Thanks for that.
The same radiator has always warmed up with only the hot water system on, but recently it has been getting as hot as when the central heating is turned on, I was wondering why the change?
The radiator is heating when the Hot Water only is selected.
The 'heat sink' which is at least one radiator with x2 lockshileds valves, and the rest with TRV's, is a 'heat sink' for the central heating to protect the boiler. And for combi's where the excess heat from the boiler when the hot water is heated - is lost down the central heating flow.
A heat sink on a gravity or fully pumped system is also required if TRV's are fitted .
However this is not the problem here, it's is a gravity hot water system.
The only difference between the central heating being on and the hot water, is when the heating is on the pump is running. The primary flow is above the heating flow, and is a larger diameter pipe, when the water is heated it rises. As the pipe is at the top of the heat exchanger and is the larger diameter pipe the water will travel up this pipe.
Even when they were new installations, heat found it's way up the heating flow and the upstairs radiators would get warm. As the system gets older there's more restrictions/friction in the primaries and more heat escapes up the heating flow.
You can prevent this, by altering the pipework where the heating flow leaves the boiler, if you form a 'trap' in the pipework (an elbow down before allowing the pipe to rise. The heating will still work as the pump will circulate the water. But it will stop the natural circulation up the central heating flow when Hot Water only is selected.
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