Radiators not turning off and overflow leak

[shoot66]

I have a central heating system with a separate zone for upstairs and downstairs each controlled by its own wall thermostat. I have a Ultrasteel 150L indirect mains pressure hot water tank with an imit dual thermosat on the side. I have 2 Drayton 2 port zone valves and these don't seem to be working properly.

The hot water is fine and through the summer we have just heated the water and there have been no leaks or problems. When I put on the radiators they are not turning off even when the wall thermostat clicks off. When it continues the pressure release overflow leaks. If I click down the hot water tank thermostat to a lower temperature both the zone valves click in and then the radiators turn off.

I was wondering whether I need a new tank thermostat or whether it's the electrics that's faulty. I thought it might be the zone valves at first but they click in and work when I turn down the tank thermostat.

Any help and advice would be appreciated. Thanks in advance.

 

[twgas]

I would think you have a wiring issue - has any work been done on system or is it a new (to you) system.

 

[shoot66]

The system has been in for over 3 years. It was leaking most of last winter but was working before that so that makes me think that the electrics must be OK or it would have been leaking from the beginning? Nothing has been added since.