Central Heating switching off early...

[Subdelirious]

I have a normal gravity fed system with a boiler downstairs, a thermostat in the hall and thermostat valve control to all but one radiator. I also have zone control valves next to the tank although I can't work out what they do.

My boiler heats the water well and when the temperature in the tank reaches the temperature selected on the thermostat, fixed to the outside of the tank, the water heating AND central heating switches off even though the house hasn't warmed up properly.
The central heating won't turn back on until the temperature has dropped in the tank. It doesn't matter what temperature is set on the thermostat in the hall.
The only way, it seems, of starting the central heating is to run some hot water to lower the temperature in the tank.
What's going on? How can I sort this out?

 

[gasandoilman]

Is it a 2 port valve or a 3 port valve
If you want to get your self out of trouble tempory until an engineer comes you can take the cylinder stat off the cylinder and heating will stay on
Turn your boiler stat down a little and stay warm

 

[AlanE]

Start by checking if the wall stat is turning on the pump. Remember there may be a motorised valve that is operated by the stat and this in turn operates the pump & boiler.

 

[Subdelirious]

There are 2 valves, both two port. One feeds into the tank and the other is behind the pump feeding the radiators.

It seems that the only way to control the pump is through the stat on the tank. Turning that up turns the pump on and turning it down turns it off.
I don't think the wall stat is controlling anything.
With the pump working I lowered the wall stat and the pump didn't stop.

 

[gasandoilman]

sounds like a wiring fault ?
Have you had anything done to the system and the wires put in the wrong place?
Is you room stat working? get a multi meter a check theres a circit

 

[namsag]

It is the heating valve worn and not motoring over enough to make the microswitch inside to bring heating on or it is jammed in open position which is why it comes on with HW, nothing to do with your wiring

 

[Subdelirious]

I don't think it's an electrical problem. The wall stat was removed for decorating a while back but it worked okay for a while after.

It sounds like I need help if it is the valve, not something I can sort out.