Water gushing from expansion tank when heating off

suz-cara

Wondering if anybody can help me on this one....water gushing (gallons) from my attic expansion tank through the overflow pipe a few hours after the oil burner has switched off. Large single storey house, oil burner on a timeclock with the circulating pump switching on and off on same circuit at same time. Ballcock seems fine and no overflow when the heating is on but problem is a few hours after it has gone off. Happening most days and immersion heater is never on. Boiler isn't new, pipework and stat haven't changed. Only thing that may have changed is that some rads have been turned down or off. Thanks for any advice.

KeithP

Could be a leak inside your hotwater cylinder.
This could allow domestic hot water into your heating system.
If your expansion tank is lower than the cold water storage tank in the loft, the water level in the expansion tank will rise and flow out throught the overflow.

suz-cara

Hi KeithP.

Thanks for the feedback. The immersion heater/cylinder is never on but the overflow still occurs, never when the heating is switched on, but usually after it has been switched off...gushes for a few minutes only.

The expansion tank and cold water storage tanks are adjacent in the attic with the overflow holes on both tanks at the same level and feeding the same overflow pipe. The ballcock on the cold water tank is low enough to ensure that no overflow is finding its way from that tank back into the expansion tank. Could it be a stat problem? Thanks for the help.