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#51400 - 11/01/03 01:25 PM Exactly how does a gas valve work?
Nestor_Kelebay Offline
Carpal Tunnel

Registered: 09/14/03
Posts: 8552
I came across this totally tubular animation of how a gas valve on a furnace or boiler works, and I thought I'd post the URL for anyone interested:

The following animation shows how the gas valve on a furnace works:


go to that page and click on the gas valve. It takes a while for the animation to load. The brown pulsating rectangles connected to the thermostat and thermocouple are supposed to be electromagnets. The thermocouple's electromagnet operates on the tiny voltage produced by the thermocouple, but the electromagnet controlling the flow to the main burners requires 24 volts AC. If the gas valve is getting 24 volts AC when the thermostat completes the circuit to the furnace, the gas valve should open. If it doesn't, the problem is in that electromagnet somewhere.

I like this animation because it provides a good understanding of what's actually happening inside the gas valve when you depress the button to light the pilot, and why a burnt out thermocouple will prevent the gas valve from opening.

#51401 - 11/02/03 12:42 AM Re: Exactly how does a gas valve work?

That's pretty cool!

But it doesn't say how the thermocouple is able to send the signal back to the pilot valve to let it know if there is a flame or not. Is there a liguid or something that expands when heated and holds the valve open, and when it contracts (from absence of heat) it closes?

#51402 - 11/02/03 11:41 PM Re: Exactly how does a gas valve work?
Nestor_Kelebay Offline
Carpal Tunnel

Registered: 09/14/03
Posts: 8552

No, a thermocouple makes use of a phenomenon originally discovered by a fellow called "Seebeck" in the 1800's. Seebeck discovered that if certain pairs of different metals are connected together, a small voltage will be produced at the junction where they're joined.


And, what's perhaps more important, is that the voltage generated at that junction will increases as the temperature of that junction goes up, and it'll go up very linearly with the temperature. That is, a graph of the voltage generated by that junction versus temperature would be a straight line.

The thermocouple itself consists of a metal conductor in the middle surrounded by an electrical insulating material and then there is a copper sheath on the outside. The metal conductor in the middle is connected to the copper sheath only at the end of the thermocouple (which sits in the pilot light flame). Because of the Seebeck effect, a small voltage will be generated at the end of the thermocouple, and that voltage causes current to flow through the metal inner conductor, through the coil of an electromagnet and then back through the exterior copper sheath of the thermocouple. And, it's that electromagnet that holds a magnetized steel plunger up against spring pressure so that gas continues to flow to the pilot light.

The kind of thermocouples used on boilers and furnaces only produce a few thousandths of a volt. Those gas valves will require 24 volts AC to energize the main electromagnet that opens the valve that allows gas to flow to the burner trays.

Until recently, gas fired water heaters didn't have electric power supplied to them. So, gas water heaters have to use the voltage generated by a thermocouple sitting in the pilot light flame to power the electromagnet that holds the pilot light valve open AND the electromagnet that holds the main burner valve open. To provide additional power, hot water heaters use something called a "thermoPILE" instead. A thermopile is nothing more than a whole bunch (dozens, I think) of thermocouples all connected in "series" (pronounced "end-to-end"). Just in the same way that you can connect eight 1.2 volt batteries end-to-end to make a 9.6 volt battery pack for a cordless drill, you can connect multiple thermocouples in series to generate a higher voltage from the heat of the pilot light. Typically, the thermopile in a hot water heater generates almost one volt. (I think it's 0.96 volts, actually.) And, it's that one volt that's used to power both electromagnets in the gas valve on a gas fired hot water heater.

And, that brings us to trouble shooting problems.

If the pilot light stays on, it means the thermocouple or thermopile is good. If the pilot light won't stay on, connect a new thermocouple or thermopile to the gas valve and have a helper heat it with a butane lighter. If the gas valve works fine when the spare thermocouple is being heated by the lighter, you need to replace the old thermocouple or thermopile.

Also, since there's negligible voltage drop along the metal inner conductor or external copper sheath of a thermocouple, you can use a longer thermocouple or thermopile to replace a short one.

Heating equipment simply uses thermocouples because they generate a voltage. You can also make use of the fact that the thermocouple's voltage increases linearly with temperature to make an electric thermometer, and thermocouples are also used for that in monitoring equiment.

Thermocouples gradually generate less and less voltage as they get older. In the field of metallurgy, there's a process known as "annealing". Just in the same way that white styrofoam appears to be made of small "beads", all metals consists of "crystals" where all the atoms within each crystal are arranged in a certain pattern, and that pattern is oriented in a particular direction. In neighboring crystals in the same metal, the atoms will be in the same pattern, but will be oriented in some other random direction. This is exactly the same as the change in the direction the streets run at from place to place when small towns grow together to form one big city. Anyhow, "annealing" is when you heat a metal up (or heat it and cool it in cycles). While the metal is hot, impurities in the metal will gradually congregate at the interfaces between the crystals in the metal, and some crystals will gradually grow in size at the expense of their neighbors. As the impurities in the metal separate out at the crystal bounaries, and you get fewer and fewer boundaries as the crystals grow, the internal electrical resistance within the
thermocouple goes up, and it's voltage output goes down. So, thermocouples don't fail like light bulbs that work perfectly until they don't work at all. Rather, they fail like most things which gradually perform less and less well.

And I think that's all I know about thermocouples.

(Newer electronic stoves use something called a "thermistor" (which changes it's resistance linearly with temperature) to control the heating element in the oven. Thermistors vary their resistance with temperature more linearly than thermocouples vary their voltage with temperature.

#51403 - 11/03/03 12:11 AM Re: Exactly how does a gas valve work?

In reply to:

Hope this explains thermocouples for you, Plumbear.

Yes. Very nice job, Nestor.

However your advice about having a buddy heat the thermocouple with a lighter is poor. I wouldn't suggest doing that for a couple of reasons: one is that the lighter is very unlikely to have the same flame characteristics and output of the pilot burner so it's too easy to get a misleading result. But more importantly is the safety aspect of it. Although there are still some safeguards in place to prevent it, it is possible for the gas valve to open to the main burner once it "thinks" it has a pilot and there's nothing worse than a burner being fed while the pilot is a few feet away!

The only other thing is merely additional and that is regarding powerpiles (thermopile, pilot generator, etc). The most common water heater in North America is the American Water Heater or AWH. These are the typical residential natural gas and propane fired water heaters that you see installed all over the place. They use a different type of gas valve that utilizes a wax pellet immersed in the heated water. The expansion or contraction of this waxy material moves a piston against a snap disc that is under spring pressure and which in turn is connected to the main burner valve. These valves use a thermocouple instead of the more expensive thermopile and are amazing in their simplicity.

Some appliances (water heaters, boilers, warm air furnaces, deep fat fryers, etc) make use of a pilot generator pile and are designed to run on less than 750mw of power. Thermostats with 24v ratings are usually incompatible with these appliances and connecting 24v to the control circuitry will damage them.

Nice essay!

#926983 - 11/14/17 04:40 PM Re: Exactly how does a gas valve work? [Re: Nestor_Kelebay]
the_chemist Offline

Registered: 01/28/04
Posts: 468
Loc: Pittsburgh
I cant find this animation
Rocks are good at being opaque and even better at being old.

#926987 - 11/14/17 04:51 PM Re: Exactly how does a gas valve work? [Re: the_chemist]
bilvihur Offline
Carpal Tunnel

Registered: 02/04/05
Posts: 3126
Loc: Hudson Valley, New York
The original post was from 14 years ago...
MSI Mobo, Phenom II CPU, Win 7/64 Premium, 8GB DDR3 Ram, Chrome, Avast, Malwarebytes

#926988 - 11/14/17 04:54 PM Re: Exactly how does a gas valve work? [Re: the_chemist]
CabinConnection Offline
Carpal Tunnel

Registered: 01/20/04
Posts: 51219
Loc: The Indianhead's Left Nostril....
Originally Posted By the_chemist
I cant find this animation

It's likely Nestor's link is no longer supported / active. However if you Google "animation of how a gas valve on a furnace or boiler works", there are a number of YouTube videos that are available.


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