Central Heating only works when Hot Water is ON

[AllenDee]

Hi,
I have a gravity fed system with 2 separate valves for hot water and central heating. Here's what's happening, and I'm really scratching my head here:
The only way to make the central heating work:
- Switch off the central heating from the controller.
- Bypass the central heating valve and lock it in
- Switch on the heating
- Pump starts
- boiler works, hot water + all radiators heat up - until the hotwater tank reaches it's max temperature.
If you don't switch off the central heating from the controller, then after few minutes the mororised valve unlocks the valve, returns to close and switches the boiler off.


If you use it as normal, and switch ON the central heating, the central heating valve motor opens the valve, boiler starts up, after 5 minutes or so, the boiler turns off, and the pump stops.
I just changed the circuit board with a new one - same thing.

HELP Please !!

 

[Madrab]

Sounds like you have a faulty CH 2 port valve

The way the system works -

1) programmer calls for heat - CH or HW or both - send a switched live out to either or both thermostats
2) the room stat or cylinder stat isn't satisfied so they pass through that switched live from the programmer to the live on the respective 2 ports valve
3) the valve opens and hits a microswitch - the microswitch then sends another switched live to the boiler and turns it on

Usually when a 2 port valve is manually locked then it doesn't activate the microswitch. When it's manually locked though and the HW is activated, the HW 2 port will fire up the boiler and the rads will heat until the cylinder stat shuts it all down.

If you know your way about mains electricity (240V), a circuit tester and are comfortable with mains testing then I'd be checking the Live + SL (orange) to and from the CH valve and out to the boiler when it shuts down.