Combo boiler sizing

It is depend on the room size flat or house. 2 bad flat 18Kw combi enough. 2 bed house 24kw. 3 bed 28kw. If you know room size corridor etc will be good. There is Application as wall you can download. Regards Indy

The calculation works on a U value of each room on the heat required. A U value depends on size of the room, type of floor, type of wall, insulation and number of outside walls. Once all rooms are calculated the combined total in KW or BTUS is the total heat requirement. There are 4,312 BTUS to 1KW. The hot water on a combination boiler is calculated in a flow rate. The hot water in storage heater is calculated at 2.2KW for a 130 litre, 3KW for a 175 litre and 4.3KW for a 250 Litre. Google Mears calculator for further info. Regards, Jason

Rule of thumb is 1 kw per radiator and 5 kw for your water cylinder ( if you have one of course and not a combi boiler. If you have wet underfloor heating then allow 50 watts per square meter.

There appears to be lots of complicated advice with btus and outside walls etc, most 3 bedroom houses with around 10 radiators will work fine with a boiler of around 28 to 30 kw output. If you want better hot water go for a higher output, say 33 -35 kw, you will use more gas for showering but the heating output will be similar to a 28 kw boiler. For larger houses a 40 kw boiler will give around 5 litres more hot water per minute, depending on the boiler model, so 16-18 litres a minute at 65 degrees, The heating output is also greater so will heat more radiators. 12 - 14 litres per minute for the previous boilers but they use less gas and heat less radiators. All depends on the the house, and the needs of the occupants.

Really really surprised at the answers above. The amount of bedrooms and radiators for a combi is basically irrelevant. The amount of bathrooms and hot water usage is what a combi is based on. Extremely surprised at such answers from supposed engineers.