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Is it KISS or constant – weather compensation vs. set flow temperature?

 
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Air Source Heat Pumps (ASHPs)
Last Post by cathodeRay 5 hours ago
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cathodeRay
 cathodeRay
(@cathoderay)
Famed Member Moderator
9974 kWhs
Joined: 3 years ago
Posts: 2011
12/03/2025 12:07 pm  

Posted by: @cliffhanger
↑

Can't pm you I haven't enough posts.

Keep it out in the open! Others will want to have a look and possibly comment.

The heat loss report estimates the heat loss to be 7.42kW at -5.28°C (blimey, that's cold!), using what appears to be standard calculated methodology (areas U values air changes etc). There is a good chance this figure is too high, especially for an old building check the wall U values, there are also some links to published data on actual (measured) U values buried somewhere on the forum. Let's take 25% off: 5.5kW. The report mentions a Vaillant heat pump, but I thinks your is a Grant, either way, I think it is rated at 9kW. If these numbers (5.5 and 9) are correct, then there is a likely explanation for your heat pumps behaviour staring us in the face.

It is possible to get an empirical (measured) heat loss providing you have historical data of fuel usage (can be any fuel, but is easiest with gas). Basically, if your house is in steady state (steady room temp), then the energy being delivered to the house (energy in adjusted for efficiency) is the heat loss at that OAT. If you have such historical data, we can go through the method in more detail.       

Midea 14kW (for now...) ASHP heating both building and DHW


   
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cathodeRay
 cathodeRay
(@cathoderay)
Famed Member Moderator
9974 kWhs
Joined: 3 years ago
Posts: 2011
12/03/2025 12:22 pm  

@cliffhanger - thoughts on how to deal with your situation: I would still start with room stat set high and all TRVS etc open and then fine tune the weather compensation curve to as low as can be, but still  sufficient to heat your home. It may take a lot of tweaking, all the more so if the heat pump is over-sized. It will struggle to lower output sufficiently (because it can only go down so far), and will end up cycling sooner than a less powerful model. Ignore what the curve numbers appear to be telling you - they are just numbers - and focus instead on the leaving water temp and room temp. 

Midea 14kW (for now...) ASHP heating both building and DHW


   
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