Vaillant combit - PCB blown ?

[Jethro_uk]

(I wouldn't mind, but I took the advice here to leave everything alone ...)

Had to drop the cover on the boiler today, as demand hot water seemed to be iffy, and last time this happened, the diverter valve needed changing.

I can't be sure, but as I opened the cover there may have been a slight "pfft". Or there may not. Either way, the display is now blank, and the boiler is acting dead.

I have removed and replaced every connector, and checked the 2 small glass fuses in on the PCB. but they are fine.

Seems I can source a recon PCB for £37. However, do PCBs just pop like that ? I guess there's always vibration and heat/cold to stress things.

Absent any test kit, would the balance of probabilities be the PCB ?

I even removed and opened the LED panel.

I've got 24 hours to decide before I order the part.

Thank god (a) it's not the depths of winter and (b) we have a gas fire and a SuperSER if SWMBO gets nippy (as I piss off to work).

Hot water might have to come from the electric shower for a while ....

[Jethro_uk]

UPDATE:

after "Testing the tester", it seemed the fuse to the spur had blown. (My "magic" screwdriver fuse-tester/mains-tester must be ****ed).

HOWEVER - the reason is has blown is when the boiler is powering up,

*something* is going short circuit - and popping the fuse (and this time tripping the MCB in the fusebox). It happens after about 30s of being powered up.

I've been lucky twice now (and run out of fuses).

Currently the boiler is powering up (so presumably PCB isn't that borked). But I daren't keep trying.

Presumably a valve or something has gone short circuit ? Are there any likely candidates ?

Curious as to how the two small glass fuses on the PCB managed to stay unblown while a 3A fuse went first.

[Harry Bloomfield, Esq.]

Jethro_uk has brought this to us :

Is the circulation pump fed from the boiler, if so, try disconnecting that.

[Dave Plowman (News]

Anti surge types? Or, of course, not protecting everything.

On my boiler, as well as obvious fuses, there are also PCB mount ones - those small round brown things which are soldered in place. Bet there's not many boiler repair men that carry spares for those, and a soldering iron. Just charge the customer 400 quid for a new PCB.

[Brian Gaff (Sofa]

Depends where the fuses are in the circuit I'd suspect. Brian

[Andrew]

Geoff will probably know -

[ARW]

When doing an EICR of his offices and workshop I watched him working on a newish to the market boiler that is well known for PCB fails. He reverse engineered the PCB in a couple of hours.

And yes he will probably know if it is a faulty fan etc that powers up

30 seconds after power is applied that causes the fuse to blow.

[Harry Bloomfield, Esq.]

Jethro_uk explained on 20/11/2021 :

Sensible thing to do, is to disconnect all mains powered items on the poiler circuit, apart from the boiler, then add them back one at a time, restoring power each time - to see what takes the fuse out. Always assuming you have no better method of testing.

[Tim+]

A dying/seized fan bearing sounds very plausible as a cause for a fuse blowing shortly after power-up. Admittedly they usually just get noisy rather then seizing though.

Tim

[Dave Plowman (News]

Just put in a larger fuse and see where the smoke comes from. ;-)

[ARW]

Yes...

Or you could try to power the fan/pump on it's own and not try to f*ck up the PCB whilst testing.

I have a bit of a bastard to sort out with some UFH and I suspect one of the zone valves goes short circuit when operated.

It will have to be done one by one.

[Jethro_uk]

Many years ago we had a car come to us that started blowing a sidelights fuse every week or so. No amount of trying to trace the wiring through the car from fusebox to lights assembly (including swapping bulbs, assemblies and fusebox) located the fault.

In the end, with the car worth £500 it was scrapped.

If it wasn't for the safety aspects, I would have put a penny in the fusebox and seen where the smoke came from.

[Jethro_uk]

2nd update.

Fucking diverter valve was borked (note: I replaced it with a brand new one about 5 or 6 years ago).

It's very slight, but there is a faint smell and the faintest traces of moisture on the back of the PCB covering (can you see where this is going). And just enough must have landed where it wasn't needed to short the mains.

Removing and cleaning *everything* and replacing and the boiler is "working" now (I should buy a lottery ticket). With the exception of the LCD display which has now gone a bit Chinesy ?

And of course the (now visibly) weeping diverter valve. Which is on order.

SWMBO is unbearably smug now, as they are saying "I said the how water wasn't right a week ago" ...

I guess this is natures way of reminding me to renew the inhibitor ....

[Tim+]

Glad you?ve tracked the problem!

Tim