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Increasing Room Temperature: Radiator Sizing Rule of Thumb Needed

 
Air Source Heat Pumps (ASHPs)
Last Post by AgentGeorge 3 months ago
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 Papahuhu
(@papahuhu)
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Topic starter 25/01/2026 10:33 am   [#2778]

Hello

I have a need to increase design temp of a single room from 18C to 20 C. Upstairs bedroom, 2 external cavity filled walls, 2 *18C adjacent rooms, 21 C room below, 300mm insulation, current rad 596W @50C. 

Is there a rule of thumb I may use to estimate the output I need please? Not looking for accuracy just an approximation. I’m assuming most of the heat comes from the room below, but that’s already a bit warm for comfort.

thanks


This topic was modified 3 months ago by Mars

   
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JamesPa
 JamesPa
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25/01/2026 10:53 am  

Heat loss is proportional to Inside Temp - Outside Temp, its that simple if you ignore heat transfer from/to surrounding rooms.  So if you design temp is -2 you need 22/20 times the output to go from 18 to 20.

However if most of the heat is coming from below and its already too warm for comfort then this wont apply, but if its already too warm why change anything?

 


This post was modified 3 months ago by JamesPa
This post was modified 3 months ago by Mars

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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 Papahuhu
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Topic starter 25/01/2026 8:48 pm  

@jamespa

That’s nice and easy, thanks, 10% ish. I have a 10% larger rad in a different room that’s a touch too warm, so I can just swap them over. Glad I checked, my gut reaction was to go about 50%. 

Have an oldie in this room, it is meeting its design spec of 18 but he’s still cold.  I don’t want to have to push the entire house +2 and be uncomfortable just to get one room a bit warmer. Did think about getting one of those thermostatic fan units that sit under the rads but figured it might be noisy and probably won’t give me the gain I need. 

thanks for the advice, again.



   
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JamesPa
 JamesPa
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25/01/2026 9:22 pm  

Posted by: @papahuhu
↑

@jamespa

That’s nice and easy, thanks, 10% ish. I have a 10% larger rad in a different room that’s a touch too warm, so I can just swap them over. Glad I checked, my gut reaction was to go about 50%. 

Have an oldie in this room, it is meeting its design spec of 18 but he’s still cold.  I don’t want to have to push the entire house +2 and be uncomfortable just to get one room a bit warmer. Did think about getting one of those thermostatic fan units that sit under the rads but figured it might be noisy and probably won’t give me the gain I need. 

thanks for the advice, again.

Just one thought if you are cutting it that fine.

Do you keep the door closed?  If not you may struggle to sustain the difference without the rad being much larger

 


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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 Papahuhu
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Topic starter 25/01/2026 9:38 pm  

@jamespa It varies as dad has some cognitive issues, the door leads to an unheated landing so that doesn’t help. I’ll edge it up to 20% then, we are about 400m elevation too, so technically -3C. Thanking you.



   
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JamesPa
 JamesPa
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25/01/2026 11:25 pm  

Posted by: @papahuhu
↑

@jamespa It varies as dad has some cognitive issues, the door leads to an unheated landing so that doesn’t help. I’ll edge it up to 20% then, we are about 400m elevation too, so technically -3C. Thanking you.

I'm presuming the lsv on this rad is open (or if not you tried that and it caused a problem elsewhere)

Good luck, maintaining temperature differences between adjacent rooms is always going to be a challenge particularly if doors are open!

 


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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 Papahuhu
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Topic starter 26/01/2026 12:21 am  

@jamespa It’s the index rad on the system. They were balanced about 4 months ago with inlet and outlet temp sensors, I’ve been loathe to fiddle with any of the lsv or tsv since then as was previously having heat pump woes and am firm believer in never making more than one change at a time when problem solving.
I suppose because it is the index rad it won’t hurt anything to open up the lsv fully, I’ll have a fiddle in the morning. 

(Heat pump has been amazingly well behaved all week, night and day. COP up about 0.5 units, getting combined 4.8 over last week, house temp very stable (deviation < +/- 0.2C) and warm 24/7, best thing has been the noise reduction. You wouldn’t believe how bad it was and how quiet it is now. Would never had thought an outside unit power reset could be transformational, must be so many in the same boat whom don’t realise. Homely needs to flag an error when this happens, I did contact them to provide feedback.

 

 

 



   
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JamesPa
 JamesPa
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26/01/2026 12:23 am  

Posted by: @papahuhu
↑

I suppose because it is the index rad it won’t hurt anything to open up the lsv fully, I’ll have a fiddle in the morning. 

Maybe mark where it was before you fiddle.  However if it is the index rad/circuit the LSV should already be fully open otherwise you are unnecessarily restricting flow through the whole system

Posted by: @papahuhu
↑

, best thing has been the noise reduction. You wouldn’t believe how bad it was and how quiet it is now

Some of that may be because its warmed up!


This post was modified 3 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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 AgentGeorge
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26/01/2026 7:32 pm  

@papahuhu absolutely correct, if you change 2 parameters, you cant prove which one made the difference. If you close a window and put the heater from 1 bar to 2, which caused the room to heat up the quickest/most



   
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