Thermocouple / gas valve question ?

[nick smith]

Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts when hot, more or less, and the new one I got was pretty much the same.

I then followed the electrical path (measuring volts output WRT ground) from the thermocouple around the boiler until it went into the gas valve. By the time it had reached the gas valve, it was down significantly, below the threshold of working, (so there had been a voltage drop along its path). I bridged the overheat cut-off switch (NC) with a small cable with croc clips and measured again. It didn't hold the pilot light valve open either, so I measured again, and was astonished to see that there was still 50% of the original volts being dropped across my croc clip lead, inferring quite a high current passing (and being produced by the thermocouple).

I replaced that with a very short thicker cable and even then still some voltage drop but enough to get it working again. I "heat cycled" the cut-off switch and now it doesn't drop any significant millivolts, and all is currently working OK...

Anyway the question is ...... can anyone tell me the typical output in millivolts and current from the average thermocouple, or what is required to keep a gas valve's pilot light solenoid open ? It must be tiny !

I forgot to measure it and my meter / leads would have probably been

too high a resistance to measure it anyway.

Thanks,

Nick

[BigWallop]

Clean the soot and other debris off the thermocouple probe. This should also help.

[nick smith]

I did the cleaning bit as a first thing - the old thermocouple probe insulator was bright white and clean in the main and was producing the same output as the new one.... Thanks anyway,

Nick

[Mike Faithfull]

I don't know how much current the thermocouple will deliver, but my Potterton manual suggests that when the pilot jet is correctly adjusted (i.e. when "the pilot flame completely envelops the top 15mm of the thermocouple"), it should generate 18-32 mV open circuit and 10-15 mV closed circuit. I presume "closed circuit" in this context means when connected to the gas valve. I have got another year's life from what I thought was a burned out thermocouple by simply re-making the connection to the gas control valve, but I've never actually measured the voltage to confirm the numbers.

[Paul Barker]

" Anyway the question is ...... can anyone tell me the typical outpu

in millivolts and current from the average thermocouple, or what is required t keep a gas valve's pilot light solenoid open ? It must be tiny !"

I find that my Fluke multimeter is temperamnental at this also. I bogh a thermocouple tester from Acorn Gas Spares in Leeds. It's a Regi product, and basically it is a solenoid same as the power unit in th multifunction control valve (yes I know, we all call it a gas valve). friend just uses a power unit from an old gas valve. It's a very simpl and effective method of proving a thermocouple.

Sometimes we can be fooled by the overheat stat, I find it's best t remove the leeds of the o.h. stat from the interuptor before shortin it, because if there is insulation break down it tracks the current t ground and a perfectly good thermocouple is discarded when a new lee for the oh stat was what was needed. I've done it myself. I've eve blamed the power unit when it was completely innocent!

I was once told how many milliamps proves a thermocouple, but th thermocouple tester is a more reliable indicator, it proves it can d the job

-- Paul Barker

[Ian White]

If you can measure continuity between the copper outer sheath of the cable and the button connected to the inner conductor, it is very likely that the thermocouple is OK. However, it operates in a very low-resistance circuit (high current but very low EMF) so attempts to make measurements with a normal meter and test prods are likely to stop the solenoid valve from working.

The most important thing is to get two solid electrical contacts to the TC. For the end contact button, it can sometimes help to build up the face of the button with a small blob of solder, so you can screw it down more firmly into the valve. The current return path is through the general metalwork and/or the outer cable sheath, and this can sometimes be the less reliable of the two contacts - there's touching contact all over the place, but often not much deliberate contact pressure anywhere.

The final question is whether the tip of the TC is in the right place relative to the pilot flame. It must obviously be hot enough to hold in reliably, but not too hot or else it will burn through prematurely. You may also find that the TC is quite a sloppy mechanical fit, so you have to muck about with bending the cable until it's willing to stay in the right place.

[Harry Bloomfield]

nick smith presented the following explanation :

That all put together sounds very much like the solenoid has become shorted in the gas valve. Rather than voltage being dropped, the solenoid (due to shorted turns) is overloading the thermocouple and pulling the voltage down.

The same symptoms occured on my boiler a few months ago, I replaced the gas valve and all was well. I pulled the old one apart and their was a brown mark on the solenoid, where it had developed a short.

[Ian White]

That possibility had never occurred to me, considering how few millivolts are available.

I've never had one of these valves apart... it must require very special engineering to make a solenoid that's even *capable* of being shorted by such low voltage.

[Harry Bloomfield]

Ian White explained :

The thermocouples might only produce a tiny voltage, but the produce a surprising quantity of current. The solenoid coil winding inside mine, was about 1" diameter, wound with very fine wire, with a metal slug inside it. You provide the energy to open the gas valve, then the metal slug just needs to trap it to hold the gas valve open. If the current decreases, the metal slug is no longer able to hold the gas valve against its spring so the valve closes.

If anyone is curious and happens to have an old valve... The solenoid on the most common type of gas valve can be accessed by removing the cover plate on the bottom, which is held on by about 6 screws, with a rubber seal beneath it. Once the seal is broken, the valve should of course be scrapped.

[Harry Bloomfield]

Paul Barker used his keyboard to write :

The current is rather difficult to measure due to the voltage being so low and the current so small....

Any attempt to measure the current will introduce resistance into the circuit from the leads, clips and instrument. The measured values will be meaningless. As Paul suggested above, the best test is one of substitution with known to be working items.

[Ed Sirett]

25mV would be typical. After that check all the connections including the one on the gas valve which can come loose quite easily.

You might even be able to prove the gas valve is OK using a universal t/couple even if the part is a bespoke unit with o/h contacts.