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Preparing for 2nd winter in new build house.

Air Source Heat Pumps (ASHPs)

Last Post by GrahamF 5 minutes ago

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sbm74

(@sbm74)

Active Member Member

Joined: 1 hour ago

Posts: 3

Topic starter 19/09/2025 8:39 am

Firstly, apologies for what is likely to be a long post. We have a complex setup that will take a bit of explanation.

We completed our new build in Sussex almost a year ago and moved in at the end of November 2024. The house is brick and block construction and was still very much drying out when we moved in given the wet weather we had had all through the build and the traditional plastering method that was used internally.

We have two separate mains power supplies entering the property: one supplies the power for the house and one EVCP and the other supplies the ASHP, some AC and another EVCP. We have solar PV and a Powerwall 2 on the house phase. This means that the ASHP does not benefit from any other power sources.

The ASHP and UFH system was designed by Nu-Heat (we didn’t have a choice in this) and they have provided a room-by-room heat loss calculation. Most of the rooms have thermostats.

Until I found this forum, we had 4 zones in the house: reception rooms, bathrooms, bedrooms and unused rooms. Having read some of the posts here, I’m reducing the number of zones. The ASHP phase is on the Cosy Octopus tariff, so we have to set back all zones during the expensive period in the afternoon. Currently the set back is significant enough to stop the ASHP coming on at all. As I mentioned, we don’t have any alternative power source on this phase, so it’s entirely dependent on the grid. I realise that such a large set back is likely to be adversely affecting the efficiency of the heat pumps. I’d be grateful for any advice from other users of this tariff.

I’ve attached a photo of our heat curve. Firstly, it looks like the maximum supply temperature that’s been set by the installer is very high at 60°. I feel like I should reduce this. I’m also considering undertaking the task that is detailed on this forum of using the thermostats as limiters and just using the weather compensation curve on the heat pump. I’ll wait until the weather is colder. The Weather Control setting is currently off. There isn’t a lot of information about what this actually does, but it would seem that it checks the weather forecast and adapts its behaviour accordingly. I would be grateful if anyone has any knowledge of this feature and whether it’s worth enabling.

If I believe the year-to-date data provided by the heat pumps, I have a SCOP of 4.24. I don’t know how accurate this is likely to be and we haven’t hit any cold weather since the spring.

My main question is am I doing everything I can to make the system as efficient as possible? I am a bit of a tech geek, but this is the first time we’ve had a heat pump so I’m still learning. On the other hand, I don’t want to be completely obsessive over this (I have that tendency) so I’m not planning to constantly tweak. I just want to be sure it’s set up efficiently and then I can leave it alone.

We are happy with the temperature in the house for the most part. Our bills were higher than we’d like last winter, but I know that is in part due to the house drying out.

Thanks in advance for your help with this.

Forum Handle: sbm74

Manufacturer: NIBE

Model: 2x F2040-16

Type: ASHP

Arrangement: Mono

Area of Country: West Sussex

Home Type: House

Size: 650m2

Insulation Level: 2024 Cavity wall new-build

Heat Loss Calculation: 22.1kW @ Amb. Temp, of: -3 °C

Type of Controller: Room thermostats

Type of Control: Room Temperature

System Type: UFH throughout

Total Output of Heat Emitters: kW

Buffer Tank or Low Loss Header: BT 215 litre

Secondary Pumps: 1 DHW return

Solar PV System: 4.2kW on different phase

Power Diverter: No

Battery System: Tesla Powerwall 2 14kWh on different phase.

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GrahamF

(@grahamf)

Estimable Member Member

Joined: 5 months ago

Posts: 38

19/09/2025 9:31 am

What an interesting and ambitious project. You certainly have built a big house - 650m2 is enormous!

Here are some pointers:

I wonder why you have 2 x single phase power supplies. With 2xEVCPs, 2x16kW heat pumps, ovens, tumble dryer etc, I suppose you might exceed 100 amps, if they were all on at the same time. In that case, one 3-phase power supply might have worked better.
Our 1968 house is a 5 bedroom detached, with three bathrooms, but it is “only” 350m2. It has a calculated heat loss of about 15kW, though I suspect it is less. Your new house must have much better insulation, so it may be worth checking the heat loss calculations in case 22kW is too pessimistic. Your system may be able to tell you how much heat the system actually generated during the winter months.
Your heat loss calculations should include a design flow temperature at the target outside temperature of -3C. I would hope for a number of maybe 35C. Certainly, a 60C flow temperature in an underfloor heating system is much too high. A hot shower is about 40C. A floor at 60C would burn your feet and ruin your floor. I am not an expert, but I think even 40C UFH is considered dangerous. Therefore, set the weather compensation curve carefully before you turn it on.
I would investigate whether your system provides any reports of compressor frequency, outlet and inlet flow temperatures, electricity consumed, power output, COP and SCOP, outside temperature, indoor temperature…. Ideally, you would download this in .csv format in 30 minute intervals over the winter months, so that you could analyse it with Excel. Some system provide their own graphical analysis, which may well be good enough. Looking at compressor frequency tells you how often the heat pumps turned on and off. Ideally, you want them to run continuously at a low flow temperature, because that consumes less electricity.

i hope that helps.

Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation

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