Radiator sizing

[F]

I'm in the process of renovating the last room in the house (thank goodness!) and need to replace the 620mm x 1415mm single radiator as it is 42 years old and looking a little dated. It may also be following its friend from another room by developing a pinhole leak.

The room is 2.3M high, 3.33M wide and 4.11M long with a 2.43M2 double glazed window. Two of the walls are external walls with cavity wall insulation, the room underneath is heated and the loft above it has ~170mm of insulation. If I enter all that into the BTU calculator at

it would seem that I need a radiator with an output of 2018BTUs. Single radiators of a similar size (Stelrad K1 Compact, 600mm x 1400mm) to the one I am replacing are listed at 4681BTU.

Has the insulation I have installed really reduced the heating I need to provide to less than half? Or am I missing something?

TIA

[Andrew Gabriel]

Possibly. Do the same check on other rooms in the house. If they are all similarly over-spec'ed, you really need to do the same.

Other factors are that radiator outputs are often given using unreasonably high delta-T values. What delta-T is yours specified at, and what temp do you actually run your boiler?

Have you got a condensing boiler? If so, oversized radiators can make for more efficient operation (providing they're all similarly oversized).

[spud]

Try toolstation

[F]

They're all based on the original 1967 installation (pre insulation) sizing. We don't have any problems getting the house up to temperature in good time on cold days.

From the website: 'All radiator outputs are calculated @DT50°C inline with European BS EN 442 testing standard'.

It's a Worcester Bosch 24(?) Ri and hasn't got a dial marked with the temperature, just 'Reset' and 'max'. I just run it as low as it will go without the house taking for ever to warm up. It's a little less than half way between the minimum (35C) and maximum (82C), so say 50C?

Yes.

OK, so the radiators I've looked at are around 5% or 10% oversized according to the calculator.

[F]

For...

[spud]

Precisely

[The Natural Philosopher]

Should be less than a third.

I'd say you are about right.

something like 100W/sq meter Just under a kilowatt probably.

[F]

Good news and, thinking about it, I've spent quite a lot on insulation over the years: the house had no double glazing or cavity wall insulation and had just 25mm of loft insulation when built so would have needed fairly chunky radiators to cope with the heat loss.

I'll get the smaller (and cheaper) radiators ordered then.

Looks like a toss-up between OnPlumb and HeatAndPlumb, though OnPlumb are cheaper with their free delivery for orders over £50 (ex-VAT).

Unless someone knows cheaper...

[David]

Is this a conventional boiler? I have been told that they have to be set at a minimum of 65C to get the hot water cylinder at the correct temperature.