In an earlier contribution to this discussion,
Dave Osborne wrote:

Roger Mills wrote:


I wonder whether it's worth adding one additional refinement to
reflect the way in which many systems are *actually* configured? I'm
referring here to pump over-run arrangements. Your diagrams
faithfully reproduce the original Honeywell 'plan' details - with
the boiler and pump wired in parallel in all cases.

Not true, see C-plan. In this arrangement, the boiler and pump are not
wired in parallel. The boiler can run without the pump when there is
call for hot water but no call for heating.

Sorry - you're right - it's not *quite* all cases - I overlooked C-Plan
systems!

However, my understanding is that this type of system (C-plan) is
deprecated and if you were to replace the boiler, then you would
automatically upgrade to fully-pumped and then you would need to
change wiring to S-plan and probably implement the boiler overrun as
discussed.

True - but there is still scope for converting a 'conventional' gravity
HW/pumped CH system to C-Plan *without* changing the boiler, in order to
gain the advantages of HW temperature control and boiler interlock. However,
these systems invariably use boilers which *don't* need pump over-run -
almost by definition, because the pump doesn't run in HW-only mode, anyway.
So, in the case of C-Plan, the existing Honeywell info will suffice without
requiring any embellishments.
--
Cheers,
Roger
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