Heating causes water to overflow into expansion tank

Please can someone help with a problem I have with my hot water/heating system?

I've got a conventional system with a boiler, hot water tank, etc. The problem I have is that when I put either the heating or the hot water on, hot water overflows into the expansion tank.

At the moment I know I have a problem with my ACL Lifestyle mid-posision valve, in as much as it doesn't come on properly when just the heating is on - you have to turn the heating on (off/on) about 20 times before the mid-posistion valve moves to the H setting and the heating comes on.

Could the overflow problem be caused by the faulty mid-position valve, or is there another problem (maybe an airlock) with the system?

Really grateful to anyone who can help.

either piped wrongly or a build up of sludge around the cold feed/open vent connections. the same sludge has probably ruined the three port valve.

increase the height of the primary vent pipe ???

twgas said:

either piped wrongly or a build up of sludge around the cold feed/open vent connections. the same sludge has probably ruined the three port valve.

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Thanks TWGAS. I'll see about getting the three port valve replaced and any sludge removed and see what happens then.