Flame failure electrode question

Any evidence of insulation breakdown or damage to the lead that connects the flame detector? (Just floating an idea).

I wonder if it would be worth opening up the main control box and looking for swollen/leaking capacitors (assuming it’s not potted)? Don’t think you have much to lose.

Tim

Could it be the body of the component is the electrical return path and there is corrosion on a screw thread on the fitting?

A flame sensor relies on conduction in the flame. The burner is grounded and a small current passes through the flame sensor electrode when the flame is lit.

The fact it lights suggests that that part is at least working ok. When lit I don't know the criterion used by the manufacturer that the flame is lit. Is it expecting a rectified current between some limits?

A boiler engineer suggested that cleaning the electrode can have a dramatic effect. Stainless steel tarnishes over time and affects the measurement. I would give the electrode, not the insulator, a good rub with emery or similar abrasive material to clean the electrode, at least the part in the flame.

Thank you gents for your suggestions. I will take the electrode out of the machine today, check the wire and clean around where it bolts to the body, I suspect it probably does earth through there Alan. I did clean the electrode yesterday Fred but I did it in place. Tim, I'll have a look in the control unit too Many thanks.for your help.

Um, if there’s a flame then the valve is already open. ;-) The flame detection comes into play after the valve opens. If no flame detected within a certain number of seconds, the valve closes.

That just tells you that the igniter is working. It doesn’t tell you anything about the flame detector.

Tim

I would have thought the gas vale would open when no flame is sensed, as a precaution to a faulty sensor failing in a positive sense.

Yer-but, it also tells us the system thinks the flame sensor is actually working and not failing in a false positive with no flame.

I'm pretty sure my gas valve won't open and indicate a faulty electrode if it senses a flame when one shouldn't be there.

While I would be tempted to put a DVM (Volts) on the sensor wire, I'm not sure if it would be high enough resistance so as to not interfere with the rest of the circuit.

^^^^ ^^^^ not

FWIW, I opened my control box today. It’s easy to get at the bottom of the circuit board but alas the HV igniter is potted into a recess in the top cover. Just makes it a bit more of a faff to look inside as you need to desolder the connections to the igniter. I’d assume that your is similar.

Tim

This might interest you Stephen.

I got my control box open. Mine looks like this.

Tim

C5 on that board also appeared to have failed - it looks as if it is bulging at the top.

I had a failure like this in old Ideal Mexico floor standing boiler. It would power up and run for a bit then the flame would cut out - it would then periodically restart, run a few a few seconds and stop again.

It turned out to be the gas valve - it basically would not stay open when hot.

With the boiler it was easy to diagnose with a multimeter. I looked at the mains input into the valve. I could see the demand appear when the programmer/stat was calling for heat, and it remained there even when the valve shut off - indicating that the valve was not doing one thing it is required to do! By looking at the input of the valve you can eliminate all the other bits like internal stats, or control system problems.

Normally it would lockout fairly quickly if flame failure is detected I would have thought... I can't see it running 4 mins in those circumstances.

It did cross my mind that it could be the gas valves but the 8 second burn time is significant as that’s more or less the nominal ignition time for the control board. A failing gas valve that would flow gas for a fixed period regularly would be an odd fault.

Tim

Thank you gents for all your suggestions. Tim, I have open the control box which is different to yours, and to my untrained eye, there's nothing obviously amiss, no leaking or puffed up capacitors or staining. Nothing obvious showing on the soldered side. I'm aware that this isn't definitive.

Most failed caps look fine. A component tester would check them after unsoldering. Bad joints are another thing you can look for, eg cracked solder.

After all my deductive reasoning I consider it very unreasonable for your circuit board not to be obviously faulty! ;-)

I’m gonna stick to my guns and say that it’s just inconspicuously faulty now.

Tim

It could well be the circuit board Tim/Animal. If that's the case, it's probably not worth the expense of replacing it (if I could find one) and it'd be beyond my capability of repairing it. My wife is chomping at the bit to go and buy a new one. I just don't like being beaten by a fault which as you say, probably something minor. Thanks to everyone for your suggestions and help. Steve.

That’s a shame but you’ve probably had your money’s worth out of your dryer. I was sad to retire mine after 33 years. If the heat pump replacement lasts 33 years I shall be very surprised (and 99 years old!).