Odd Boiler Problem

[Syd Rumpo]

This is a Baxi Megaflow boiler running a pressurised hot water system.

This morning, when I turned the heating on, the timer/programmer started switching off and then back on every few seconds. Even the display backlight was going off. The hot water was on before I turned the heating on, and turning the hot water off on the programmer allowed the system to function normally for the heating. After a while, I turned the hot water back on and all seemed to be ok.

However, it started again this evening. Same symptoms, the timer was cycling off and on every few seconds. I turned the hot water off and again the heating works normally.

Pressure is ok. I'm baffled, especially by the backlight on the programmer going off and on. The system is about four years old and has always worked well.

Any ideas?

Cheers

[Andrew Gabriel]

May have partially lost its supply (disconnected neutral?) which is being backfed or substituted by one of the other connections, providing that connection is not turned on.

[John Rumm]

I have seen a Honeywell programmer do something similar - turned out to be a failed electrolytic in its (very crude) mains PSU. THis particular one could cope with either output calling for heat in isolation, but not both at once.

[Andrew Mawson]

I had exactly those symptoms recently with a Sangamo 3 channel programmer when it had been installed just under three years. I rang them up and they replaced it free of charge on a next day delivery - very impressive service - didn't even demand proof of age. From their reaction I suspect it was a known issue.

(tech speak: I suspect an internal capacitor had gone 'dry' in the power supply circuit. When it had the additional load of switching the relay the volts dropped causing a reset, whereupon it started the cycle again)

Andrew

[Syd Rumpo]

I looked at the boiler control board as Rod had suggested and others from the web, but it all looked good, in fact much better than I was expecting. The last time I saw one of these it was a filthy crappy bonded paper PCB with poorly soldered through-hole parts, this one looked clean and modern.

So I Googled the timer which is a Honeywell ST9400C and saw that there had been problems with a batch of these caused by faulty capacitors. Mine was not in the range of serial numbers given, but I had a look anyway.

The power supply to the uC is from the mains, I'm guessing through a dropper capacitor, into what looks like a bridge then to two paralleled capacitors. These are aluminium electrolytic SM parts, about 6mm diameter by 8mm high. I was unable to positively identify the values from the markings, but they both measured at less than 10nF - clearly wrong.

The markings did include '22' and '63E', so I guessed at 22uF 63V which would about fit with the size. None of those around, of course, so I soldered in an ancient 47uF 63V axial on flying leads as there was insufficient headroom at the existing location.

And viola, a large fiddle!

Thanks all for the help.

Cheers

[Syd Rumpo]

Thanks, John, that was exactly the problem.

Cheers

[g3yjr]

replying to Syd Rumpo, g3yjr wrote: I had a problem with the series dropper capacitor & I replaced the elecrolytics too:

[Brian Gaff]

2015? Brian