Hi there,

Our house has a total 16kw - 17kw of radiators, the boiler is capable
of supplying 28kw. On the boilers diagnostic menu you can adjust the
maximum kw from 12kw right to 28kw, it came factory set at 28kw but I
thought we could use a little less gas by running in exactly at 17kw
which is the maximum ever required, however this what I have observed:

When on 28kw the boiler fires at 12kw for a minute, ramps right up to
28kw and quickly achieves the flow temperature of 70ºc, it then
modulates down slowly and happily maintains 70ºc until the thermostat
stops calling for heat. The house warms up pretty quickly.

When on 17kw, again the boiler starts on 12kw, ramps up to 17kw and
takes a fair bit longer to get to 70ºc, once at 70ºc it will modulate
down and maintain 70ºc as it does when on 28kw. The house takes a bit
longer to get warm but still reaches the set temperature in a
satisfactory amount of time, just not quite as rapidly as before.

So the question is what is the better way to run the boiler, will it
save gas running on 28kw as the boiler modulates down quicker? I have
heard it's best to run at 17kw as it stops putting unnecessary strain
on the main heat exchanger, how true is this?

The boiler is a Vaillant Ecotec plus 937 (37kw Dhw, 12kw-28kw Ch)

Thanks.