In an earlier contribution to this discussion,
Scott wrote:

Ok - been in my house a year and its got a new Potterton boiler, with
std hot water tank, with expansion tank in the loft. I have always
had a leaking over flow but if was pushed to the back of my mind
while I ahd an extension done.

Anyway - know its doing my head in.

I ahve checked its not the any corroded parts n the expansioin tank,
causing water to be filled up even though the counter balance is down.

What seems to be happending taking taoday as an example. Not central
heating on - and we have on timer - several hours in morning and
slightly more at night.
So today its just hot water being timed on - and hence am getting
overflow, upthrough the expansion tank and hence into overflow and
then coming out the pipe at the side of my house.

What has caused this - air in the system?

Advice please so I can cur ir

Scott

I assume that it's the feed and expansion tank which is overflowing? Is the
level in the main cold header tank (for the hot water system) higher than
that in the f&e tank? If so, as others have suggested, the likely culprit is
a leak in the internal coil in your hot water cylinder - allowing the
primary and secondary water to mix, and forcing secondary water, at slightly
higher pressure than primary, into the primary circuit. If this *is* is
problem, it means replacing the hot water cylinder.

As someone else has said, it could *just* be caused by the ballvalve in the
f&e tank being set too high - so that when the system heats up and expands
it overflows - and when it cools down and contracts, fresh cold water is let
in by the ballvalve. This is the first thing to check, but is probably *not*
the problem, unfortunately.
--
Cheers,
Set Square
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