Rad for conservatory

[The Medway Handyman]

Looking to put a rad in a new conservatory which is 3m x 3m ceiling height about 2.4m. Three outside walls.

One wall is double skin brick block with insulation, two walls are double glazed floor to ceiling. Ceiling is transparent plastic - apparently with a high insulation factor (but its not the triple wall stuff - its smooth). Floor is chipboard - don't know whats underneath.

Tried this site

which reckons about 6,000 btu's - but it doesn't have a pull down option for 'conservatory'.

Assuming this is right, its gonna need a 900 long x 620 high double or there abouts.

Does this sound about right? I'd prefer to err on the side of caution & fit a TRV.

TIA

[Roger Mills]

I haven't seen the calculations - but I would be very dubious about the result. Bearing in mind that 6000 BTU/Hr is somewhat less than 2kW, I doubt whether that's enough[1]. I assume this is for a customer, 'cos if it was yours, you'd know what was under the floor? Can you recover the cost of doing the calcs, as well as of supplying and fitting the rad? If so, I'd try to get hold of a better heatloss program (such as those available from Barlo or Myson)[2] which allow you to specify U-values and other factors, and which provide a breakdown of the calc rather than a single result.

[1] You could try putting a couple of 1kW oil-filled radiators in there on a cold day to see how 'toasty' they make it. [2] You can download the Barlo program from and, if you speak nicely to Andy Hall, he'll email you a copy of the Myson program

[Andy Hall]

I don't trust any of the web site calculators and certainly not any that work in BTUs, particularly if they don't say BTU/hr.

That aside, 6000 of the deprecated units comes to 1.75kW.

I always use the tables of U values and a calculator for each surface or one of the radiator manufacturer's calculators - the Myson one is the best IME.

You can look up the U values of each surface, multiply by the area and the temperature difference. That gives the loss for that surface. Then simply add the losses together. There is a similar calculation for air changes based on volumetric measurement and air changes.

Assuming that the rear wall is against a heated room, you can effectively ignore that.

Without doing the sums, based on my conservatory, which is quite a bit larger and has a glass roof, and scaling to the size of this one,

1.75kW is rather light. 2.5kW is nearer to the mark on this basis, assuming -3 outside and 21 inside.

I would tend to oversize for this application anyway. According to building control rules, a TRV is needed anyway, so given that, it's a reasonable approach.

[John Rumm]

Another option here would be one of the Myson fan assisted rads. These will very much faster warm up times which can be handy for a conservatory if it is not used as full time living space.