ReedRichards

6 W or 6 kW? If it's the former I can do you one cheaper.

There was bound to be a squeeze as the RHI comes to an end. If it had not been s shortage of ASHPs it would have been a shortage of MCS accredited installers.

I've had the 12 kW version for 13 months now. I have not had any problems with it except once when I accidentally switched off the hot water and did not realise what I had done or how I had done it. The instructions were lacking at first but then I got hold of an installers manual. The AI feature is "Weather Compensation". Eventually I adjusted the parameters set by the installer and the new more ambitious ones seem to be still keeping the house warm whilst consuming less energy. I was sad that the controller does not do Load Compensation but that feature seems to be relatively uncommon on heat pumps.

Strictly it's the rate of energy transfer that counts, the Watts rather than the Joules or kWh. If you increase the pump speed each litre of water going round the loop will lose less energy but you will be sending more litres of water round in any given time so the net effect is to give you more Watts of heat.

Is your UFH on all 24 hours? If yes and 17.5 C is too chilly for you, can you raise the flow rate to boost the return temperature so the average temperature is higher?

I replaced an oil boiler with a heat pump. My old oil boiler used about 21500 kWh per year's worth of oil (@10.35 kWh per litre) so less than that in actual heat because my boiler was not 100% efficient. My heat pump used 6900 kWh of electricity in its first year of use.

Why did you need a High Temperature model of heat pump? Why do you think over 30 kWh per day is wrong? I have 12 kW ASHP. For the last couple of days it has been quite mild for the time of year and the daily consumption was about 26 kWh. There have been a couple of cold days when consumption was over 40 kWh in a day. But my average for the entirety of last year was 19 kWh per day.

So if both zones or just the radiators are demanding heat then the output water temperature will be 40 C but if only the UFH zone is demanding heat then the output water temperature will be 35 C? And the heat pump is consuming a lot of energy but appearing not to do so because the return water temperature is almost the same as the leaving water temperature? Could the pump speed be too high? Reducing the pump speed ought to increase the delta T.

No, I only really know about the LG heat pump I have myself; I know it is true of that. This does not have "degree minutes" nor a supplementary (immersion) heater. I also have a third party controller so the heat pump has no way of knowing how hard it should be working because it doesn't know what temperature I have set the room to be at. It's a pretty safe bet that anybody else using a third party controller will be in the same boat. Before buying I looked at Grant heat pumps also and they seemed to be the same as mine.

Vaillant is one of the few makes of heat pump where this could actually be true. Using the right control algorithm the heat pump will raise it's water temperature to raise the internal temperature more quickly, although having reached the set temperature it will then reduce the water temperature to maintain the set temperature. But, as I see it, the majority of heat pumps determine their flow temperature either by the outside temperature or using a pre-set value. So most heat pumps won't run less efficiently when the thermal demand is greater; they will just run for longer at the same level of efficiency (COP).

Or alternatively perhaps they could be built to conform to the same Opentherm standard as many gas boilers are - in which case any third party controller that supports Opentherm would be suitable. My LG heat pump has the option of control based on the leaving water temperature and/or the room temperature but what the "and" option actually does is completely undocumented.

Yes, @J1mbo , I think I would have gone with Vaillant had I known this at the right time. If more manufacturers of ASHPs also made gas boilers then maybe we would see Load Compensation more widely available.

Thinking some more, the more generic reason why @pdf27's second point is wrong is that it assumes that the heat pump knows how hard it has to work and can change the water temperature accordingly. It could do this by monitoring the temperature in the room with the control thermostat and modifying the water temperature according to how far the set temperature was above the actual temperature. This would be a form of "Load Compensation", which is a feature that is becoming quite common on gas boilers. But unfortunately it is not a common feature on ASHPs, most of them don't seem to have it. That is a great pity because it is a means of getting more economical operation. But I think you can hang on to your TRVs unless you have Load Compensation.

My heat pump does not work like yours, @pdf27 It runs in 15 minute cycles, The fan comes on A minute of two later the compressor comes on. The leaving water temperature is raised to a maximum which is either determined by a fixed setting or by the weather compensation algorithm. The heat pump tries to maintain this temperature If the room thermostat is satisfied (possibly also if the return water temperature gets too high) the compressor and fan are switched off. The cycle repeats 15 minutes from the start of the previous one. If the house needs more heat then the duration of the time at the maximum water temperature is increased. But there is no change in this water temperature and so no change in the efficiency, whatever the demand.

See https://www.ofgem.gov.uk/sites/default/files/docs/2017/11/drhi_faqs_about_epcs_v2_0_29_nov_2017.pdf

Sunamp You will also need a special version specific to a heat pump; their standard Phase Change Material changes phase at too high a temperature to work with a heat pump.

To qualify for the RHI you need an MCS accredited installer who can complete the job before the deadline. They may be in short supply but I don't know this. You also need an EPC that does not recommend additional loft or cavity wall insulation. Otherwise you don't need good insulation to have a heat pump, you just need a bigger one (and bigger radiators) if your insulation is poor. If you can get the RHI I'm not sure if you would derive any benefit from not heating your water with the heat pump. Otherwise the running cost will be at least half that of your immersion heater and, like with the immersion heater, you only have to use it to heat your water when you want to.

You need to be absolutely clear that electrical UFH has nothing whatsoever to do with heat pumps or boilers. It runs entirely on electricity, you get about 1 kW of heat for every 1 kW of electricity (a little heat goes down and is wasted) so it is very expensive to run. With a heat pump you would get 3 to 4 kW of heat for every kW of electricity. With an oil or gas boiler you are heating with a much cheaper fuel.

This may be right but I'm not convinced. If you have UFH is not each room controlled by its own thermostat with the equivalent action to a TRV? If one room is slightly hotter than the rest of the house then other rooms will derive a small benefit from this and need less heat. If one room is slightly cooler the the rest of the house will need slightly more heat. But surely this is more to do with the size of the radiator relative to the need of the room, TRVs can compensate for oversized radiators so your house needs less heat than it otherwise would. My heat pump has a weather compensation feature but this just sets the leaving water temperature according to the outside temperature. It knows nothing about what the TRVs are doing, just whether the room thermostat is calling for heat or not.

The installer does not have to prove that the radiators are adequately sized on a per room basis. My kitchen and a small en-suite bathroom did not have adequately sized radiators but in both cases it was difficult to replace them with larger ones. The shortfall was brought to my attention and I had to agree to it. Other rooms had somewhat oversized radiators so the house as a whole was fine. Both rooms do now sometimes feel a little cool if their doors have been closed for a long period so with hindsight maybe I should have tried a little harder to find better radiators.

If you opt for a Gas Boiler, consider a "Low Temperature" boiler.

I meant this: Delta T (°C) Correction factor Correction factor relative to Delta T = 30°C 5 0.05 0.097 10 0.123 0.239 15 0.209 0.406 20 0.304 0.590 25 0.406 0.788 25.49 0.41666667 0.809 30 0.515 1.000 35 0.629 1.221 40 0.748 1.452 45 0.872 1.693 50 1 1.942 55 1.132 2.198 60 1.267 2.460 65 1.406 2.730 70 1.549 3.008 75 1.694 3.289

So what water temperature did the MCS calculations assume? And why do you doubt them? Was your original aim to see if you can use a higher water temperature and your existing radiators? That's probably a false economy, as I hope you have now realised. If you want a correction factor table based on T30, just divide all the numbers by 0.515.

To qualify for the RHI you need to use an MCS certified installer. Part of the installation process is that they should evaluate the heat loss from every room in the house and how big a radiator it need. You can make an input as to what design flow temperature you want but your installer should then do the radiator calculations for you.

Delta T (°C) Correction factor 5 0.05 10 0.123 15 0.209 20 0.304 25 0.406 25.49 0.41666667 30 0.515 35 0.629 40 0.748 45 0.872 50 1 55 1.132 60 1.267 65 1.406 70 1.549 75 1.694 I