Dripping F&E Tank O/F

Hi -
My gravity fed open vented Central Heating, connected to a Potterton Netaheat2 has been and still working fine - all rads nice and hot, all bleeding ok, boiler and room stat working normally. BUT recently hot water started ejecting through F&E outlet into tank on heating start up.. This is causing the overflow to drip - drip - drip for about the first hour or so that the heating is on for then this stops. The ball valve on the supply to the tank is functioning ok. I checked the other posts on this site and couldnt find one exactly like this, so very much hope one of you can help.
Thanks. GP

vent pipe could not be high enough.....

too much water in the f and e cistern also...need to have room to expand etc...so level should not be too near the warning pipe outlet

Usually any slight expansion will actually flow up the feed pipe, as it is a bit lower, so I wonder if that has a blockage of sediment? (very common) so that it is being forced up the 22mm expansion instaed? If you draw a bucket from the system drain cock, does the F&E water level drop by a similar amount (with the ball tied up)?

22mm expansion

Click to expand...

Hey guys, let's get this right. The 22mm pipe is the vent pipe. the 15mm pipe connecting to the base of the tank is the feed and expansion pipe.

The pedant.

Of course it is sorry, I knew that really