How do "two wire" thermostats work?

[Palindr☻me]

Hi,

During a friend's gas fire service, the technician replaced a thermostat and left the old one. I saw it, picked it up and have been puzzling for days how it works..

It doesn't have a single tube with a bulb to heat at one end and an actuator at the other. Those I understand. It has two thin "wires"- one soldered/welded to the outer of a small short tube at each end.

The other wire goes through the small short tubes. At the actuator end, it slided freely in the tube but is terminated immediately after - looking as if it just has a small ring soldered to the end. At the bulb end, it just goes into the tube - presumably connected to the bulb in some way.

Now, clearly, if the thin wires were rigid bars, it would be obvious how it works. Heat the bulb bar, it expands relative to the other bar and exerts force on the actuator. But these are just very thin, coiled, wires. I really cant see how any force is transferred to the actuator.

I've tried heating the bulb - but there is no visible movement at the other end. Which proves nothing, as the thing was replaced - presumably because it didn't work..

Anyone know how these work?

[Andrew Gabriel]

A picture would help a lot, but it sounds like it might be a flame failure thermocouple. The end is heated in a the pilot flame and it generates just enough current to hold a gas solenoid valve open. If the pilot light goes out, the gas solenoid valve closes, cutting off the gas.

[Palindr☻me]

LOL, thanks Andrew. I think that you have the answer.

I hadn't thought that they might actually be wires and that it could be electrical rather than mechanical.

They actually can make a solenoid that operates off the current generated by heating a thermocouple? How many mV do they give out? Or does it have an amplifer and separate power souce?

Presumably the guy tested its output and discarded it as below spec.

So,how do they fail? Thermocouples are pretty reliable, IME.

-- Sue

[Andrew Gabriel]

I did measure one once -- IIRC, it was about 20mV. I don't have one here to check now.

No.

After 5-10 years of a pilot flame playing on it, they fall to bits. Often the end gets eaten away, which should be obvious if it's not symetrical any more. OTOH, you might find it still works fine. They cost so little that engineer might simply replace one to see if it was the cause of a problem.

[Palindr☻me]

I don't doubt you, but I am still trying to get my head around how 20mV can produce enough mechanical force to move anything, eg a solenoid. If it was compressed air, and not gas,it could maybe have moved a needle into a tiny bleed hole, that then allowed the build up of static air pressure to move the valve. But this would leak gas all the time that the needle wasn't blocking the hole.

[ricardianno]

The two metal strips are only in contact when the flame heats them therefore the valve is only open and the gas flows only when the metal strips are heated. If the flame goes out (or the power fails) the metal strips seperate, the valve closes, the gas is cut off and the flame is extinguished

[Andrew Gabriel]

The current doesn't move anything. It provides only the holding current for the solenoid valve against a spring. The valve is initially opened by pressing a button and holding it in until the thermocouple warms up (a few seconds) and can hold it open. If the flame goes out, the thermocouple must cool and release the solenoid within 30 or 60 seconds (I forget which), shutting off the gas.

[Palindr☻me]

OK, got it. I hadn't realised a button had to be pressed. That makes sense - the energy comes from the person and not the thermo-generated power. With no air gap, I suppose not much power is needed to hold against a little spring.

Out here in the sticks there is no mains gas and I am not familiar with gas fires! Last one I had, you turned a little brass tap and stuck a match in the front.

Many thanks,

Sue

[Palindr☻me]

Manyy thanks for your reply. I think I understand how it works now!

[Graham]

Andrew has given you the correct answer in this case, but in general don't "diss" low voltage/high current scenarios.

The clasic one I am thinking of is those Weller soldering guns. The voltage across the bit is almost immeasurable but the current induced into what is effectively a shorted-turn is plenty to heat the bit.

[Palindr☻me]

It is rather easier to imagine that something plugged into a mains supply can do useful work..

In this case, a small metal cylinder has its tip heated in a small gas flame and this produces enough power to hold a solenoid against a spring. And the spring has to be strong enough to *always* pull the armature away in the case of the power failing - as it is a safety device.

So not only producing power, but enough power to hold against a reasonably strong spring.

And doing this with 20mV. Even if it produced an amp, that is just 20mW. That's less than an ant having a bad ant day..

[Graham]

Ah I see what you are saying, but the return spring for the button you press is much stronger, and separate from, the spring that closes the pilot valve.

I pulled a faulty ITT gas valve apart before disposing of it a few months ago and I would estimate the weight of a couple of pound coins would be enough to cancel the spring force. The pilot valve itself was a (synthetic)? rubber membrane that required inlt a few mm travel.

[Gio]

Try looking at this page and the associated hyperlinks to thermocouple etc.

know it is for a propane installation but the principle is the same. On the two diagrams the gas enters the valve from the left and exits from the lower port on the right.

Gio

[Palindr☻me]

I wasn't actually thinking of the spring for the button return. I still don't understand how these valves work and don't have one to look at (or even gas out here in the sticks).

IIUC, you turn the knob and push. This push lifts a hammer against a spring and then releases it, striking a piezo element and causing a spark at the pilot jet to light it. The push must first open the pilot light valve, pushing a plate in the valve against the armature of an elctromagnet and supplying gas to the pilot light. This plate is presumably at the centre of the rubber membrane you mentioned.

The pilot lights, heating a thermopile and generating electricty which is fed to the electromagnet. That holds the plate against the armature, keeping the pilot light valve open and it stays open until the gas supply is interrupted or the pilor light is blown out and the thermopile cools.

The operator doesn't know all this - but merely keeps the button pushed for ten or so seconds and then releases it - at which point the pilot should stay lit.

The operator then turns the valve to a "heat" setting.

Somehow the plate must be interlocked with the main heater gas supply valve, so that only if the plate is against the armature, will the main heating gas supply go to the burner, where it will be lit by the pilot flame. If the thermopile cools and the plate is released, somehow it has to then stop the main heating gas supply. Presumably by moving something, using energy stored in a spring. That spring is presumably compressed by the operator pushing the button - but has to be held back by the electromagnet - and when released has enough energy stored within it to close the main gas valve, and then some for a safety factor.

That is my quandry - the thermocouple/thermopile has to produce enough power to not only hold the pilot light valve open, but to resist the spring energy that has to be stored so that the main heater gas valve is closed if the thermocouple cools.

Unless, of course, I am totally misunderstanding what is going on...

[Palindr☻me]

Aha, thanks Gio.

So the valve actually interrupts the total gas supply, and not just the pilot supply! Presumably that means:

The gas goes through the ESV to the inlet of the rotary gas valve that controls the amount of gas going to the burners. In the "light" position, the burner supply is zero, becuase of this rotary valve. Once the pilot is lit, the knob is turned to regulate the amount of gas going to the burners.

If the pilot light is out, even though the rotary gas valve would allow gas to the burners, the ESV has the supply interrupted.

There must be a physical stop to prevent the knob being pushed in far enough to mechanically close the ESV, at any point other than the "light" position.

The penny has finally dropped, perhaps.