"Peter Cherry" wrote in message
om...
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.

How does this work? Gravity fed CH systems have no stat on the tank. There
is nothing for them to connect to. Are you sure there are no valves?

Adam