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.

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.