Thermocouple on Gloworm Spacesave

[Jon Rowlan]

I am trying to understand how the thermocouple on my boiler works as I have bent the copper pipe and need to know whether this is stopping the thermocouple from working.

As far as I can tell, the end that resides near the flame is just a pointy outside bit and needs to be heated up for 20 seconds to keep the pilot light going. There is a long length of thin copper pipe that bolts into the side of the manifold that is the source point for the pilot gas supply.

On the end is what seems to be a metal or lead ball which rests on a plastic plate that slots into place. This plastic plate has a strip of metal on it. in the thermocouple copper pipe there seems to be an internal cable that must somehow contact with metal on the plastic plate.

Now, heres the cruncher ... with no power turned onto the boiler where does the electrical charge that must bridge the copper and plastic metal strip come from?

I'm baffled ... it seems that this is an electrical switch but there is no power???

Can anyone enlighten me??

jON

[Christian McArdle]

I'm not sure if I quite follow what you describe, having never seen inside the gas valve. However, a thermocouple actually generates electricity directly from the heat. They are used in old boiler designs because they work even without a power supply. Old boilers tend to only require a call for heat signal and have no permanent live, so the pilot control must work without the use of mains electricity.

Christian.

[IMM]

Correct.

The basic safety gas controls of a boiler/gas appliance are:

1. Thermocouple valve
2. Governor
3. gas solenoid valve.

These were all separate valves on a pipe inside the appliance. In large boilers they still are. In the 1960s they were all integrated into one casting called the multifunction control valve, which is on the Spacesaver

The thermocouple generates a small current, enough to power a solenoid valve which supplies the pilot light, but only when the valve is on its seating. Enough to hold it down. That is why you have to turn a knob (the pilot gas tap) and hold down the same the knob for 30 seconds) pushing the valve onto its seating; enough time for the thermocouple to energise the solenoid and hold the valve on its seating maintaining a pilot light. If the pilot light disappears the solenoid de-energises and drops to its seating stopping all gas to the appliance. Another solenoid valve further upstream is in the main gas valve which is usually mains powered.

[Martin]

A Thermocouple generates a very small voltage (mV) which is then used to sense heat; the voltage is generated by the thermal excitation of a junction of two dis-similar metals. Run a Google on Thermocouple if you want to delve deeper.

[Jon Rowlan]

The multifunction valve is the manifold at the other end of the Pilot Pipe from the Piezo I presume?

I understand the principle now, but is a kink in the thermocouple outer sheath likely to stop the thermocouple from working ...

we tried to get a spanner around the pilot nut and pushed the thermocouple outer sheath into a little "v" shape and broke the insulation on the Piezo at the same time which now refuses to work.

It looks like the Piezo, thermocouple head and Pilot are in a block and need to be replaced together - is it possible to buy and fairly eay to fit each of these components individually?

If so, how can I check that its all working ... is there a way to check the charge generated by the thermocouple?

jON

[IMM]

It depends on how sharp the kink is. Thermocouples are cheap. get another two and have one inside the boiler in case.

Then buy an new peizo electrode.

They slide out of the block. Look harder, they are usually secured in some way to the block.

There are some device to check these, but expensive. Get another thermocouple and a spare in case. They are cheap. ...and new peizo ignition electrode.

[IMM]

Uh!