Hi, I've got a problem with a hot water expansion tank overflowing at
a property I let out. Its a 5 yr old GCH gravity system i.e. the type
that must heat the hot water when the CH is on. The tank that is
overflowing is the large hot water expansion tank in the loft not the
small C/H tank.

Water is not entering the expansion tank via the pipe that hooks over
the top (tested by putting paper over it), it looks like it is coming
back up from the hot water tank itself. The problem often happens
overnight so I have changed the ball-cock in case the increase in
water pressure was making it leak. My plumber suggested that the
thermostat on the boiler was probably overheating the water in the
tank causing it to overheat and expand too much. I have changed the
boiler thermostat and set the boiler to the lowest setting, still no
change.

The cylinder also has a thermostat strapped around the outside, I have
tested this by turning it down and checking that it cuts out the
boiler - it does.

My other thought was that the coil in the hot water tank had a hole in
it but I don't think it does. I have tested this by cutting off the
water supply to the C/H overflow tank (holding the ball cock up) and
the level does not drop over a couple of days.

I have set the thermostat strapped to the tank down to about 55
degrees and the hot water does not seem to get very hot, but still
when I look in the expansion tank I can see hot water coming up from
the hot tank. I am convinced that the hot water tank is not
over-heating.

My only theory left is of someone elses thought (an old posting on
this newsgroup) that it could be down to a mixer tap problem ... I
installed a thermostatic mixer tap in the bathroom a couple of years
ago & are wondering if that might be causing the problem. There is
also a mixer tap on the kitchen sink, just a basic one. But as the
problem often manifests itself at night i.e. when nobody is using the
shower, I am not certain this is the problem.

I'm not a trained plumber, just somebody using my own bits of limited
knowledge and logic. Have I missed something, before I start ripping
out expensive thermostatic mixer taps? If you do suspect the
thermostatic mixer tap, has anyone any idea how I can test it easily
(not easy to isolate it for 24 hours as the tenants use the shower
often).

Many thanks for reading this,

Peter