On your specific questions: the primary pipework looks like plastic, roughly 22mm

Is that a single pair only?  If so it may be part if the problem.  How long?  What flow rate do you have ( your heat pump will tell you) and what dt flow to return (again your heat pump will tell you).

I’ll also follow your advice about contacting the LPA directly. I hadn’t appreciated that they still hold the enforcement powers even when NHBC handled the inspections, so I’ll put everything in writing with photos and diagrams so the Councillors can see the issues clearly. The commissioning requirements you’ve highlighted from Part L are another gap, I’ve only been given generic manuals, nothing site‑specific, and certainly no commissioning plan or completion notice. Combined with the insulation defects already identified, it’s becoming clearer why the system is struggling.

I’ll start pulling all of this together into a proper evidence pack. Thanks again for taking the time to set this out so clearly, it’s given me a much clearer route forward.

Thanks for all of that is really helpful context. I agree that if the house can’t get above 16°C in cold weather with a 7 kW unit, then something fundamental isn’t right, and the insulation/air‑tightness side is looking more likely. I’ll definitely follow up with Building Control as you and @transparent suggested, especially around the insulation evidence and whether the application fell before or after the photo‑requirement change. I’ll also ask what they hold on file in terms of plans, SAP and the under‑screed build‑up, because none of that has been provided by the builder.

On the heating side, the floor and rads aren’t getting above room temperature during cold spells, which lines up with what you’re saying about heat loss. Your comparison with your own builds really highlights how far off things are here, even with modest flow temps you’re getting stable indoor conditions, whereas mine drops away quickly. I’ll take proper surface readings and keep gathering data, but at this point the insulation route looks like the first thing that needs resolving before anything else can be sensibly assessed.

On your specific questions: the primary pipework looks like plastic, roughly 22mm, with 15mm plastic to the rads. The COP figure of around 2 was just a rough comparison between the heat pump’s reported output and the electricity used on the coldest days. As far as I can tell there’s no buffer, LLH or plate heat exchanger in the system. I’ll start logging proper data now and will post whatever I can, including the thermal images. I agree the insulation needs sorting first, that’s already in motion and hopefully once that’s fixed the rest will be easier to pin down.

Thanks,

That’s really useful. I’ve got the Daikin MMI controller and will increase the WC curve to 55°C @ -3°C on the next cold spell. I’ll log room temps, flow/return, outdoor temp and energy use as suggested and report back with results.

Just waiting for NHBC to get back so will give it a go.

Should be interesting.  Do watch to see if the UFH gets too hot, although if it does then thats a positive outcome!

That’s really useful. I’ve got the Daikin MMI controller and will increase the WC curve to 55°C @ -3°C on the next cold spell. I’ll log room temps, flow/return, outdoor temp and energy use as suggested and report back with results.

Just waiting for NHBC to get back so will give it a go.

I'm also pleased to see that you've been commenting on the quality of insulation which has been installed. Outside of the publicly accessible part of this Forum, @jamespa has been looking into what's involved with retrofitting existing housing stock to achieve greater energy efficiency. Although yours is a new-build, and the insulation specification is probably 'correct' on the plans, it's the shoddy installation which then undermines the theoretical SAP score.

That's going to be the case irrespective of the age of the property.
Poor installation practice renders the survey calculations utterly useless.

1: Now let me highlight a separate issue regarding new-builds done by the larger firms who are members of the NHBC.

These homes are not necessarily built to the current standards for either Building Regs or SAP scores.

At any time in the past, the Developer can 'pre-register' the site with NHBC for a small nominal fee.

When that happens the clock stops for all Building Control issues. Whatever was the then-current set of regulations is the standard to which the Developer is required to comply.

The site doesn't have to be owned by the Developer at that moment, nor does Outline Planning Permission need to have been sought.

A Developer who is a member of the Vistry Group is presently constructing new houses near me which only meet the 2006-level of Building Regs, and the 2009 edition of the SAP (Standard Assessment Procedure) energy-efficiency criteria. At the point of sale, the new owners are provided with an EPC Certificate, whilst unaware that it isn't based on the current standards.

In your case you should ask to see a multi-page printout of the SAP which was undertaken for your new home, based on the approved plans. At the foot of each page the software prints the edition/version of the SAP which is being applied, and the software licence number of the assessor. That can be very useful information in a dispute.

2: Irrespective of these homes being signed off by NHBC, you should still be communicating the issues to your elected Councillors at the level of LPA (Local Planning Authority). Unless you're in a Unitary or Metropolitan Authority, the LPA is the District or Borough Council.

Since that Council holds the powers of enforcement, they need to be put in the picture regarding breaches of regulations.

Always do this by email. Not only is it then in writing, but the date-time stamp might later be required as part of dispute-resolution.

Elected Councillors are not expected to know the technicalities of the regulations, but they have the power to require Council Officers to undertake any investigation. In most cases that will start by them forwarding your email to the Head of Planning, and requesting a response (to them, not you!).

For that reason, I always provide Councillors with photographs and diagrams within my emailed PDF. They will find that very useful.

Here's an example of how I've raised a complaint about a Developer wanting to plant species with edible fruit on an estate where the soil is contaminated with arsenic.

I've taken a snapshot of the Developer's Landscaping Drawing (presented during the planning application), and superimposed screenshots of the 'key' from the side of that Plan, with arrows pointing to the problematic locations on-site:

That allows a Councillor to immediately see what I'm concerned about, without having to wade through 100+ pages of Application Documents.

They can forward that to their Planning Officers with a cover-note which simply says "Investigate this complaint and send me a report detailing the safety risks".

The section of the page which I've used here by way of illustration is the start of a 5-page PDF which progressively gets more technical. I quote my source references and any relevant 3rd-party experts. There's no wriggle-room for the Planners to dismiss what I'm stating.

3: Here's the relevant sections you need from the current Part-L of the Building Regs. That Approved Document addresses Heating and Power, and will therefore cover all issues regarding a heat pump.

and Section 9

The two sections stipulate what should've been done to properly commission the system, and documents which they are required to hand over to you.

Those documents must be site-specific, for your particular house. It's not sufficient to give you manuals from a manufacturer and generic operation instructions. You need to have sufficient training/instruction to operate the heating system so as to minimise the amount of energy (electricity) it uses.

I could write more, but I'm going to stop there and let you respond to the above.

This Post from me forms part of your 'evidence', and can be sent to your Local Councillors.
They may wish to also post here, requesting clarifications. That's fine, of course!

I would follow @transparent advice as your heating doesn't meet building regs. It has to be properly sized, not over or under. (seeing the amount of new builds with large HP's the building inspectors can't be following up on this) If you can't get the indoor temperature above 16° when it is cold with a 7kW HP then I expect your insulation doesn't meet regs. As Transparent said, building control are the ones to enforce this. I don't know where NHBC's loyalties lie as I have no experience of them but if you have an NHBC warranty and they did the building control inspections contacting your LBC (local building control) would be another path to explore if you aren't getting anywhere with the builder. 

If the building control application was submitted after October 2023 then the builder should have been required to submit photograph evidence to confirm the insulation was properly fitted. These photo's are supposed to be geo-tagged to prove they were taken at your property and not one set of photos to cover several houses on the site. They don't require a picture of every part of the building so there is no proof every area is fully insulated but it would be a start. The application can be submitted in advance and it is quite common for developers to submit BC applications in advance of building regulation changes so your 2025 build could be submitted previous to the photo's being required. Previous to this the inspector will have taken pictures of the foundations and the build up to damp proof course and this should give a good indication of the space that was allowed for insulation under the concrete screed that has your UFH pipes in, but if the BC inspections were done by NHBC this won't be held by your LBC.  There should be a set of plans submitted with the BC application that detail the insulation it was designed with to meet the regs, along with SAP calculations etc that I realise you can't get from the builder.

LBC inspect at structure up and then don't come back until final inspection.  Our house was a timber frame shell when he last inspected so no insulation, not much at all really. There is a lot that goes on between the structure being signed off and the building being completed that isn't physically inspected, hence now requiring photos to confirm the heat loss part of the regs is being adhered to.

There is a requirement to submit a building control application to the LBC even if the developer used an independent inspector so your LBC will have a record of your site and when the application was submitted. They may know who the building control inspections were done by, I'm not sure on that one.

Do you have neighbours on your development or is it a one off? I wondered how they are getting on if you have any.

How warm does your floor get if you lay a thermometer on the floor?  What temperature are the rads at? I would describe mine as luke warm or even cool as at the moment the water leaving the HP is mostly 25-26° so the floor tiles don't feel very warm. I think the highest I have seen the floor tiles is 25° when we have a cold snap, I had a temperature gauge laying on the floor most of the first winter.

To me this sounds like a problem with the insulation and/or air tightness but if your floor and rads are not above indoor temperature obviously there's a problem.

As an example of our heat requirements. When we had the initial assessment for the heating, from the plans based on meeting building regs standard figures the heat loss for our 185 sq mtr house at -2° was just under 5kW. The design assessed SAP gave a heat loss of 4.2kW, so we are building to insulation levels that are about 15% better than 2023 building reg minimums. Our bedrooms have a heat loss of around 200 watts each so the rads only need to be small.  Our main bedroom has a vaulted ceiling and 3 external wall and this pushed the heat loss up to just over 500W, still small rads even at 30° flow temperature.

I have 5kW HP's for both houses, 2 heat pumps not shared. I'm in East Anglia so it isn't a cold part of the country. The heating was commissioned in the 185 sq mtr house in February this year as the cold snap happened, minus 2-3° at night for a few days.  Previous to this I had been using the woodburner at evenings and weekends to try to keep the chill off but it was only getting to around 13°. Within 36 hours the temperature was up to 18° downstairs with a max flow temperature of 35°. After the first 48 hours I switched to running the heating for 6 hours overnight only, when the electric is cheap. We don't have the upstairs heating connected yet so this was just from the UFH downstairs but the heat spread through the whole house after about a week. The UFH pipes downstairs are 16mm dia and at a standard 150 centres.  I turned it off after about 2 weeks as the house was getting over 20°.  Since then the sun on the windows and the woodburner at weekends has been enough to keep it between 16-18°. Our insulation is a little better than building regs but not by much and I still have issues with the french doors and a window not closing fully so we have a little extra ventilation.

We are currently living in the smaller house (130 sq mtrs) and don't notice the cold outside unless the temperature drops below -5° or there's a very cold wind, but even then the house is still warm inside. The heating has not needed to run 24 hrs a day at any point to keep the house warm and this also has a max WLT of 35°. I haven't fitted the rads in the bedrooms and our bedroom does get down to 14° if it is cold and windy but most of the time is 17-18°. Our boys sleep with their windows open most of the winter as they think it is always too hot.

OK here are my considered thoughts based on info so far:

I’m going down the formal route rather than trying to redesign the system myself. While that is in process in the background, I’m trying to understand the system better so that, when someone finally turns up with a clipboard, I’m not fobbed off.

Very sensible!

General comments:

I think its unlikely, but not impossible, that you house loss, when properly insulated, exceeds 7kW and if it does it will be by a small margin.  This being the case I would NOT push for the unit to be uprated at present, because you will end up with the 16kW Daikin (they do intermediate models but they are just the 16kW derated) which is double the required size.  Only if the house loss is proven to be greater than 7kW would I go for an upgrade, and then I would be reluctant to accept a Daikin because of the large jump to the next model.

I do think its worth doing a heat loss calc, but Im not sure it will change your course.  An estimate of the extra loss due to the failed insulation is definitely interesting.

It would help to do the experiment I suggest above (up FT to 55C) to get a better indication whether its rads or heat pump that is limiting.  It wont be definitive though given the insulation failures!  I dont think you have said anything about pipework diameter or material, it could be that so perhaps post some info.  For 7kW your primaries leaving the heat pump should be 28mm copper, dividing perhaps into 2 pairs of 22mm copper with 15mm drops to rads.  If they are less than this or plastic please post full details for assessment  

If you are really seeing a COP of 2 then it could be a faulty heat pump which is not unknown.  Worth trying to ferret this out by interrogating the pump further.  Likewise posting any of the data @cathoderay has mentioned may help narrow down.  Also where does the figure of 2 come from?

I think you confirmed that there was no buffer, LLH or plate heat exchanger between heat pump and emitters but could you reconfirm.

First step for me remains getting the insulation fixed.  Until it is (a) you are losing lots of heat and (b) you cant redesign the heating system, if that is what it needs.

I will post more if I think of more, in the mean time I hope that helps.  The mystery is clear - 7kW is almost certainly enough or very nearly enough for the house if correctly insulated, so why is the house falling so far from being heated.