Troubleshooting: Worcester CDi boiler power supply

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Hello all, this seems an excellent forum - I've been through lots of threads to try to find an answer to my problem with no success so I hope someone can help.

My Worcester 28CDi RSF combi boiler stopped working this week. I noticed the pressure had dropped to 0 (but the power to the controls was fine). I turned the unit off at the wall, then refilled it with the key to the correct level (1). When I switched it on again, the controls were dead.
So here my troubleshooting so far:
Fuses on mains board tested: fine.
Fuses on circuit board tested: fine.
Tested voltage to power supply into the boiler and it's 4V - strange because it's low, rather than being nil.
The switch at the wall is just a switch - no plug, no extractable fuse, just the wire going into the unit. So I tried to take the whole switch unit off the wall but it's been cemented in by a tiler!

Can anyone please suggest what I should do next? It seems that the problem is not the boiler but the elec supply to it, which has somehow been damaged by me turning the switch off then on again. Has anyone encountered this problem before? Do I need to tunnel into the wall to remove and perhaps replace the switch unit?
Any advice v much appreciated.
 
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HI


4v is acceptable it does not have to zero all the time.should be 240v at the boiler.if less than this then the fault is external.check all external fuses again.
 
Hi LUNAT, yes it's 4V across the connections into the boiler. Very odd as would expect it to be zero if fuses gone in external circuits.
Have checked all external fuses again - all fine (and all other appliances in kitchen fine - eg: oven) but can't access the boiler's external switch unit to check if there's a fuse in that because it's been cemented into wall.
What's next to check? Or should I get an electrician in to prise the box off the wall and check?
Thanks v much.
 
To be sure you need to measure between the permanent live and earth, and the neutral and earth. Any pipe or extraneous metal should do. Then measure between L and N.

You could be picking up 4v on induction from other circuits for instance the thermostat wiring BUT

Always check a multimeter on something you know is live before making any rash assumptions.

Does the water pressure switch on the 28CDi interrupt the mains supply, anyone? I can't remember......
 
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Hi SimonD, yes there is a cut-off if the pressure drops too low, which it had when I realised I needed to fill it up. However, the mains was still going to the control unit (clock etc and the LEDs were lit as normal). It was only after switching it off at the wall to fill it, then switching back on, that it stopped supplying the control unit.

Hmm, unlikely to be induction current because power to boiler *and* control unit seems dead. I did wonder about the meter so tested it with the socket I use for my kettle! Worked fine...
 
Hi spacethegas, what do you mean by spur? Yes it's the incoming lx which seems to be the problem. (I'm a she not a he!)
It sounds like I'd better ask an electrician for help...
Will post reply here about the solution. Thanx v much all.
 

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