Can anyone tell me why I would have two HW valves fitted in series to my system? I was having a problem with the pump running intermittently and for no apparent reason after the system had satisfied its heat demands (not overrun, we are talking of an hour or more). The pump would run on occasionally even after the system had switched off completely. Things came to a head the other day when it came on and would not go off. I eventually traced the fault to a second H/W actuator valve fitted adjacent to the hot water tank. The motor had burnt out and as the water heated up it appeared to make the contacts close and cause the pump to run. I removed the head and the system has worked fine ever since. But why would this second valve have been installed in the first place? The actuator that is U/S is a SUNVIC SZ2301F. The other two are standard Honeywell. Do I need to replace the SUNVIC as everything appears to be running OK (apart from the expansion vessel problem!)