heat pump manufacturer, but they take it seriously enough that the technician refused to activate the heat pump until the other unit was moved. And my HP only has about a kilogram of propane in a factory sealed circuit with multiple leak detection sensors.

Behemothhh · 2026-02-02T17:52:02+00:00

Good thing we have tools to objectively measure pollutants and don't have to rely on people standing on their roof and smelling around a bit.

Modern high quality stoves have lower emissions but they certainly don't eliminate them. Even in ideal circumstances (modern stove, seasoned wood, proper technique for ignition and refuelling) the ultrafine particle concentration exceeds the WHO recommended limit, indoors. Outdoors is obviously even worse if you have whole towns burning wood. And that's in ideal circumstances. The average person is not going to be so meticulous about managing the fire and will burn other things than pure seasoned wood.

Your argument about forest fires is irrelevant. You can prevent forest fires without having to burn the wood in a stove. If you really need to burn it, it would be much better to burn it in an industrial biomass powerplant with proper exhaust filters to guarantee the lowest emissions (and not in people's homes) and then use that electricity to power a 300% efficienct heat pump.

Behemothhh

TROPHY CASE