The case of the mysterious expansion tank overflow

[paulinpinner]

I have a plumbing problem that is baffling my plumber, and wonder if any gurus out there can help.

We have a fairly old gravity system, and our expansion tank has begun constantly overflowing. It's just a drip when everything is off, but when the boiler kicks in it speeds up to a fast dribble. It does not appear to be a faulty ballcock in the expnasion tank. I don't know about the drip, but certainly when the boiler is on, the excess water is coming UP through the cold feed pipe and into the expansion tank. Nothing seems to be coming from the boiler vent, and the water is not scorching hot, so I think we've ruled out a faulty thermostat.

Our plumber now thinks the heating coil must have developed a leak. The cold water storage level is higher than the expansion tank, so I suppose it would leak in that direction, but he's not convinced because why would the flow increase considerably when the boiler's on?

Any ideas, oh wise ones?

 

[fitz1]

Good chance hes right.Heat causes expansion.Therefore hole expands.Also check expansion pipe not blocked.

 

[ChrisR]

You have a pump in your system which creates a higher pressure when it's turned on. What else would a pump do?