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.
Please advise.
Possible defective "keep alive" sensor. Gas company should be able to replace it for you.
The thermocouple has probably failed and needs to be replaced.
Most common, easy fix, is the thermocouple
See
This part is interesting: what makes the clunk? it's own gas valve or something else in the room? Do you have a second appliance sharing the same flue? A water heater for example? This sounds like there may be either a blow-back from it coming on or otherwise a downdraft from when it shuts off. See if you can time the clunk/pilot going out with the second appliance (if there is one of course) because the remedies will be different for when it turns on or off.
Lacking the second appliance, it can be a defective thermopile/pair shutting the gas valve off "thinking" pilot went out while the pilot is still OK.
Hi, Or connection is loose at the gas valve body.
Especially if it's the 1969 original. O_o
TDD
+1
Thermocouple is the most likely cause. It can be tested for putting out the correct voltage when the pilot light is lit.
If replacing the thermocouple doesn?t work then try removing and cleaning the pilot orifice. Little spiders love making a nest there. Just because t he flame is blue doesn?t necessarily mean that its adequate.
Does the flame contact the thermocouple, and keep the sensor bulb hot?
Doesn't do much, unless the flame is heating the sensor bulb.
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Hi, Or connection is loose at the gas valve body.
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.
something else in the room? Do you have a second appliance sharing the same flue? A water heater for example? This sounds like there may be either a blow-back from it coming on or otherwise a downdraft from when it shuts off. See if you can time the clunk/pilot going out with the second appliance (if there is one of course) because the remedies will be different for when it turns on or off.
Presumably it is a solenoid valve closing so as to shut off gas to the pilot light when it detects an unsafe condition. Does anyone agree with that?
the gas valve off "thinking" pilot went out while the pilot is still OK.
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Strongly suspect you have a bad thermocouple, if it has one, or flame sensor.
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.
It is a "thermocouple" which by definition is electric - 2 dis-similar metals joined which produce a voltage when heated.
THAT is the thermocouple. The entire copper line and bulb are a single unit - and must be replaced without damage. Measure the length and buy a "universal replacement thermocouple" of the same or next longer length. The "universal" kit has all the required parts to connect it to virtually any gas valve and pilot ass'y.
something else in the room? Do you have a second appliance sharing the same flue? A water heater for example? This sounds like there may be either a blow-back from it coming on or otherwise a downdraft from when it shuts off. See if you can time the clunk/pilot going out with the second appliance (if there is one of course) because the remedies will be different for when it turns on or off.
That is what the thermocouple is for - to tell the gas valve to stay on as long as the pilot is lit, and shut down when it fails.
There is a SLIM chance that removing the connection from the gas valve and gently cleaning the contact on the end, then re-tightening it MIGHT make it work. I've had ONE that did, at least for a couple of months.
the gas valve off "thinking" pilot went out while the pilot is still OK.
Kudos for a good explanation but you left out the fact that a thermocouple operates when a junction of two dissimilar metals is heated. ^_^
TDD
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You did mention or explain "thermo-piles" (not necessarily a burning hemorrhoid)!
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".
Actually, it doesn't matter how long your replacement thermocouple is, provided it's long enough to reach from the pilot light to the gas valve. Thermocouples are like extension cords in that respect; you can always use a longer one to replace a shorter one.
If it's a Friday night before a long weekend, and the only store in town that's open has a 6 foot long thermocouple, you can still use it to replace an old 24 inch thermocouple.
I just thought I'd clarify that point in case any newbies reading this got the idea that you had to replace the thermocouple with one of similar length. If you can get the same length as the old one, great. But, any longer thermocouples will also work for you.
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