How many BTUs to I need? (Yes, that question again.)

I'm thinking of replacing an exiting radiator (see photo) with something a bit more exciting. A bit of Googling suggests the existing one is rated at about 5600 BTUs. (It's 1100mm by 500mm, type 22.)

I've used about 10 online BTU calculators to try to get an idea of what's actually needed. Some ask for only a few details - others seem a bit more detailed - and the results range from 3200 to 4400 BTUs with an average of around 4000 BTUs.

I have a great looking radiator on mind which happens to be rated at 4000 BTUs but before pressing the "Buy Now" button I wanted to check with somebody who knows what they're talking about that it's going to be suitable.

Could anyone advise? Thanks.

All these rads are based on a "50deg" rating which is the mean rad temperature minus the required room temp which is normally taken as 20C. If you have a boiler temp of 75C with a rad dT of 10C, then you will have a ((75+65)/2) - 20, "50deg" so rad will deliver its rated output, if you keep your boiler at 65C for condensing benefits then the you will have a ((65+55)/2-20, "40deg" so the rad output is reduced to 75% of its rated output. In your case, 5600BTUs to 4200BTus, 1.23Kw. If you opt for that 4000BTU,1.17Kw rad then the output is only 0.88Kw, fairly measly IMO except you have a very small, very well insulated room. You also have a TRV on it so why not go for one with the same output or even slightly higher in case you ever install a Heat Pump, if I ever have to replace a rad in the future, I will oversize by a factor of at least 1.5 of my existing rad(s) as I will be expecting my new heat source to be so low (even a condensing oil boiler) as to run "25deg" rads, 40% of rated output so, theoretically require oversizing X 2.5 but my rads are grossly oversized now (all have TRVs) so X 1.5 will do fine.

Thanks for the reply John. The room is 4.6m x 3.3m, 2.4m high with two external walls (two course, no cavity), just those windows and heated rooms above and below.

Oversizing would mean getting a wider version of the preferred radiator which would mean getting the floorboards up and that would add cost to an already expensive "wouldn't it be nice ..." exercise! (Or you could presumably put a dog leg in each pipe without having to bother yourself with floorboards, but then I'm worried it would look a bit of a botch job which is the opposite of the desired effect.)

A small room so should be OK with the 4000BTU, anyway you have the perfect sizing tool to hand, your existing rad, if its mean rad temperature is 59C then it is emitting 4000watts (a 38.5deg rad), if its less that 59C to give a 20C room temperature then the 4000Watt rad is fine.