Should you oversize a radiator?

[Justice League]

Replacing type 11 with type 21 and trying to get as close as to the original output as possible - should you oversize or undersize if it's not exact? I am talking less than 5% either way.

In one radator its btu was 5159 but I can either go for 4888 or 5702 - the original was a type 11 450 x 2000 and I want to replace with a type 21 500 x 1200 or 500 x 1400.

 

[steadyon]

1. You can always turn down an oversized radiator. You can't get extra heat out of an undersized one.
2. So go for a slightly oversized one if you have to.

 

[SJB060685]

With an undersized radiator you would have to increase the flow temperature to get a slightly higher output, this of course would have the same affect on all the other radiators. I would oversize slightly, you can always reduce the flow rates through it to widen the delta t and reduce the radiator mean temperature and slightly reduce its output.

 

[Stu-B]

Does the existing radiator warm the room comfortably? I’d Check if it’s correctly sized in the first place before you start thinking about bigger/smaller.

 

[John.g]

I would tend to oversize by at least 25% or more and fit TRVs, you will then get a very fast warm up and the highest possible efficiency from a condensing boiler as the return temp(s) will/can be very low once the room temps are satisfied even with a relatively high flow temp of say 75C. Also if weather compensation is installed then gas boilers can run with flow temperatures as low as 30/35C so again IMO another reason to oversize.

 

[Dan1990]

What’s the delta T for your system? Bear in mind that a lot of heating systems in the U.K. were designed around delta 60, but the current standard introduced in the last decade is delta 50. For alternative energy sources the delta T is 40 or lower.

 

[John.g]

A "40 deg" rad would have be oversized by a factor of 1.7 based on a "60 deg" rad and by 1.34 based on a "50 deg" rad. The 50 deg rad base is fairly european wide now i would think.

 

[SimonG]

Definitely oversize slightly.

 

[Justice League]

I will be oversizing slightly - it makes sense once its been explained.