Patb wrote:

OK, not an expert but....

the water can be heated by either the immersion of the Gas boiler.
The central heating system is vented.
Re the water getting into the tank, not sure about this. It is coming
in from a large pipe that connects to the tank in the loft via the
side of the tank at the bottom. Do I have to track this pipe to see
where it connects to the immersion?

No, if it is coming into the base of the cold tank, then that is almost
certainly the feed out to the hot cylinder.

Options would seem to be, primary heating water escaping from the the
heat exchanger coil in the cylinder. Check the relative heads of the
cold tank and heating header tank. Is it possible for the water level in
the header to be higher than in the cold tank? If it looks possible that
this is happening then a new cylinder would be the cure.

Another option would be if you have any mixer taps that have a mains
cold feed to one side, and the tap is "letting by" - forcing mains cold
water back up the hot pipe. Can be particularly noticeable if you have a
mixer like this that has a shower on it. (you should not use true mixer
taps in these situations unless you have non return valves also fitted)

--
Cheers,

John.

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