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New Build ASHP Defects - Looking for Advice, Support & Shared Experiences

 
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Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 1 month ago
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 Newby
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Topic starter 12/03/2026 12:50 pm  

@deltona I have raised it with their resolution service



   
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 Newby
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Topic starter 12/03/2026 1:12 pm  

@jamespa

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.



   
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JamesPa
 JamesPa
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12/03/2026 2:27 pm  

Posted by: @newby
↑

@jamespa

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!

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Newby
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Topic starter 12/03/2026 5:03 pm  

@jamespa

Thanks for the detailed thoughts, that all makes sense. I’m going down the formal route for now, but I’m trying to understand the system better in the meantime, so I’m not caught out when someone eventually turns up. I don’t have all the technical info yet because some of what I’ve been told came from a handyman who had a quick look, but I’ll get firmer details as I go. I’ll also try the 55°C flow test on the next cold spell to see whether it points more towards the rads or the heat pump.

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.



   
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 Newby
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Topic starter 12/03/2026 5:12 pm  

@johnnyb

 

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.



   
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Topic starter 12/03/2026 5:26 pm  

@transparent

Thanks, this is extremely helpful, especially the explanation about NHBC pre‑registration and older SAP/Building Regs potentially being the standard my plot was assessed against. I’ll be asking for the full SAP printout so I can see exactly which edition was used and whether the design assumptions match what’s actually been built. That alone could explain why the theoretical performance looks fine on paper while the house can’t hold temperature in reality.

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.

 



   
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JamesPa
 JamesPa
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12/03/2026 5:59 pm  

Posted by: @newby
↑

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).


This post was modified 1 month ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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