Diagnostic diversion – Parts Cannon fail

I don’t usually suffer failure when diagnosing faults, but last week was an utter disaster.

I’d had an odd problem in recent weeks, as one fuse blew, followed by random triggering of the breaker feeding my boiler ‘house’. I couldn’t do any fault-finding as things were always fine when I restored power.

Thankfully, this became repeatable when the breaker would open shortly after power was applied to the circuit, although it was still not a ‘hard’ fault, taking a variable, but at least short, time before the breaker opened.

For reference, nothing seemed odd when measured.

I set things up so I could be beside the boiler when the power was interrupted, and could hear something arcing just before the breaker operated.

There’s very little to fail – some wiring, a thermostat, and the boiler control valve.

The sound appeared to be coming from the control valve, which has a single solenoid, sealed inside the body – which seemed to explain why I could hear the arcing, but not see anything.

Anticipating a simple fix, I ordered a new control valve, cursed not being able to buy just the solenoid (bolted to the valve body and with a part number), and fitted it.

Seems that was a complete and total ‘Parts Cannon Fail’, and as soon as I applied power – the arcing was STILL present.

Arc mystery

This was bad news, apart from the loss of £130 for a valve I didn’t need, and couldn’t return (having installed/used), I had an obvious arc I couldn’t see, even in the pitch darkness of a closed room.

There was just no evidence to be seen, although I could see all the wiring and parts, and the sound STILL appeared to becoming from the area of the valve.

After a bout of disconnecting wires one at a time, to find out when the arcing sound appeared and disappeared, I eventually tracked down the source.

The arcing was originating inside a connection box mounted BEHIND the boiler, and was being funnelled along the space between the boiler’s casing and the boiler itself, making it appear to come from the control valve mounted on the front.

The freezing cold and damp weather of the past few weeks had resulted in the connection box filling with condensation, which couldn’t escape and eventually worked its way into a cable termination, where two wires exit from the termination very close – usually separated/sealed by a gooey mastic. However, in this case, the moisture had worked its way along the termination and mastic, on the left in the pic below.

I would have been mystified as to the source of the moisture, but for having the process explained previously. It seems that moisture can make its way into such boxes, but not escape in the same way, so builds up over time, until it condenses and causes problems.

The arcing just led to more arcing once it had started, Once I’d removed the box cover, I could see the arcing inside the termination. By the time I was finished, it had managed to erode the wire, and sever the connection.

Worst thing was, this wasn’t even a necessary circuit, and just lead to a light (not even used).

After drying out the interior of the box, and disconnecting the rest of the offending wiring (not needed anyway), everything was fine.

Apart, that is, from the loss of £130 fired from the Parts Cannon 😦

This one’s doubly irritating, as I restored this valve unit some years ago, when it would normally have had to be replaced after one of the control buttons seized completely.

Although it seemed to be a complete failure, I decided to strip it down before getting a new one, and found all that happened was the gunk applied in the factory, to lubricate the button’s actuator shaft, had set solid, locking it in place.

Cleaning this off, and applying new lubricant restored the button’s operation, and it remains perfect to this day.

I suppose the only positive from this waste of money is that I now have a perfect spare on hand, if the new valve ever fails.


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