Ping Geoff : Boiler question ...

[Arfa Daily]

Mate of mine asked me a question about his boiler today. I told him that I didn't know the answer, but knew someone who probably did ! Background on him. Pro electronics design man, very practical. Also, advanced DIY'er. Basically built his whole house himself.

The boiler in question is apparently quite old - before the days of combis, or even electronic control. Unfortunately, I didn't get the make and model from him, but I think it is probably a general enough question to not need it. I'll get it if required, though.

Basically, he has been having trouble with the pilot going out randomly. He bought a replacement generic pyro sensor, and fitted it. He is completely au fait with the principles of these devices, and how they should be positioned in the flame. The specs said that he should get around 30mV output off load, and 15mV on load, and this is exactly what he got when it was correctly positioned. However, the problem has persisted, and this has led to his question. The output of the sensor does not go directly to the gas valve. Instead, it goes up two feet of wire to a thermal switch mounted on the boiler output manifold, and then back down another two feet of wire to the gas valve. He reckons that he is measuring around 7mV of copper drop on this wire. The drop across the switch itself is negligible. So he ends up with only about 8mV at the gas valve, and he is wondering if this is enough to keep the valve latched under normal circumstances. He is figuring that copper drop is copper drop, and must be the same as it has always been, but his main question is what is the sensor on the output manifold actually for ? Under what circumstances could the temperature of that manifold ever exceed 100 deg C, given that this is an open vented non-pressurised system ? What is it there to sense ? Dry boiler maybe ? He is wondering whether it would be safe to bypass the switch wire run temporarily, to try to prove that the gas valve itself is ok.

Thoughts on any of it ? Anyone ?

Arfa

[Dave Liquorice]

A few feet of copper wire shouldn't drop that much volts. My first thought was "tired" contacts in the thermal switch but that appears to be ruled out. So that leaves checking that all the connections are clean dry and firm so not dropping the volts. Also free of any corrsion deposits so there is no leakage path. Check that the restsitance of the wires is barely measurable just in case there is a hidden fracture.

[David WE Roberts]

When our old boiler pilot light used to go out randomly it was due to crap in the gas pipe. This can be quite difficult to trace, since if a small piece of dirt is blown up into the pilot light jet it can block the pilot light but then any tinkering such as removing the jet for inspection can dislodge it back down the pipe so the jet appears clear.

A random time later the dirt (or a friend) gets blown up into the jet again and the pilot light goes out.

Used to happen once or twice a year - can't remember if we ever managed to get the crap out of the gas line.

Cheers

Dave R

[harry]

There may be a problem with the installation that allows the pilot light to be blown out by wind. Obviously happens in windy weather especially wind from a certain direction. Or the boiler is too close to the house corner/other turbulence generating object. The flue terminal may be damaged/not correctly fitted/upside down

Some designs have a pilot flame shield that may be missing / misorientated.

There may be muck in the pilot jet hole that moves around.

The other (expensive) option is that the gas valve is faulty/ intermittant connection. (Unlikely)

[Arfa Daily]

That was my initial thought as well Dave, but this is a guy who understands things like wire resistance and voltage drops to the nth degree. He is as old as the rest of us, and has been involved in high level military grade electronic design, all his working life. I will try to discuss this with him further. He has test equipment that can measure accurately to milliohms, so he should be able to determine whether the resistance of the wire, and its connections, is acceptable. I guess at the end of the day, it's all down to source and load impedances.

Arfa

[Arfa Daily]

That's an interesting thought. I'll put it to him.

Arfa

[Arfa Daily]

This is a boiler that has been in place and working correctly for many years. The guy fitted it himself originally, so nothing is fitted wrongly. If anything had become damaged, it is very likely that he would have spotted it. I discussed orientation of any shields and brackets involved in the pilot flame and pyro sensor positioning, but he assured me that everything was as original, and that he had been able to adjust the positioning of the sensor to get exactly the output voltage stated on the device's spec sheet.

Yes. That is an idea that I like

That is his last thought also. But, like you, he feels that it is unlikely

Arfa

[harry]

Another thought. The pilot flame may be adjustable. Where the small gas pipe for the pilot flame enters the valve block, there may be an adjusting screw next to it.

The size of the pilot flame can be turned up and down with this screw (if there is one) If it is set too low, the flame can blow out. If too high, the flame can burn out the thermocouple.

[newshound]

I've seen this too (on a multipoint)

[John Rumm]

I found a dodgy gas valve on a mates Ideal Mexico a while back. The symptoms were a little different though. Pilot would stay lit, but the main burner would shutdown after a couple of mins running for no apparent reason. The call for heat was still present, and mains was still reaching the valve even though it was off.

[geoff]

In message , Arfa Daily writes

As others have said - sounds like the pilot flame is being diverted from the thermocouple for whatever reason. Either that or a break in the wiring at a connector for example. The only way to test for this is to give the wire a tug and check for elasticity, insulation AND wire both being crimped to the connector.

Has he been by the boiler when it drops out? He should be able to hear the slug dropping out and find out what is going on in real time

best I can offer, I'm afraid

[Arfa Daily]

Thanks, Geoff. Do you have any thoughts on or knowledge of the reasons for having that additional thermal switch attached to the output manifold, and in series with the sensor ? That was his main query, as he couldn't think of any kind of overheat condition detectable at the output, which would necessitate shutting down the boiler by means of the gas valve latching voltage.

Arfa

[geoff]

In message , Arfa Daily writes

Other than its a statutory requirement to have a manually resettable safety cut out on the boiler, no

[Doctor Drivel]

Could be the "pilot stat", as they called it, in the Multifunction gas valve. If pilot is stable and the thermocouple lead is new it points to the valve. Whether parts are available for these valves or a complete valve is needed I am not sure.

If the boiler is so old it has one of these valves he nest getting a real quality condensing boiler. Direct him to an Atmos or a cheaper Intergas - same thing.