Expansion Tank

[John]

My combi is upstairs and is an open vented installation. Recently the expansion tank has been overflowing (through the warning pipe) .

I think it is due to the fact that when the water expands it obviously fills the tank - when it contracts it allows more water in. Then the next time the heating goes on it expands and overflows.

The tank is very small and I think this is the problem. However, space is a bit limited due to all the cross timbers. Are there any other solutions - I am even thinking of converting it to a closed system (it is a Worcester Heatslave) but I am a bit reluctant in case the 12 year old systems starts to leak if the pressure is higher. I guess I would have to plumb in a pressure release vent pipe to the outside wall.

[Dave Liquorice]

But it would have always done that expansion/contraction thing so would have always overflowed. You say this a recent...

Sure it's not just the ball valve sticking due to a build up of scale, a slow leak due to grit or a failing washer?

I take it that you don't have a hotwater cylinder as you state "combi" another good cause of an overflowing expansion tank is a leak in the coil in the cylinder. The level of water in the CW storage tank is normally higher than that in the expansion tank, water always tries to find a level.

[John]

Thanks - I did replace the ball valve diaphragm on Sunday. I reckon that I need to have the tank almost empty to allow for the expansion that takes place. I will monitor it over the next few days. I don't have a tank.