Central Heating Staying on Full

[stem]

If the 'Boiler On' light is not lit when the problem occurs, that suggests that the thermostats and motorised valves are working and turning off the control signal to run the boiler.

If you want to be absolutely sure, as a test, you could install a switch in the wire going into the boiler 'switched live' terminal (the far right terminal next to the thermostat knob) leave the switch in the 'on' position and run the boiler as normal. When the fault occurs, operate the switch. Does the boiler stop? if so, it is something outside of the boiler that is causing the problem (thermostat or motorised valve). If it continues to run, something inside the boiler is at fault. After that, it may take someone who has a good working knowledge of circuits and a multimeter to find the actual fault, but this should narrow it down a bit.

 

[picasso]

One idea is to stop throwing parts at your boiler which thermistor did you change ? honeywell 3 port valves can have 100v on the switched live with no call for heat, with some boilers (namely worcester bosch) this can be enough to bring the boiler on, google capacitor/honeywell 3 port valve .

 

[Agile]

I would do something a little easier to manage.

That is putting a 230v neon onto the call for heat contact.

That will enable anyone like the au pair to just look at it and report it.

But if the PCB has been changed I cannot think of anything in the boiler apart from temp sensors which could cause the problem.

A way to monitor a sensor in situ is to measure the voltage across it. This will change as the resistance varies because of the way those circuits are designed.

Tony

 

[stem]

If the fault was internally generated and fed back to the call for heat contact, then the new neon light would stay on and prove nothing.

 

[JF77]

Thanks for all the help.

I changed the thermistor (thermostat sensor) I got a Honeywell replacement.
If I disconnect the thermostat sensor would the boiler still fire?
I will check the resistance with the meter.
The boiler is in the garage so doesn't get that cold.

The 3 way valve is a Drayton.

As for changing all the parts it's getting like Triggers broom! The only thing left to change is the overheat sensor and it can't be that!

 

[Agile]

If the fault was internally generated and fed back to the call for heat contact, then the new neon light would stay on and prove nothing.

Well that is quite true.

However, the circuit that detects the mains voltage is a low voltage arrangement and in itself not capable of producing 230v to feed back to the SL terminal.

 

[JF77]

Winter is coming.....so the heating is on and ......same problem!