Heat Loss v Outside Air Temperature

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 36 minutes ago

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bontwoody

(@bontwoody)

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Joined: 4 years ago

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Topic starter 10/11/2025 9:27 am

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 ......

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
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Steelbadger

(@steelbadger)

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10/11/2025 10:15 am

I did something similar when I was trying to get to grips with our old Frankensystem. I wasn't able to model it based on the actual heat delivered, however, because I'm not aware of any way to track the heat delivered by a heat pump hot water cylinder unless it actually does it itself.

I do think if you're taking the total daily energy, that you need to divide by 24 for the heat loss, not just the 18 your heat pump is running. In theory it needs to make up for the 6 hours it's not running during the 18 hours it is, so it'll run a little harder in that time to maintain the target temperature (or, alternatively, your heat curve is set a little higher than it technically needs to be if it was operating 24 hours a day).

You're measuring loss, using heat-in as a proxy and assuming a steady indoor temperature. Heat Loss doesn't stop just because your heat pump stopped, it just has be made-up when the unit is running.

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JamesPa

(@jamespa)

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10/11/2025 10:29 am

Posted by: @bontwoody

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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

Thats exactly how I sized my heat pump. Surveys (2 off, each taking 3 hrs, one of which I paid £300 for) said 16kW, measurements of gas consumption said 7kW. The challenge was to persuade installers to take notice, but fortunately I found two were willing to do so. I fitted the Vaillant 7kW (which can do about 8.4kW, although drops to 7kW or even a bit less with defrost). Perfect match.

Personally I think it should be compulsory to compare measured loss with surveyed loss and, if they don't reconcile to sufficient accuracy - enough to resolve any major design decisions - work out why. If installers were obliged to do this we would see far fewer oversized heat pump installations.

This post was modified 36 minutes 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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