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How Accurate Is My DIY Heat Loss Calculation?

 
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Air Source Heat Pumps (ASHPs)
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 Old_Scientist
(@old_scientist)
Honorable Member Member
Joined: 2 years ago
Posts: 529
26/08/2026 5:09 am  

Posted by: @kilgorexv
↑

@old_scientist i had a look at this, all my gas consumption for New Years Eve was 85kw. Thats the highest day for two years. Winters have been mild though. -3 was as low as it got, and even then not all day. Even then allowing 6 for hot water and cooking, leaves about 3.33kw an hour.

My boiler is a reasonably odern condensing  combi boiler, installed in 2012, but se3rvices once a year and still seems ok, no major problems, so for my calcs i assume around 85 to 88%

Thanks for your feedback.

85kWh of gas usage, divided by 24 gives 3.54kW and adjusted for 85% boiler efficiency gives a heat loss of 4.16kWh. Allowing for DHW and cooking brings the heat loss down to 3.9kW and your calculated figure is right in the middle at 4kW.

I think that's very consistent, which should give you high conviction in your calculated heat loss.

Posted by: @kilgorexv
↑

@jamespa Also this coming winter I am going to run my combi system at as low as it will modulate and see how we get on.

This is exactly what we did before getting our heat pump. We had an oil boiler, so unfortunately didn't have accurate gas usage data like you do, so it was more of a finger in the air for us, but we turned the flow temps down to 50C on a really cold day over winter to get a sense check that our radiators (at their present sizes) could still output the required amounts of heat at lower flow temps, and they were fine.

We ended up replacing all our radiators with the largest sizes we could reasonably accommodate in each room which allows us to run at lower flow temps, and now we rarely raise the flow temp above 32-33C all winter (35C if it gets really cold). If you are replacing radiators anyway, fit the biggest you can accommodate in the space available. Don't worry if the heat output doesn't exactly match the heat loss on a room by room basis as heat will flow throughout the house - better to get the flow temps down as low as possible.

 

 


This post was modified 1 hour ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW). Net Zero


   
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