Thank you @grahamf, nice explanation. I understand the wd curve, I was just wanting to know if it was worth setting it to maybe 20⁰c @ 14⁰c? Or is that not a good idea?

@craigh I would say it is a bad idea.  I can’t see any logical justification for setting the flow temperature to 20C when it is 14C outside.

Obviously, the WC curve has two ends.  You need to get both of them set correctly.  Otherwise, the house won’t keep to a steady indoor temperature when the outdoor temperature changes.

Setting it to 20C flow temperature when it is 20C outdoors has a clear logic to it, as I explained.

It is simply not practical to set the flow temp to 20C for a desired room temp of 20C.

The radiators (emitters) cannot emit heat into the room if they are the same temperature as the room.

Start from the amount of heat (kW) your heat pump will generate/output when running on minimum. That is the minimum amount of heat you need to dissipate into the room through your emitters. Knowing the rated heat output of your emitters, you can then calculate the flow temperature that will be required to emit that amount of heat. It is pointless trying to set the flow temp lower. The heat has to go somewhere. If the system cannot dissipate the heat, then the heat pump will simply cycle off.

Posted by: @old_scientist

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It is simply not practical to set the flow temp to 20C for a desired room temp of 20C.

Er thats exactly what the weather compensation curves on my Vaillant do doubtless because, as Graham F says, its a logical thing to do.  You can overlay a minimum flow temperature, but you dont have to.

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Posted by: @old_scientist

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The radiators (emitters) cannot emit heat into the room if they are the same temperature as the room.

er thats the point I think, when the room is 20C you dont want them emitting heat into the room (assuming 20C is your target temperature)

Posted by: @old_scientist

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Start from the amount of heat (kW) your heat pump will generate/output when running on minimum. That is the minimum amount of heat you need to dissipate into the room through your emitters. Knowing the rated heat output of your emitters, you can then calculate the flow temperature that will be required to emit that amount of heat. It is pointless trying to set the flow temp lower. The heat has to go somewhere. If the system cannot dissipate the heat, then the heat pump will simply cycle off.

Once the OAT is sufficiently high so that the demand is less than the minimum output, either you must allow the room temperature to rise above target or accept that cycling is inevitable, whether controlled by the heat pump or controlled by some temperature sensor somewhere.  Why is it better to impose cycling externally to the heat pump than it is to let the heat pump optimise its own cycling?  Do we think that somehow we are better at designing an optimum cycling regime than the heat pump manufacturers themselves are, and if so why?

Afternoon everyone.

Thank you @bobflux , @ashp-bobba , @f1p , @old_scientist , @grahamf and @jamespa for replying with such a wealth of knowledge. I didn't realise so many people had responded as I only had maybe 3 email notifications, so I've gone back to the beginning and re-read the lot.

Really good information, and everything explained in terms even I can understand, so thank you all for your contributions.

Regards 

Craig