Two Port Zone Valve Passing

2 Mar 2020
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United Kingdom
I've just recently replaced a Tower 2 port zone valve with a Honeywell V4043H1056 22mm as the Tower had been leaking and was passing a fair amount of heated water.

System boiler, two heating zones and one DHW zone valve.

Is it normal for them to pass a little under pressure? All seems okay when removed and blowing through them as a test. Even the replacement valve appears to pass a bit and I do not think it is heat transfer or reverse flow.
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Bypass is in the roof space. The three zone valves are in the cylinder cupboard off a common supply. When either the DHW primary or a heating zone is calling then the line is hot. There is another bypass in so far as a towel radiator and hall radiator so nothing can pump against a dead end. The pipe beyond the suspected passing zone valve is only hot for a few feet but a bit more than transferred heat I think.

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