When designing a heating system, the designer should calculate the heat loss
of the building. That will be in a certain set of conditions - eg average
minimum winter temperature outside at your location, humidity/wind,
draughts/chimneys, popular internal temperatures (eg 18C in bedrooms, 21C in
living rooms). That gives you a heat loss figure in kW. For example, for
my house when I did the numbers it was 5kW.
In other words, in those conditions you need 5kW running just to fight the
losses and keep it at steady state.
Of course, if you actually want to warm the place up from cold, you need
more than that. If your heating was outputting 5.1kW it would warm up, but
very very slowly.
So the heating system is sized to be able to swing your house around faster.
For example, my heat pump installer calculated that as 8.5kW and the next
model up was 10kW. That also provides a bit of headroom in case the calcs
If you're on gas the upfront cost of installing a larger boiler is often
relatively minimal, so installers tend to oversize, often skipping the
calculations and going for a finger in the air. That's why people end up
with 24kW or 40kW boilers. Those also have to provide instant hot water, or
if you have a cylinder you need a boiler large enough to heat the water.
The heat is also limited by your emitters - you need to dump the heat into
your rooms, otherwise it just goes around and back to the boiler. So the
boiler output temp (condensing can be cooler, also heat pumps) and the size
of your radiators etc also sets how fast the system can swing your
Once your house has reached steady state your boiler will modulate - it
won't run constantly, it'll turn on and off to maintain the temperature.
That's fine, it's designed for that. Meanwhile you'll need full power when
heating water, so oversizing isn't wasted for that.
 it's actually a bit more complicated, since conductive losses depend on
delta-T so a cool house has lower losses than a hot one, and so the
temperature would ramp faster to begin with and then struggle as the house