Thermocouple question

[andrewjo]

I have a problem with my boiler because the pilot light will not stay alight. From what I've read on other posts, it could just be the thermocouple. So I went and bought a new universal one from B&Q for about £5, and fitted it. However it does not appear to have made any difference.

I removed the new thermocouple and examined it and I noticed it has a small fracture near the end of the thin copper pipework (not the probe end). Would this affect the way it works? I thought a thermocouple was an electical device, but then again why does it need the thin copper pipework?

 

[chrishutt]

The thermocouple lead has two conductors - the outer copper tube and an inner steel (?) wire, with insulation between the two. It is common for the outer tube to be "squashed" flat near the connector end to allow connector nuts of different sizes to be slipped on.

The pilot flame impinges onto the tip of the thermocouple, where two dissimilar metals are joined. the heat generates a weak electrical voltage which is sufficient to hold open a solenoid valve in the gas valve.

If yours is not working, check that the pilot flame impinges properly onto the t/couple tip, and check that the connector at the other end makes a good electrical connection. Note if the connection is directly to the gas valve or via a splitter and wires to an overheat stat or similar.

 

[HarrogateGas]

Would help if we knew what boiler you had......may have a duff Over heat stat.

 

[andrewjo]

Thanks for the comments. Boiler is a Thorn Olympic 38/50B.

So chrishutt are you saying it shouldn't matter if the thin copper pipework of the thermocouple is fractured (it has a tiny break in it) ?

 

[Bahco]

In my very limited experience of thermocouples I would not purchase a universal one again.

My old boiler used to eat them every 6 months or so.

 

[chrishutt]

So chrishutt are you saying it shouldn't matter if the thin copper pipework of the thermocouple is fractured (it has a tiny break in it) ?

What matters is the electrical continuity. If the fracture causes the inner and outer conductors to touch, you might have a short circuit. Also if either conductor is broken completely it would be open circuit. The voltages and current involved are very low (millivolts and milliamps I think) so even a reduced cross section of conductor might impair performance.

 

[HarrogateGas]

If you have a multimeter you can chek the Thermocouples performance.................you should be getting around 24millivolts from a new/decent thermocouple. The gas valve will hold however anything down to around 14-16 millivolts.

 

[andrewjo]

I tested the old and the new thermocouple, and under ideal conditions (I held them in the flame from the gas hob), they are both kicking out 25-30mV.

So I thought maybe I'm not getting a good electrical connection with the electrical contacts at the gas valve. Trouble is, where the thermocouple meets the gas valve, it's right up against the wall and access is very tight. I broke off part of a wall tile to make it easier, and now I can just about get the tips of me fingers in there to start turning the holding screw. Actually I just used a little dentists mirror and a torch to check out the contacts and they seem fine, nice and clean. I can see one contact is right at the end of the thermocouple, the other must be made between the copper pipework of the thermocouple and the body of the gas valve, via the holding screw? This may be important because the copper pipework on the new thermocouple that I bought is a lot thinner than the old one, and I had to use the old holding screw with the new thermocouple (new screw has a different thread?).

Anyway, I was hoping it was just the thermocouple, but realistically it looks like it's something else that's failed. Is it possible to test the gas valve independently of the thermocouple?

 

[ChrisR]

The thickness of the copper tube won't matter. The fixing nut comes down against a little flange at the button end of the tc. As long as that's intact you should be ok.

Is it possible to test the gas valve independently of the thermocouple?

The resistance of the bit the tc connects to is too small to measure - it looks like a short circuit, which you can check. Bent paperclips in your case? That won't tell you if it really IS a short, of course.

It's easy to get a bad connection, but your gas valve sounds suspect. Corgi dept.

 

[HarrogateGas]

Sounds like the gas valve has gone. Dont think that boiler has an over heat stat??

 

[ChrisR]

No it hasn't - I had to check - Partsarena.com/baxi .

 

[Agile]

Although the gas valve CAN fail to let the thermocouple hold the magnetovalve ON, that is very unusual!

This fellow clearly done not know anything about thermocouples and it would be better risk management for him to get the proper manufacturers spare part and fit it before calling in a CORGI registered boiler engineer.

Merely measuring the o/c voltage of a thermocouple does not mean it is working, it needs to be measured on load!

Tony Glazier