How large a radiator do I need?

[jmaxwell]

I'm looking at replacing an old radiator in an office/bedroom (see attached photo) with a modern convector radiator. I've been trying to work out the BTU/Watts requirement for the room but there is a large discrepancy between the various calculators available online (with some suggesting as low as 3000 BTUs and others as high as 6000, though most in the 4000-5000 range) so I wanted to ask on here to get a clearer idea of what I need.

The room is in a ground floor Victorian maisonette and measures 3.6 x 4 x 2.4m. Three walls are internal, apart from one which is south facing, having two large single glazed wooden sash windows, with a total glazed area of 2.56m2. The exterior wall is solid brick construction and 35cm thick, being a bit narrower than this underneath the windows. Underfoot are exposed pine floorboards over joists, with an unheated cellar below (the boiler is situated there but there are no rads). The ceiling is lowered for acoustic purposes with the original ceiling of lath and plaster 30cm above. Above that is another flat.

The present radiator which provides good heat in the winter measures 840 x 780mm and I assume that if I go with a double panel double convector unit, I could go with something a little smaller. I was thinking of this:

http://www.screwfix.com/p/kudox-pre...ct-convector-radiator-white-700-x-700mm/66028

Is this along the right lines? Or is it over/underkill?

 

[DP]

Easy answer is, if present radiator does not have fins, is single panel, go for 800 wide by 600 high single panel with convector fins

 

[jmaxwell]

Thanks, present radiator is double panel without fins.

 

[DP]

Go for double panel single convector 800x 600 high

 

[jmaxwell]

Would this provide sufficient output even in mid-winter? The flat can get very cold...

 

[ianmcd]

if you opt for a convector radiator you will be best to remove the shelf from above the radiator

 

[motorbiking]

Or you could calculate it correctly

 

[Agile]

Radiator outputs have been measured in kW since 1980.

If existing is adequate then go for same size with fins.

But a longer single panel would give better distribution of heat and that might give better comfort if used as an office. That is about 1800 x 600.

Since you should be fitting a TRV with a new rad its better to rather oversize as the cost of the rad is small compared with the labour time/cost.

For an older single skin wall using about 100 W per sq m of room area usually gives adequate heat output.

But a draughty old sash window can lose a lot of heat when windy. Better to seal any parts you don't need to open with silicone and consider tape during the winter. The worst part is the horizontal join in the centre.

Tony

 

[JohnD]

if you opt for a convector radiator you will be best to remove the shelf from above the radiator

Why's that?

 

[Agile]

Because if too close it will impede the convection air flow up the back!

But you knew that so why are you asking?

But generally a spacing equal to the rad panel to the wall spacing is not going to have too much effect.

But better to have at least 100 mm. Remember also that its going to get very hot so unsuitable for plants or wax statues!

 

[motorbiking]

As Agile says its the windows that are the killer, the AC/h is going to be quite high. I'd be looking at 5-6k btu, but it depends also on the temp differentiation in other parts of the home. I disagree on the shelf based on the likely drafts (AC/h)

 

[JohnD]

But you knew that so why are you asking?

because the gap looks OK to me.

 

[jmaxwell]

I disagree on the shelf based on the likely drafts (AC/h)]

You mean that you don't think it'd be a problem to have one?

 

[Agile]

because the gap looks OK to me.

The gap is just about OK for the existing radiator.

But that is 780 high which is not a current height.

Current standard height is 600 mm and a few sizes are also made in 700 mm too.

Tony

 

[mcmoby69]

Better too big than too small. Double panel, double convector if you don't want to get involved with pipework. Any decent plumbers merchant will help you out with sizing.
You can always turn an oversized rad down, you can't turn an undersized rad up.