Dripping Overflow From CH Expansion Tank

[John Evans]

Hi

This has probably been discussed before. If so I will be grateful for a pointer to the information.

Over a period of a few weeks a partly full CH expansion tank reaches overflow outlet level and then constanly drips, but only when the boiler is running.

This is not a leaking float valve as I've turned off the water to the float valve and kept a constant check on the system - it still slowly fills up. It isn't likely to be a leaking heat exchanger coil in the domestic hot water storage tank as this is only a few years old.

Is this likely to be air entering the system at some point? Is there a recommended size of expansion tank for a given size of heating installation?

Any advice on how to pinpoint the problem will be appreciated.

John Evans

[Roger Mills]

The most likely cause is a leaking coil in the DHW cylinder - particularly if the water level in the cold header tank is higher than that in the CH F&E tank.

Otherwise, you may just be running the level in the F&E tank too high - so that there's insufficient room for expansion before the overflow level is reached. There only needs to be a couple of inches of water in the tank when the system is cold - just enough to cover the outlet which feeds the system.

[John Evans]

The DHW cylinder is only a few years old so I don't suspect that and I have virtually emptied the F&E tank but it refills over a period of days. Thje legth of time depends on wether the CH is on or not. So I would think it was something more unusual. (The CH boiler is newish but there was signs of the problem before it was replaced).

[Roger Mills]

The fact that the DHW cylinder is only a few (how many?) years old isn't

*necessarily* a valid reason for discounting it. There are not that many possibilities!

How certain are you that the ballcock isn't leaking. I think you said that you've turned off the supply. How confident are you that the tap actually works?

I suppose that if air (or, more likely, hydrogen) were being generated, that could displace some of the water into the F&E tank raising the level. But the 'air' would collect somewhere. How often do you bleed the radiators? Does anything other than water come out? If it does, by how much does the level in the F&E tank go down after a bleeding (assuming that the ballcock is turned off)?

[TheScullster]

"John Evans" wrote

Very long shot on a system with F&E tank, but is there another fill point somewhere with a valve (which is passing). If not, then the DHW cylinder is the likely culprit I'm afraid whether 1 or

100 years old. Air entering the system would not cause the level in the tank to rise, as once it reaches the tank it has effectively escaped.

Phil

Phil

[Andrew Gabriel]

There's a clue in the subject line -- it's an expansion tank. When it fills to the ball-valve shut-off level, how much clearance is there between the water level and the overflow, to allow the central heating water to expand into the tank? The ball-valve shut-off level can be quite low in the tank, but there must be space above that to allow the tank to take the expanding water volume as the heating system gets to max temperature, without it getting to the overflow.

If this is the problem, then it will be expanding past the overflow level (except it can't as the excess water is lost), and then when it cools back down, due to the lost water it will cause the ballvalve to top it up again. (I bit like a sealed system with a failed expansion vessel.) In this case, adjust the ballvalve to fill to a lower level, and add new inhibitor as it will have been lost over time.