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Extend the colder range of the weather compensation curve?

Air Source Heat Pumps (ASHPs)

Last Post by RadWhisperer 5 months ago

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TheBinMan

(@thebinman)

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Joined: 2 years ago

Posts: 18

Topic starter 12/11/2025 3:22 pm [#2589]

In theory everyone has a heatpump designed to deliver the heat required at a certain outside design temperature for whatever part of the country you are in. Say for example a system is designed for 45DegC at ODT -4DegC.

Now when it comes to the weather comp curve my system will allow me to input a flow temp down to an outside temp of -20DegC. Are there any issues in extending the linear line beyond -4DegC?

At the end of the day if it's colder than -4DegC then the heat pump input will surely need to increase flow temp to maintain internal house temp? And for arguments sake we are going to assume my ASHP can deliver the heat required down to -20DegC.

Earlier this year I do recall some days of -8DegC and always below freezing for several days. From memory I don't recall any complaints but just made me wonder especially if was even colder.

This topic was modified 5 months ago 4 times by TheBinMan

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JamesPa

(@jamespa)

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Posts: 4714

12/11/2025 6:37 pm

Posted by: @thebinman

↑

Are there any issues in extending the linear line beyond -4DegC?

No issues at all in extending it so far as I can see, certainly until the point where you are asking it to exceed it's max flow temperature

Obviously the heat pump will eventually run out if steam but you might as well squeeze as much out as you can.

Vaillant (and some other heat pumps) have a series of pre-defined curves (which you choose between with a kind of slider) that are actually shown in the manual as going beyond their max flow temp.

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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TechnoGeek

(@technogeek)

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Joined: 3 years ago

Posts: 174

12/11/2025 6:53 pm

@thebinman

Posted by: @thebinman

↑

Now when it comes to the weather comp curve my system will allow me to input a flow temp down to an outside temp of -20DegC. Are there any issues in extending the linear line beyond -4DegC?

In answer to this and personal experience, nope. My system is set up whereby my house needs a flow temperature of 52C at OT -2C. In order to get the best fitting curve over the heating season I have the weather comp points set to OT +15C ; 35C flow and OT -5C ; 55C flow. It will obviously use a little more power as it gets colder but at least the rate of change of the flow temp matches the rate of change of the heat loss of the house better.

It also came in handy when we had an overnight temperature of -5C to -6C, the house still remained warm. If the weather comp point was set to 52C at OT -2C the flow temp would have flat lined at temperatures below -2C OT on my system and the house would have started loosing temperature by my reckoning.

Regards

This post was modified 5 months ago by TechnoGeek

5 Bedroom House in Cambridgeshire, double glazing, 300mm loft insulation and cavity wall insulation
Design temperature 21C @ OAT -2C = 10.2Kw heat loss, deltaT = 8 degrees
Bivalent system containing:
12Kw Samsung High Temperature Quiet (Gen 6) heat pump
26Kw Grant Blue Flame Oil Boiler
4.1Kw Solar Panel Array
34Kwh GivEnergy Stackable Battery System

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TheBinMan

(@thebinman)

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Posts: 18

Topic starter 20/11/2025 7:36 pm

Thanks for replies. That's good to hear and wanted to make sure I wasn't overlooking something massive.

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Grantmethestrength

(@grantmethestrength)

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Joined: 1 year ago

Posts: 184

22/11/2025 8:22 pm

Sorry if this is a daft question but doesn’t the heating curve continue on to the max output of the ASHP i.e. if you set say set 32@-3c for example, won’t it just continue up the same trajectory (if that is the right word) up the graph so -4c is say 34c, -5c is 36c all the way up to max system output say 55c@-20c for example? If not is it better to extend the line of your heat curve on paper and look where your max output would land and use that as your setting or would that be the wrong way to look at it?

Kind Regards
Si
——————————————————————————
Grant Aerona3 13kW
13 x 435w + 13x 480w Solar Panels
Sigenergy 10kW Inverter
25kWh Sigenstor battery

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JamesPa

(@jamespa)

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Posts: 4714

22/11/2025 8:35 pm

Posted by: @grantmethestrength

↑

Sorry if this is a daft question but doesn’t the heating curve continue on to the max output of the ASHP i.e. if you set say set 32@-3c for example, won’t it just continue up the same trajectory (if that is the right word) up the graph so -4c is say 34c, -5c is 36c all the way up to max system output say 55c@-20c for example? If not is it better to extend the line of your heat curve on paper and look where your max output would land and use that as your setting or would that be the wrong way to look at it?

Not necessarily. Some heat pumps flatline at either end of the defined 'curve', at least according to the manual. Depends on the heat pump

This post was modified 5 months 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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@sand

(@sand)

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Joined: 4 years ago

Posts: 88

23/11/2025 11:50 am

This is how i worked out my weather compensation curve, cold end 42⁰@ -6⁰ warm end 30⁰ @ 7⁰, our heat pump doesn't play nicely under 29⁰/30⁰ LWT its oversized, plus downstairs ufh, upstairs rads and upstairs is smaller than downstairs, dorma bungalow.

So I had to get the warm end of the curve correct, which has been tricky. I think every home is unique.

Its an Ecodan so I can off set the whole curve up or down. + or -

Not sure if formula will be saved when I upload so this is the how i worked it out, can only upload image.

All info. I got from this forum, which was way over my head, so I put it in a spreadsheet to help me understand it.

42 - 30 =12
6 - -6 = 13
Divide 13 into 12 to give increments of 0.9231🤔

Know that probably incorrect as there needs to be brackets etc, but I'm no maths genius 🤪

So my heat curve doesn't go up in a full number increments, but heat pump rounds up the LWT number anyway.

Then this gives the formula
Column 4 x column 5 = column 6
Colum 1 + colum 6 = column 7

You could try to hit a full number for the increments but that would overheat our house due to ufh downstairs and probably be more costly.

Hope this helps someone.

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RadWhisperer

(@radwhisperer)

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Joined: 6 months ago

Posts: 35

23/11/2025 4:19 pm

Daikin has a slope WDC which changes per degree of temp change I think. Never used that one myself. The more commonly used one has the two points at each end. That one does appear to flat line at each end.

Mine was set to 50C @ -3C and 21C @ 20C I think due to the Octopus design. I added more insulation and a couple of upgraded rads so brought it down to 45C. However until the temps dropped I didn't have a clue what it would run at. Over the last week I kept checking down to 0C and it never showed above 34C so adjusted mine to 36C at -5C at the cold end. With modulation enabled that can go +/- 4C on demand or if outside conditions change the effective WD curve.

My house is small and well insulated so I found at the warm end anything above 12C and the house sat at 19C without any heating. So I set that end to 26C @ 12C.

This post was modified 5 months ago by RadWhisperer

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