Originally posted by bruce_miranda
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1) Not all my radiators were HR92 controlled at the time. (But are now)
2) A heating zone valve allows control of the average heat transfer through the CH circuit to be maintained during hot water reheat.
When hot water is not reheating the boiler relay on my system TPI modulates the boiler to control average heat flow to the radiators. When hot water reheat activates the boiler relay goes to 100% resulting in the flow temperature reaching the set flow temperature (typically about 70C) and that would result in all the radiators going over temperature during a hot water reheat.
However with a heating zone valve the intermittent opening of the valve during hot water reheat allows the average heat flow into the radiators to be roughly maintained so radiators do not go over temperature, but nor do they go cold like they would with hot water priority and a prolonged hot water reheat time...
If you already have wired hot water priority and all radiators are HR92 controlled you won't see an advantage from a heating zone valve.
But for those with hot water cylinders who do not want hot water priority (ahem) and/or don't have all radiators controlled by HR92's, a heating zone valve is desirable.
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