Heat Loss v Outside Air Temperature

bontwoody · 2025-11-10T09:27:55Z

Im doing some work on a friends heat pump installation which I feel is oversized, but until we get some cold weather I cant be sure how much. However it got me to thinking that if I know the total heating energy each day and the average Outside Air Temperature, I could plot a scattergram of hourly heat loss v Mean OAT, add a trend line and extrapolate that down to -3C. As a test I tried it with some data from my own installation from last winter. I thought the results were quite useful. I dont run my heat pump 24/7 so divided the total heating energy by 18 instead of 24 to get my hourly rate. That might not be the correct thing to do but the graph shows my anticipated heat loss at zero outside would be be around 3.5kW, and extrapolating it down further would indicate a heat loss of about 4kW at -3C. This is well within my 5kW output and ties in quite nicely with my observed usage at the OATs we tend to get near the sea. I just need some more data points on my friends heat pump ......

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Air Source Heat Pumps (ASHPs)

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bontwoody

(@bontwoody)

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Topic starter 11/11/2025 5:06 pm

I also havent made any allowance for heating from bodies and other electrical sources, but it was only really meant as a quick and dirty method to get a rough estimate of a suitable heat pump size.

The property I am looking at, is heated 24/7 and the internal temperature is pretty constant. We are nowhere near needing any defrosts yet.

Even with all the caveats though, Im very impressed that when looking at my own data, the result matches my heat meter measured output very closely so I think is a useful tool.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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cathodeRay

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11/11/2025 5:47 pm

Posted by: @bontwoody

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The property I am looking at, is heated 24/7 and the internal temperature is pretty constant. We are nowhere near needing any defrosts yet.

Ideal conditions to collect the data! Although the left hand side of the scatter plot will be empty, the right hand side should be nine and tight, increasing confidence in the result.

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

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