Poor ASHP performance this morning, adding additional rads

So after this mornings colder spell recorded 2˚C here in the early hours. ASHP COP has taken a nose dive.

So I’ve been going through things this afternoon to try and work out what’s going on.

The conclusion I have come to is the living room where the room stat is for the ASHP, which also happens to be the furthest point from the ASHP (26m away - pipe length). With a shortfall of 700w (room requires 1955w) combined with the length of the pipe run is what is causing the issue with the ASHP, much higher electric usage and the room not actually reaching temp.

So I’m looking at an additional radiator and upgrading one to give me an output of 2404w

My question is, can you have to many / to much water volume on an ASHP, I have a buffer of 100L in addition to the rads.

Current radiators on the system are (all K2 unless stated otherwise)
1600 x 750 x2
1000 x 700 x2
1800 x 450 to be replaced with 2400 x 500
1000 x 900
1000 x 600 (P+)
900 x 600 (P+)
900 x 600 x 2
1200 x 700

In addition to the one replaced above also looking to add a 1600 x 900 as well.

thanks for any insights / feedback.

I can’t imagine the volume change from increasing the size of one and adding another will be much in %age terms.

I’ve got around 26 emitters and 90l of buffer - all K2 plus three large towel rail.


Just been checking ultraheat spec sheet, they would add an additional 24 liters, so in comparison to how many you have already it should be fine then.


1 Like

This isn’t particularly unusual. What do you normally get? The 4 defrosts happening will be borrowing some heat from the house, which will knock COP down a bit too.

Adding radiators may allow you to reduce the flow temperatures, though you seem to managing to run in the low 30s, and there’s no indication you’re limited by emitter surface.

Adding volume to the system will help reduce how often it cycles in mild weather, but I’m not seeing much of that happening either.

You might not need to add any radiators…

It’s normally sitting around the low 4’s

4 defrosts seem quite a few for the period of time, though have nothing to base that off really, I guess if it needs them then well it best do them.

I only came to the conclusion of additional radiators due to the difference in room heat loss and current output and the room stat has only in the last few hours reached the set temp. (ASHP overshoots by .3˚C and then kicks back in once temp has dropped by .3˚C below room stat temp).

The lowest the flow temp goes is 29˚C and usage is then 480 watts.

Maybe I’ll just have to accept it needs 800ish watts and my home battery won’t make it through the day.

Thanks for the reply.

This is my system on Sunday.Lots of defrost cycles which knock the CoP down to low 3s rather than the low 4s that it was earlier.

The humidity was, in effect, 100% so freezing up is to be expected. The heat pump is possibly undersized for the building and the loft insulation hasn’t been fully reinstated yet so it is working flat out.

1 Like

Thanks for the reply billt, I was looking for other pumps which were doing defrosts, but didn’t come across any, though after looking at 3 thought it must just be mine.

Still a decent COP though, in spite of all the defrosts.

My unit is 7kW, but never seems to go above 5, even on DHW runs.