Slightly unusual one this. Got a call for help from a friend to say the heating in his house is not working correctly.
Its a conventional vented Y plan type system, Ideal Mexico 2 RS80 boiler. Symptoms were rads not getting very hot.
I checked out all the controls and they seem in order. Room stat calls for heat, 3 port valve feeds call onto the boiler, and the boiler fires in response to it. However not for long. After a few mins the boiler cycles off as if its being disabled by its internal stat. It stays off for 10 mins or so before firing for a bit and then shutting down again.
My first thought was the boiler stat must be cutting in too soon or there is a lack of water flow causing overheat. However the pump seems to be running (max speed) - and the pipe temperature at the pump soon falls as the boiler cycles off.
Looking at stat its a bulb type sensor pushed into a pocket at the top of the HE, and that feeds into a control with the temperature knob on it. It feeds mains to the gas valve. I presume when it reaches it limit it would cut the power supply to the gas valve. However that is not happening. When the boiler cycles off, there is still mains present at the main gas valve.
Now to my mind, that should be enough to cause it to fire. However is there some other input to the gas valve that can inhibit it even when its got volts on its input? Or do you think the gas valve is doing something daft when it warms up?