Worcester 24i combi - DHW but no heating...unless DHW on 1st

[diabologuy]

I can only get the central heating to work if the DHW has been running first to get the gas to flow and ignite, when the water is turned off the boiler stays alight and the radiators heat up.

I figure this rules out the programmer (timer on the boiler) as I can also use this to turn the heating off once it's on.

Rules out the thermostat - no roomstat and the stat on the boiler is turned up to max otherwise the CH wouldn't work at all...?

There's 240V going into the gas valve for DHW and stays like that when the CH is set to on when DHW turned off again, but no voltage seen at the gas valve when I try to get CH by itself. But the fan is going.

Does this point to the pcb or is there something else I can try?

Started a couple of days ago, no changes made to any of the system.

 

[diabologuy]

Looking through other posts I'm wonering if this can be caused by the CH thermistor?

But wouldn't that stop any CH at all?

 

[Agile]

If the CH thermistor is o/c then it makes the red light flash.

Why dont you measure the resistance of both sensors?

Tony

 

[diabologuy]

Engineer reckoned thermistors were ok and swapped out the pcb and this seems to have sloved it...

What I didn't realise was that you have to wait 4 minutes after the heating has beeen switched off before it will fire up again...the engineer said he'd checked that before changing the pcb.