vent pipe/ expansion tank causing overflow

I have an open vented system.
The expansion vent on my central heating expansion tank is constantly dumping hot water when the heating/hot water is on.
I found gunk in the pump cleaned that out.
I have put cleaner through the whole system.
All radiators etc.. work fine.

This overflow is causing penatrative damp in one of my walls.

Does anybody know the cause/solution?

The expansion tank is designed to take up the enlarged volume of water when it heats up, so there are a few things that could be causing it, here are a few for you to check:-

• The ball valve is letting by
  The ball valve is incorrectly positioned/adjusted so that there is too much water in the tank when cold
  The pump is set at too higher a speed
  The open vent is on the wrong side of the pump and is pumping over
  The heating coil in the HWC is leaking
  The high limit stat on the boiler is malfunctioning

Artfuldodger,

I have checked all of the above and none of them seem to be a problem.

The heating system has worked fine for several years.

Before I double check these, have you any other suggestions.

Had the same problem when the pump was on any higher setting than 1, even then I got steam coming out of the expansion pipe into the tank, condensing in the cooler overflow pipe causing it to constantly drip. Have now fixed it and can run pump upto speed 3 without overpumping, 2 fixes but both addressing the same problem;
1) Water level in expansion tank was too high, re-adjusted the float to put water level just above the low pipe feed at the bottom of the tank (because this is an open system, the level of water in the expansion pipe will be the same as that in the tank, therefore, if it is high, it will take very little expansion of the water in the pipe for it to flow into the tank.
2) Expansion pipe into tank was too low, only just about 6 inches above tank before bending through 180 degrees into tank. Same as point 1 above, very little expansion still required before water flowed into tank. Increased the height of this 'bend' so that it is approx 3' above tank before the 180 turn back down to tank.

Sorted it for me, hope it is of some help....

Have you checked and double checked the Ball Cock isn't leaking. If i were you i would ask someone to double check it just for peace of mind. Maybe a neighbour! Other than the above it most likely will be the sludge in the system and if you give your chemicals time to work i'm sure all will be ok.

I lowered the water level in my expansion tank.

Problem solved!

Cheers

Whoooooooohoooooooooooo, dohhhhh.

i have just moved into a house with an open vented system hotwater /heating..

the heating works fine up to 20 degrees, if i want it hotter steam and hot water over flow into the expansion tank.
along with this i get noises from the pump as if it has run dry.

all radiators get hot, even so i have bled them with the system off..

one odd thing is that bend in the overflow pipe is touching the top of the expansion tank. to my thinking the pipe doesn't rise above the tank enough. but would this cause the pump to run dry?
Help