Boiler/ HW fine, but heating not starting up? Clock, t/stat or something else?

[DP]

Taking stock of what the OP has posted, my money will be failure of component shown in the picture below

240 volts from your thermostat is reduced and conditioned to light an LED in the device. Remaining two terminals are shorted when the led lights to thermostat operation. For positive diagnostics, need a multimeter. Have replaced above device in the past to get boiler working.

 

[Fez111]

could be the timer itself, it will have a L&N and two switching wires, join the two switching wires together same as you have done with the stat, now you have ruled out all heating controls so if it doesnt come on its a boiler fault

Thanks Ian, I have now checked with multimeter. No reading at all at the stat (would explain why shorting at the stat didn't change anything). But 240 at both the terminal block/pcb and also at the timer/clock.

If i understood you correctly - I should try shorting the two black switch wires here and that would eliminate if it is the timer? cheers

 

[Fez111]

Taking stock of what the OP has posted, my money will be failure of component shown in the picture below

View attachment 250303

240 volts from your thermostat is reduced and conditioned to light an LED in the device. Remaining two terminals are shorted when the led lights to thermostat operation. For positive diagnostics, need a multimeter. Have replaced above device in the past to get boiler working.

Thanks DP, What is this part?
If its relevant, I have 240 as normal at both the terminal block/pcb and also at the timer, but zero reading at the stat.

 

[CBW]

Thanks Ian, I have now checked with multimeter. No reading at all at the stat (would explain why shorting at the stat didn't change anything). But 240 at both the terminal block/pcb and also at the timer/clock.

If this was at the thermostat end, try linking out at the pcb end, if it fires up, you have found your fault.

 

[DP]

As per above diagram, one meter lead on N, other at Black 2, you should see 240 volts, again 240 N to black 1
If that is the case, then new circuit board time.

Going across the two wires at the thermostat will show nothing. If temperature is low on the dial, you will see some voltage, not if calling for heat

With power off, put a link across bk 1 and bk2. If boiler works, thermostat faulty. If no joy, short the two terminals at the clock NOT live and neutral. If still no joy for heating, new pcb time.

The component in my picture is called an optoisolator. 240 volts or whatever the stat voltage is, is reduced, rectified and smoothing carried out, applied to two terminals. In the package, these two terminals light up an led. Led illuminates a semiconductor switch which then turns on and tells the electronics that thermostat or whatever is calling