Calculating wattage for a combi boiler

[joules99]

Does anyone know what relationship the kW spec of a combi boiler has in relation to number of raditators required. For example we have 9 rads, they vary in size but assume that 4 of them are double size, the rest are medium, what output will we require if we were to buy a combi bolier?? Also we have one bath, and the shower is electric, therefore irrelavent! Thanks people...

[stanleyman]

I'm afraid it's not as simple as that. The radiator sizes are related to the heatloss of each room and the heatloss is related to the size of the room, amount of insulation in ceiling, walls and floor, number of outside walls, number of air changes etc., etc.

The rads are then sized according to the heatloss and running temperature of the boiler. The total of house heatloss plus an allowance for any extra exposed position heatloss plus any separate hot water requirement gives the minimum boiler size in KW or BTU's. This is all best done on a heatloss calculator and the competant gas installer can do this.

However, if you intend to keep the 9 radiators then they maybe sized ok. Combination boilers are oversized for most installations because they need a large output to cope with the hot water demand so you may not need to worry about the size. I would guess the heat requirement for 9 rads would be about 40,000 to 50,000 BTU's or roughly 12 to 14.5 KW.

New regulations require a new gas combination boiler to be of the condensing type to be fitted along with a timeswitch, room thermostat and thermostatic radiator valves on all rads but the thermostat room. You need to inform building control before starting work unless you use a CORGI self certificating installer. The proposed house pack will need all this paperwork when you sell.