Boiler size calulation (oil boiler)

no,to do it properly ,measure your rooms lxwxh,windows,orientation of the walls ie north south etc,composition of walls,go on the internet there are many sites where you just put it all in it will tell what you need,measuring your rads will not do it.contrary to advice of the ex royal marine the existing rads may be undersized or oversized ,giving you a false estimate for your boiler,heating needs for a building are calculated by room size,u values of walls,size of windows (double glazed or single glaze) how many outside walls,witch way they face,ground floor or first floor.

There are a few ways to calculate boiler size needed. What he is asking can be used to work out how many kw your radiators produce. If you had no radiators, then you would need to know room sizes, but as you already have radiators and I assume they aren't being replaced then you need to know their size. Knowing the room size is completely pointless. You need to know how much power each radiator needs in order to size the boiler. Knowing the room size will tell you what radiator size you need to heat the room.. This has already been calculated when they were first installed. Knowing the room size now is irrelevant, the installer needs to know the current radiator sizes. Room size gives amount of power needed for each room. Each room's power added together gives the boiler power needed. You already have radiators so the installer needs to know total power added together to give total boiler power needed. This is calculated from knowing radiator sizes. For example: You have 10 rooms with 1 radiator in each. I work out a room size to need 1kw power to heat the room. So you need a 10kw boiler (This is just an example as there are other things you have to take into account) You have 2kw radiators in each room so actually need a 20kw boiler. A 10kw boiler is installed based on room size and you have radiators that don't have enough power to feed them. You'll have cold radiators in the house. If you base it on knowing you have 20kw of radiators then you'll get a 20kw boiler. It will heat the radiators fine. Extremely poor advice and knowledge from anyone saying to base it on room size. I would steer clear of any company who doesn't know these basics as you'd have a underpowered boiler installed.

It’s the room sizes you need floor/wall construction etc NOT the existing radiator sizes

With modern gas boilers you can range the boiler at source so it isn't as important, oil however tends to have a kw range of 12-18, 18-25 etc as it cannot truly modulate. I would be more comfortable if the company did a heat loss calculation of the property, then sized the appliance off that as I assume the old radiators are likely sized based on fitting under the window rather than requirements.