On an S plan system with a gas boiler that has a constant live feed to
a boiler over run circuit, in order to keep the circulation pump
running to remove residual heat in the boiler, is it normal to control
the pump at all times via the boiler? Or does the over run circuit
only activate as the boiler stops burning till the boiler is below a
set level, with the normal circulation still controlled by the
The boiler in question is a 3 year old 50kW New Ideal, non condensing
in the boiler room of a large rambing 4 bedroom house with ballroom.
On my fairly new WB, the boiler is always in control of the pump.
Mains in to boiler, demand into boiler, boiler out to pump.
I would assume this is logical as only the boiler knows when the heat
exchanger has cooled enough to be able to switch off the pump.
Are you sure the circulation pump is not for the HW? This would mean your
system has two pumps.
Normally a pump overrun is controlled by the boiler and with an S plan a
bypass is used as both valves are closed when the boiler turns off.
Sorry for delayed reply: it's a big house and there are twin
paralleled pipes feeding a 40mm main, these are the only pumps..
For some reason the room stats trigger boiler (which is wrong but
something to do with the motor on motor off 40mm Belimo valve not
being compatible with the honeywell spring release on the HW circuit)
but as most of the rooms have TRVs the pumps can run for long periods
while the boiler is not firing. This is why I wondered if I could
control the pumps directly from the boiler.
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