Boiler only fires when the hot water is on Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[JAmesclow27]

I have recently had the MV on the CH replaced. I have replaced the controller with a spare, the controller works. When I switch the CH on, I can hear the MV click and can hear water moving in the pipes, but the boiler does not fire. If I then switch the HW on, the boiler fires and the radiators get hot (v hot), there is no problem with Hot water. The arm on the CH MV does not move when I switch the CH on, it feels loose and there is no resistance when I move it manually. The arm is on the right hand side of the MV.

I think the CH MV is permanently on (siezed open)? Is it the "opening" of the valve that fires the boiler?

[DEEARR2]

Depending on how the system is wired, it is usual to signal the boiler from the MV micro switches. Judging by your description of the CH MV the action sounds as though the valve is faulty. Also check the wiring at the MV, should have 5 core cable. 2 driving the valve motor and 2 signalling the boiler and an earth.

[Whitcop]

When a MV opens or closes it operates a micro switch. This switch closes when the valve is open. The pump and boiler ignigition are connecetd in parallel via this switch so that the pump starts at the same time as the boiler calls for heat. It sounds like you missed a wire off somewhere.

I hope this helps

Whitcop