CH expansion tank overflowing

10 Jan 2013
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United Kingdom
Hello. I have an old backboiler with the typical tanks in the loft (HW and CH) and a cylinder in the bedroom (vertically between the boiler and the header tanks)

The CH expansion tank is continually overflowing:

> water level is at the overflow point
> a steady drip out of the plastic overflow pipe emerging from the house.
> if bailed out, water levels rise again after a couple of hours

This new water is NOT coming from the cold feed to the CH expansion tank; if I isolate the cold feed, the water still rises.

If I drain the HW header tank until it is below the level of the CH expansion tank (and isolate the cold feed to the HW header tank, of course), the CH expansion tank does not refill/overflow. When I allow the HW header tank to refill, after a little while the CH expansion tank also rises.

Does this mean, conclusively, that the coil is leaking in the cylinder?

After a long time (maybe a year?) of the whole system being isolated at most junctures, when I reopened everything the problem did NOT occur for a whole winter. Then, after another period of isolation, it DID occur again with several days of re-commissioning.

It will set me back about £3500 to put in a combi boiler, due to a lack of suitable locations and absence of plumbing in those areas, etc. I'll prefer to keep the old system as it is if possible, since I really don't use it heating and hot water very much anyway.

Thanks for any suggestions.
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