Originally posted by bruce_miranda
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While that original idea of mine is better than just mechanically locking the heating zone valve open, (it allows full flow and also occasionally exercises the valve to stop seizing) I prefer the scheme I show in the circuit diagram above, as it is equivalent to what you are doing except when hot water reheating is enabled, at which point it reverts to normal TPI to prevent radiators going over temperature. Of course if you have hot water priority this isn't needed.
The above circuit is a further logical progression from the original idea to run the heating zone valve from the pump.
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