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.

@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

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

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?

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 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.

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.