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Thanks for getting back to me. The problem only occurs when it's asking the system to heat water. Overflow through the vent pipe occurs when heating water on its own and if heating water and heating the rads. There is no overflow if heating rads only. hope this is clear.

No maintenance or any other work done for several years.

I have an open vented central heating system fitted with a three way valve and a Grundfos Selectric pump. System has worked well for years. Recently there has been continuous flow of warm water to the expansion tank through the vent pipe. This only happens when the system calls for heating of...