Central heating boiler doesn't close down when it should

[Redheadpeter]

I have this intermittent problem with my central heating boiler (Potterton Suprima 120). It runs fine and heats water and rads but when all controllers turn off and the boiler should shut down showing only a constant red light sometimes (more times than not) the boiler doesn't shut down and sits there cycling through its own run/stop sequence determined (I imagine) by its internal thermostat.

Maintenance engineer came round and said there must be a short because the switched live had current even when all controllers were shut down. We have 2 radiator zones plus water so controllers are boiler stat, 1 combined controller and thermostat for upstairs heating, separate controler and thermostat for downstairs heating and hot water time.

Engineer said I must have knocked a nail through a wire! Not possible given where wires run.

Sometimes the boiler does shut down properly - generally when I have changed settings on controller for downstairs heat and hot water and turned heat on that controller to OFF.

I've studied wiring diagrams until my head spins - and I don't think it is a short (installation was professionally done about 6 years ago).

Does anyone have any other thoughts on what might be causing this?

 

[Lectrician]

Usually a zone valve sticking.

If it is an S Plan, you will have two zone valves, one for HW and one for CH (possibly more if the heating is zoned). These have microswitches inside which fire the boiler. These microswitches can get stuck when they get crudded up - usually due to a leak or greening etc.

 

[Redheadpeter]

Now that's interesting - because when the engineer called we had another fault with the downstairs rads getting hot when they weren't supposed to. He took the motor off the zone valve and turned the tap back and forth a few times manually, but said he would order a new motor so we will see if that does the trick.
Thanks for your help.

 

[gas4you]

get another engineer

You can check which actuator head is faulty by checking which orange wire is supplying the 240V to the wiring centre.

 

[Redheadpeter]

Thanks to everyone for help. Once new actuator arrived it took me about half an hour to work out which of the existing ones was still passing current and then to change it (awful lot of wires in that wiring centre!). But now seems to working the way it should. I'll wait for the service engineer to return and gently point out the error of his ways!

 

[Diyisfun]

Thanks to everyone for help. Once new actuator arrived it took me about half an hour to work out which of the existing ones was still passing current and then to change it (awful lot of wires in that wiring centre!). But now seems to working the way it should. I'll wait for the service engineer to return and gently point out the error of his ways!

And ask for your money back