Radiator sizing

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We have a large room 5m x 4m which has always been a little cool. It has a partial pitched roof with a bedroom over 3/4 of it. Window about 3m2 and 2 outside walls (cavity insulated) and one wall to garage probably insulated. Online calculators show about 6000 btu required. A thermometer is showing room temp at 20 C which is cooler than the rest of the house. The valve is wide open with all the other rads almost closed..... I have been into the roof space and added some more insulation and made sure what was there is properly fitted(no voids or draft pathways).

It has a 1200 x 430 twin modern radiator with two sets of fins, no air in it, no sludge - Toolstation rads 1200x400 are 5000 btu so I am beginning to think that the rad is too small.

As I know what the current rad is delivering would installing a 1400 wide rad improve the temperature? Do I need to worry about the “delta”?
 
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Hi, my lounge is 4 x 5½ metres, and the type 22 rad is 500 x 1800 mm (approx 9000 BTU) perhaps a little too large.
Could you have a taller rad, as well as a longer one?
As Doogle says a 10% or more larger one is the answer.
I would also check the rad sizes of all your other rads and rooms - if they are all oversized (by say 10%) then your lounge rad needs to be the same ratio. I found that at my daughters house, most rads were 20% oversized, so the replacement rad for a bedroom was scaled accordingly. (Original rad far too small).
 
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Height is limited by a window with a low sill, could go wider.

Might get a 450mm Stelrad in but 500 won’t fit.
 
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I asked on another thread but received no conclusive answer.
Are Stelrad radiators the best as they say?
Are they worth the money for my own house?
 
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We have a large room 5m x 4m which has always been a little cool. It has a partial pitched roof with a bedroom over 3/4 of it. Window about 3m2 and 2 outside walls (cavity insulated) and one wall to garage probably insulated. Online calculators show about 6000 btu required. A thermometer is showing room temp at 20 C which is cooler than the rest of the house. The valve is wide open with all the other rads almost closed..... I have been into the roof space and added some more insulation and made sure what was there is properly fitted(no voids or draft pathways).

It has a 1200 x 430 twin modern radiator with two sets of fins, no air in it, no sludge - Toolstation rads 1200x400 are 5000 btu so I am beginning to think that the rad is too small.

As I know what the current rad is delivering would installing a 1400 wide rad improve the temperature? Do I need to worry about the “delta”?
There was a thread on this subject not long ago https://www.diynot.com/diy/posts/5150314/
Might be worth asking Alan 483256 what rads he bought.
 
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Getting the insulation sorted helped and saw 20C before changing the radiator - warmer than it has been before.

Stelrad 1600 radiator delivered (eventually) from Screwfix. Tuffnells claimed to have delivered it when they hadn't so got it 2 days late.
To help flow as much as possible it was installed with 15mm elbows on the inlet/outlet instead of 10mm that the builders uses and a 15mm lockshield valve in place of the 10mm. It now has a thermostatic valve (there are still 3 radiators in the house without them) and have seen 21.5C in there with the valve set around 5 so much better. Wife doesn't sit under a blanket any more!

Anyone want a 1200x450 Permo radiator?
 

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