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-c*ck 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 c*ck 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