In an earlier contribution to this discussion,
Sam wrote:


My boiler looks ancient and only has live and neutral, so I guess I'm
ok? It's also oil rather than gas; does that make a difference?

I don't know much about oil-fired boilers - but imagine that some *do* need
pump over-run. If yours is old, and holds a lot of water, it probably
doesn't. Especially since it only appears to have a switched live.

In situations that require it, how is it achieved?

Invariably the pump is connected to some dedicated pump control terminals on
the boiler, and the boiler decides when the pump needs to run - quite
independently of what the external timers and stats are doing. The actual
logic varies from boiler to boiler. Mine has a thermostat and, in addition
to running the pump whenever the external 'call for heat' demand is on, also
runs it whenever the boiler temperature is above a certain value - until
such time as it has cooled sufficiently. Some simply have a timer which
continues to run the pump for a specified time after the demand is removed -
similar to a bathroom fan remaining on for a while after switching off the
light. As I said in an earlier post, the boiler needs a permanent live so
that it *can* power the pump even when the external (switched live) demand
is off.


I've heard of these bypass valves. Do you also use them if TRVs are on
all radiators? I've always left TRVs off the hall rad (thermostat) and
bathroom (towel rad) hoping that would be sufficient for whent he
other TRVs close. Do I need a bypass in addition or is that enough?


From what you say, your boiler doesn't need a pump over-run - so the pump
stops when the demand is removed, and the water doesn't need anywhere to go.
Systems which have TRVs on *all* radiators (not recommended!) and which use
a boiler which needs pump over-run may well need a by-pass - even when using
a 3-port valve because in CH-only mode with all TRVs closed there would
otherwise be no flow path.
--
Cheers,
Roger
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