My three story house has a 20 kW oil boiler which cycles on/off so clearly larger than required. Every time a boiler switches off any heat in the boiler either goes out of the flue, or with some like my C Plan it can heat domestic hot water if not already hot, but basic idea is for it to switch off as little as possible.
Oil boilers don't modulate, they only turn on/off, a combi oil boiler just has a tank inside the boiler, gas however can modulate, typical is 6 - 26 kW, but the higher to maximum output also in general the higher the minimum output, there is a tendency to use a larger boiler to give a good output for domestic hot water, but can't see why, before the combination boiler I had two independent boilers one for central heating and one for domestic hot water, the latter was a Main 7, seem to remember maximum output was 18 kW I know when I decided to install a shower it was no large enough to have a shower with side jets, but for the standard shower I had to be careful to have a shower head large enough as if it didn't use enough hot water the boiler would cycle off/on, not what you want in a shower.
Mother had a combi boiler and it had two options, eco on or eco off, with eco off there was a small reservoir of hot water, but in the shower the reservoir would run out, so shower would go warm, cold, warm again. But with eco on this did not happen, once warm stayed warm, but if sink taps not turned on full would not get hot water.
This is all hinged around boiler minimum output, boilers can be too big.
Also radiator temperature, with oil is does not matter, but with condensing gas whole idea is the return water is cool enough to extract latent heat from the flue gases, so each radiator is set with a temperature gradient so return water is cool enough, so whole radiator is not at 70°C likely at 55°C at the return end, this is also required for the TRV to work well, as my TRV's when they exercise to stop them sticking, take around 3.5 minutes to go through the cycle, if the lock shield is not set, the radiator can get stinking hot before the TRV can close.
Last house when bought late 70's had no central heating, open plan, and whole house (three bedroom) was heated with a 4.5 kW gas fire vented with a flue brick, at this time no cavity wall insulation and no double glazing, the big problem was no automated control, so latter did fit central heating, but a 4.5 kW gas fire heated whole house, kitchen was a little on cool side, but it worked, which gives you a good idea how large a heater is required.
Having a larger heater allows for faster recovery, essential if using geofencing, whole idea is not to heat home when not in use, but if your living room radiator is 6 kW and your using electronic TRV heads so only heating rooms in use, then you need boiler to turn down to 6 kW.
Even with timed switch on, I have sequence, kitchen, dinning room, living room then bedrooms, so on switch on all boiler output going to kitchen, there is no need for a massive output boiler today. Minimum output has become more important than maximum output.
