Before the days of Combi boiler I had two boilers one for central heating and one for domestic hot water is was called a Main 7, and it was I think 18 kW, and for a 3 bedroom house this was ample, although slow filling a bath. Before fitting the central heating the house was fitted with a single gas fire, open plan house so did heat all rooms, this was 4.5 kW. It was not thermostatic controlled and with bedroom doors closed the bedrooms were cold, so central heating was fitted.
But you need very little power to maintain the heat in a well insulated home, I now live in a three story house with an 18 kW oil boiler and within an hour the boiler is cycling off/on.
A large radiator is around 3.5 kW, smaller rooms can be down to 1 kW, so if I add together all the radiators in the main house, I am looking at around 18 kW, if the flat under the house is used OK higher, but today really the radiators should be around double the size of the boiler as we only use radiators in rooms being used, so really I need larger radiators in both living room and kitchen as I as it stands can't heat the house fast as the radiators are not big enough.
And it is speed of heating from a eco setting to comfort setting not the maintaining of the house temperature. There is no point in having geofencing to turn on heating if it takes 3 hours to heat rooms first occupied on arriving home, in the main we have ½ hour to reheat first rooms to be used when using geofencing, so the kitchen, dinning room, and living room need to be able to use whole output of boiler, bedrooms are not used until latter and by that time the kitchen, dinning and living room are just ticking over.
So maximum demand in any house is likely the shower, bath does not matter if it takes longer to fill, and so a shower needs around 16 kW standard, 24 kW with side spray units, as to two showers running together I have never tried. So the 18 kW oil boiler is ample for this three story house, 5 bedrooms, two kitchens, two living rooms, dinning room, two bathrooms and a shower room, although the showers are electric in two locations only one from DHW if this house can run on 18 kW can't see why any smaller house would need more.
