One room is colder than the rest of the house

Pie_Eater · 2025-10-27T09:51:44Z

Hi all heating season starting- exciting times! my AsHp daikin 8kw installed by octopus has performed excellently over the last year . It’s used around 400 kw more power than the quote but we have our house at 21 not 20. So all good really only positive things to say. one minor issue is that our loft /spare bedroom is around 0.5-1 degree cooler than the rest of the house (1 loop all on same temp). It was sort of expected, initially radiators we recommended to be replaced but we pushed back (all the other rooms are fine as expected).however the room is only used as a study 2 or 3 days a week and rest of the time empty. And as a bedroom the temp at 19.5 ish is totally fine. So to heat it to 21 the whole week feel totally wasteful.My wife uses it and feels the cold more than I do so would want to improve it. In this case , better to use a portable small heater to provide heat on days she works there? Or still better to upgrade radiators?

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AndrewJ

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28/10/2025 2:54 pm

Posted by: @old_scientist

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Is that because it's putting out more heat than the property requires at this time of year? ASHPs can only modulate their output so low (typically to around a third to a quarter of full output) and achieving continuous running is not always possible this time of year as even at minimum output the heat pump may still be cycling or turning on/off by the thermostat (depending how it's been set up).

Honestly, I don't know. I can see that it is ramping up to 1.75kWh of power but on other occasions it might be around 0.7kWh.

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AndrewJ

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28/10/2025 3:03 pm

Posted by: @jamespa

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Yes. There is nothing sacrosanct about DT5 even on aggregate, and certainly not for any individual radiator. Furthermore if all the rads have been balanced for DT5 then its possible that all have LSVs turned down, whereas at least one, preferably several, should be fully open.

Installers, if they balance at all, will usually balance for equal deltaT on the basis that this is how the system was designed, it can be done relatively quickly, and if all the calculations are correct this should result in the design room temperatures. But design loss and actual loss are two different things so in practice a basic balance for deltaT may need to be tweaked to get the desired room temps. Turning down one LSV is unlikely to have a major effect on others and yes its generally a better approach than turning down the TRV if only because it ensures that the volume of the radiator is always in circuit to help with cycling and defrost.

If all your upstairs rooms are reaching 24-25 then turning down all the LSVs may not be the thing to do (but neither is having them all switching on and off on TRVs). More likely you could turn up some of the LSVs downstairs (reducing deltaT and thus raising output), perhaps turn them down a bit upstairs, and reduce the WC curve a tad.

@toodles has written an article on balancing for equal room temp/desired temp differences (as opposed to balancing for DeltaT); its worth a read.

That said one doesn't have to get obsessive about eliminating TRVs. I have one out of ~14 rads on a TRV, its in the 'visitor' bedroom and I do it as a courtesy to visitors. I started off with all the upstairs on TRVs and all the downstairs open loop, but gradually adjusted the LSVs until the TRVs became unnecessary. Some installers will say TRVs upstairs, open downstairs. That's not optimum but not completely silly, however it does risk roughly halving the system volume if they all shut down simultaneously, which will half cycle time (bad) at any given set of conditions and reduces the water available for defrost.

@jamespa thanks for all the advice. Just so you know, I'm not ignoring it but pulling it together to try and work out a plan of action. I'm currently measuring what the rooms are heating to so I have an idea at least of what might need doing. So far, the Lounge, which contains the Primary Pod is ranging between 21.8c and 22.5c; the kitchen is between 21.6c and 21.9c; the dining area follows the lounge quite closely (perhaps 0.2c warmer); the hallway is running at around 0.5c warmer than the lounge. Upstairs the rooms I've measured are running at around 23c to 25c.

Anyway, as I say I'm pulling together all the advice to act on. I also have a call out with Octopus to try and speak to someone in more detail about how the Cosy works.

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