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Preparing for 2nd winter in new build house.

sbm74 · 2025-09-19T07:39:51Z

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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sbm74

(@sbm74)

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Joined: 1 month ago

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Topic starter 19/09/2025 11:34 am

The setting is on the room stats. That room has 2 loops. The ASHP is using weather compensation so the OAT would have to be around -15° before it got to 60°. According to the heat loss calcs the maximum flow temp should be 50°.

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Carpenterstation

(@carpenterstation)

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Joined: 9 months ago

Posts: 45

19/09/2025 11:56 am

Welcome to the forum. I am commenting as there are similarities with our property. Relatively “new” build and 450m2 and 250m2 ground floor is all UFH.

Our install was not great (temps far too hot, excessive cycling, heat literally thrown away by a diverter when circulating temp was too high). We fixed by setting all in room thermostats to 20 degrees all the time acting as limiters, set the upstairs to 18deg, reduced flow temp to 32degees and switched on weather compensation. After a few weeks of tweaking w/c, system settled down - and the house is now permanently warm enough.

efficiency isn’t everything. You need to be careful that you aren’t just efficiently producing too much heat.

👍

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sbm74

(@sbm74)

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Joined: 1 month ago

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Topic starter 19/09/2025 12:46 pm

@carpenterstation Good to know. Looks like that's the way forward.

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Transparent

(@transparent)

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Joined: 4 years ago

Posts: 2680

19/09/2025 1:43 pm

Posted by: @sbm74

↑

The setting is on the room stats.

Yuch!

So that's basically just an on/off control.

It either lets water flow through a loop at anything up to 60°C, or it doesn't.

The thermal lag for UFH within screed will be at least an hour.
My kitchen/diner area (9m x 4m) has a lag of around 4hrs with <40°C flow.

Measuring the air temperature in the room is next to useless.

The 'correct' approach is to remove the thermostatically-operated valve on the manifold.
Fit a manual valve, and adjust it to maintain a steady flow.

IOW you're using it like a lock-shield valve on radiators, to balance the flow throughout all UFH loops.

The longer the loop, the more open the manual valve will need to be.

Save energy... recycle electrons!

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