Yes, but that surely assumes that the boiler controls the pump at all times - with the pump running whenever the boiler is firing - plus a bit. However, in a C-Plan system, the pump doesn't run at all in HW-only mode - it's just gravity!
I think what I'm saying is that you *can't* have gravity HW in a system whose boiler needs pump over-run - it *must* be fully pumped.
Nope there is no connection to the pump, that is only controlled by the room stat and CH timer. The new connection above is to the motor of the valve, the switch of which only controls boiler. Uh Oh, have we got into a latch up situation? Yes, we have, whoops!
You can but you can't have a standard C-Plan setup as the valve will shut and stop the cooling via the gravity loop... The over run signal must keep the valve open but not let the boiler fire unless there is a demand for heat. Currently that is easier to type than work out how to do! That last "unless" is the killer.
Fully pumped is certainly easier. B-)
In that case, what you're describing isn't pump over-run - it's valve over-run. In other words, you're saying that the boiler needs an open path for gravity circulation until it has cooled. This wouldn't be sufficient for boilers which truly need a pump over-run since they need a higher flow rate to cool them than gravity is likely to deliver.
If you *did* implement valve over-run, you'd almost certainly need some relay logic - otherwise you'd open the valve - and over-heat the HW - whenever the boiler was firing for CH.
No - that's NOT pump over-run, it's valve over-run (see above)
Agreed! [At least when installing a new system].
But going back to where we started (!), if a system is currently working ok with open-loop gravity HW, it's very unlikely *not* to work if it's converted to C-Plan - assuming, of course, that the valve is put in a sensible position so as not to block the vent path!
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