In article ,
Harry Bloomfield wrote:
The Natural Philosopher formulated on Sunday :
The pump is controlled by main thermostat.

Not in modern boilers. In modern boilers the pump is controlled by the
boiler and the boilers determines when the pump needs to run. Basically
as soon as flame is detected, through to burn turned off and remaining
heat removed from the boilers heat exchanger. Very sensible too, why
waste the heat? My boiler also exercises the pump and the fan, if they
haven't had need to run for a while.

you say modern boilers. Mine, which I installed in 1989, has this. very
necessary if you have zone valves.

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