In article ,
Harry Bloomfield wrote:
on 17/01/2021, The Natural Philosopher supposed :
The boiler firing is *also* controlled by water circulation temperature.

Modern boilers do what is called modulate - the Kw output can be varied
over a range of output, to suit the heat for HW or CH requirement.

I installed a modulating boiler in our last house in the early 1970s.

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle