Replacing old radiators with new more efficient ones.

[Dollychris]

I need to replace 2 lounge radiators (1 = 6ft 4" x 2ft single & 1 = 6ft 4" x 2ft double) with new more efficient ones. The old ones are about 20 plus years old now. The room is 26 ft 6" long x 12 ft wide. The height of floor to the ceiling is 9ft. Can anyone advise what replacement radiator sizes I would require please? Many thanks.
Dollychris

[nonomaybe]

I reckon that the most efficient radiators are the old cast iron types if that isn't what you already have( if so treasure them!!) you will find them at architectural salvage companies.

[buildersmate]

One wonders why you are doing this - it is the boiler in your system that is the most important link in the 'efficiency' chain.
If you are doing it on 'efficiency' grounds only - don't.
If you are doing it because the rads are starting to show signs of rust spots (rusted through from the inside) or they look manky (technical term) then that's more understandable.
What makes modern rads more efficient than their older counterparts is the increase in metal surface area, coupled with the design that encourages warm air to be drawn from the bottom thro' the fins of the rad (hot air rises), creating a nice warm heat flow from bottom to top that 'sucks' the thermal energy from the rad.
So in theory you are right.
But if you replace with a narrower rad, what are you going to do with the standing pipes that fix to the ends? These are going to look pretty silly if coupled with a straight horizontal length of pipe into the new smaller rad.
With modern systems including the use of TRVs on them, all that happens if you over-egg the size is that it gets to the set temperature quicker then the TRV turns it off.
I'd make life easy on yourself and replace with a similar size, assuming the heat output to date off the old ones has been satisfactory (and you do actually need to change them in the first place).

[gucciman]

when you say more efficient i am guessing that you mean you want more heat from them rather than using less gas to heat them up?

as buildersmate has already said if your doing this as a diy job then your best bet is to try and buy a radiator that is as close in length to the old ones as possible otherwise they are not going to mate up to the exisiting pipework. it is better to go very slightly smaller than too long as you can get extension pieces for the valves. your old rads are probably imperial size and the new ones will be metric so its not always easy finding a comparable one. go to your local plumbers merchants for help as they should have more choice than the diy stores.
a new modern radiator is probably going to give out more heat than your old ones anyway and you could also swop the single panel rad for a double one.

your other option is to get a heating installer out to replace the rads as they could then adapt your pipework to suit any size rad and they can also calculate what size rads you actually need to heat your room correctly.

[heathfield]

Check the rated heat output of any new radiator before you buy. I went looking for a new radiator and found two by different manufacturers that looked identical at first glance. But one gave out the equivalent of 2.4kW, while the other only gave out 1.8kW. Though they were both the same size, closer examination showed that the water channels in the 1.8kW one were narrower than in the other.
There are websites for working out the radiator size needed for a room. See one here.
(BTU x 0.000293 = kW)

[tezestwing]

Hi Dollychris I recently up dated our central heating old rads with new condensing boiler and because years ago a radiator in the lounge was only reckoned as background heating was surprised when it came to calculate the radiator sizes in the the rooms that had existing gas fires they did not comply with the calculations that have come out and I had to fit new double panels similar in length as old ones to the latest GOVENMENT L1 energy standards (nearly double kW that of the old rad) that are required from the rad to provide all the heating for that room also what you have to bear in mind is that if you have a roomstat in the room with the convecting fire then the heat from it when you have it on will interfere with the room stat hth Tez

[tezestwing]

the roomstat that controls the boiler hth Tez