DIY Doctor UK DIY Forums

Can anyone give me some advice. Recently I noticed water dripping from the overflow pipe coming from the expansion tank in the loft. It is not the ballcock leaking. It is only happening when the central heating or hot water is on and the pump is running, then after a short period hot water comes out of the vent tube that flows into the expansion tank, and then to the outside via the overflow when the tank becomes full. This vent tube seems to disappear down towards the pump area in the airing cupboard. After calling out British Gas under our heating cover, the engineer who didn't apperar to do much, just said we needed the system flushing, which is obviously not covered, but eyewatering at £700 quoted. Is there anything I can look at, or attempt to do to try to fix this problem. I have taken all the radiators off the wall and flushed them outside, because i've been wanting to fit TRV's to the radiators for a while, there was a bit of black sludge, but the problem came straight back. After getting a few quotes, which are a lot cheaper than British Gas I was told by one lady plumber that she didin't think the problem was something that a power flush would solve. If anyone has any help, ideas or advice I would appreciate it.

If it only vents when your pump's on you could try turning down your pump (they usually have 3 speeds and are set on the middle setting)
It could be worth a try to stop it pumping over.
If it's just the the overflowing you're worried about you could just turn down the amount of water in the system with the float valve

at a guess i would say it is hot water venting look on your hot water cylinder and turn the cylinder stat down a few degrees this should cure the problem