1969 Lennox gas furnace
The pilot light won't stay lit.
I light it easily enough, and it makes a nice two-inch blue flame.
Ten minutes later, with the thermostat still turned off, I hear a clunk,
and the pilot light goes out.
It has happened about like that three times.
Thanks for that video.
The guy in the video calls it a "wire". But is it electrical? I
thought it was probably a tube filled with gas and that the pilot heats
the gas, which operates a pressure switch in the valve body. Is that right?
The pilot gas line is aluminum tubing about 1/4" o.d., which was
replaced a couple years ago without good reason.
There are two 1/8" copper lines (which I presume are tubes, not wires)
going from the valve body They are connected with compression fittings
having 5/16" and 3/8" nuts.
Those two tubes together with the pilot gas line go to a little metal
box (say 1.5 in. x 1.5 in. x 2 in.).
There is a bulb from one of the copper lines situated in the flame of
the pilot when it is burning.
> thought it was probably a tube filled with gas and that the pilot heats
> the gas, which operates a pressure switch in the valve body. Is that
No, your understanding is not correct.
Welcome to Gas Valves 101:
That copper "wire" that goes from the thermocouple to the gas valve is a
coaxial cable. It has a central insulated copper conductor and and
external copper jacket. When heated, the thermocouple generates a tiny
voltage between those two copper "wires" which carry that voltage back
to the gas valve.
A gas valve may look complicated, but it's really just two
electromagnetic valves in series built into the same block of aluminum.
The upstream electromagnetic valve is called the "safety" valve or
"safety magnet" and the downstream valve is called the "main" valve or
"main magnet". The small diameter pipe that allows gas to flow to the
pilot light is connected to the gas valve BETWEEN the safety valve and
the main valve so that gas will always flow to the pilot light as long
as the safety valve remains open. It's the power generated by the
thermocouple that keeps the safety valve open. Power is applied to the
main valve to get it to open only when there's a demand for hot water or
a demand for space heating from a boiler or furnace.
The pilot light/thermocouple combination is a safety system. If the
pilot light goes out (for whatever reason, including an old
thermocouple) that safety valve loses power and closes. That prevents
gas from flowing through the main valve to the burners even if there's a
demand for hot water or space heating. That's why with so many furnaces
and hot water heaters being fired by natural gas in North America, there
are relatively few house explosions.
If the thermocouple is in good condition, then it generates enough
voltage to keep the safety valve open, and whenever the thermostat on
your water heater calls for heat, power is then applied to the main
electromagnetic valve which opens to allow gas to flow to the burner
trays. The pilot light ignites that gas to heat the water or provide
space heating to the house.
Quite often, water heaters which rely on the electrical power generated
in the pilot light to operate both the safety valve and the main valve
will use a "thermopile" instead of a thermocouple. A thermopile is an
electronic device that is really just a hundred or so miniature
thermocouples built into the same metal housing. The result is that a
thermopile generates about 1.2 volts instead of the few millivolts
producted by a thermocouple. A thermopile sits right inside the pilot
light flame just like a thermocouple.
Unlike water heaters which normally don't have 120 volt power available
to them, the gas valves on furnaces and boilers will typically use a
thermocouple to power the safety valve and 24 volts AC to power the main
electromagnetic valve. It's important to have at least the safety valve
powered by the heat of a pilot light to ensure there's a flame present
to ignite any gas that's released into your house.
The voltage produced by thermocouples gradually diminish with use, and
when the power they produce is no longer sufficient to keep the safety
valve open, the pilot light will repeatedly go out until that
thermocouple is replaced. The pilot light will always be easy to
relight because the button your push down or dial you turn on the gas
valve to re-light the pilot simply over rides the electromagnet and
opens the safety valve manually so that gas can flow through the safety
valve to the pilot light.
A pilot light burning continuously uses a fair bit of energy. High
efficiency water heaters, boilers and furnaces do away with standing
pilot lights and use a spark or hot surface ignitor to ignite the fuel
when there's a call for heat. Consequently, high efficiency appliances
won't have a pilot light or thermocouple.
There, now you know more about how the gas valve on your water heater
and/or boiler or furnace works than most homeowners.
Coax cable, valves in series, port for pilot is between the two valves,
thermocouples wear out.
Thank you for a great explanation, nestork
I think I just have one more question. There are two 1/8" copper lines
coming out of the valve body and going to the little pilot box along
with the 1/4" aluminum pilot-supply gas line. Both copper lines are
connected to the valve body by what look like (but apparently are not)
compression fittings. Does that mean that there are two thermocouples?
One fitting has a 5/16" nut, and the other nut is 3/8".
Most thermocouple I've seen, have one copper tube, and the electrical is
pin and shield. Might this themocouple have two wires? I've never see none
like that. But, most 1969 equipment near me has long since been replaced.
Christopher A. Young
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It takes two connections to make a circuit. Each of those
two is one of them.
I just took a look at my water heater and you're right.
The thermocouple is just one nut connection. It's been
a long time since I replaced one and I guess the two
connections thing I had in my mind was from the two
connections I needed to take off apart to do the repair.
One was the TC, the other the pilot light gas line.
So, what he has is a mystery. When he removes it and
looks at it should be easy to determine if it's one TC or
two. Depending on the climate where that 1969 furnace
is located, it might be time for a new one. Can't imagine
the efficiency of that beat. I replaced by 25 year old gas
furnace two years ago. It made a huge difference in
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