Confusion or Catch-22?

I’m in the middle of the HVAC course and one section has me perplexed.


A thermocouple is a device that senses heat. It’s used in gas furnaces having a standing-pilot light. It determines whether the pilot flame is lit before the main gas valve is opened to supply gas to the burners. The flame must be lit before the valve is opened.

The heat of the pilot flame is converted to electricity by the thermocouple. It turns heat into an electrical current. The current is strong enough to open the main gas valve. After being opened, the gas flows to the pilot light. If the thermocouple does not detect a pilot flame, it will turn off the gas supply to the pilot. The electrical current from a 24-volt AC transformer operates the main gas valve."

Now if the pilot light has to be lit for the thermocouple to convert the heat into electricity which then opens the gas valve to allow gas to flow to the pilot light… What was keeping the pilot light lit in the first place???

The heat from the pilot light itself is converted into electricity and that runs to a small electromagnetic valve and holds it open to keep the pilot lit.

Ever start a pilot light? You have to hold the valve in until the thermocouple warms enought to keep the pilot lit on it’'s own, if you release the valve (let it out) too soon, the pilot light goes out and you have to start the proccess over again.

It can be confusing.


The explanation in the HVAC course leaves out some details but gives you the jist of the purpose of the thermocouple. There is a manual override for igniting the pilot flame (some gas appliances like some fireplaces have auto ignition for the pilot flame).

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As stated above, in many case manual override of the valve. Pushing it in for a period of time and then moving it into the “On” set position. Also as stated, newer appliances may have built in features to do this. I know I have tried lighting the pilot on older appliances that I literally had to wait a minute for the thermocouple to activate in order to turn the valve off “Pilot” to “On”

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Note that a gas furnace also has a flame sensor which is a thermocouple that senses the main burner flame and keeps the gas on. In the event that the burners do not light (hot wire ignition) the flame sensor turns the gas off. This process: inducer fan runs, igniter glows, gas comes on, lights, goes out; and then repeats indicates a dirty flame sensor. Cleaning the flame sensor will prevent it from turning off the gas. After the gas is lit for a while the blower fan comes on. At this time careful observation of the flame and any “rollout” that may occur (flame leaves the heat exchanger body) may indicate a cracked heat exchanger.

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Thanks guys! I just moved here from the Caribbean. I’ve never used a gas furnace in my life!

Don’t know where here is. If your home has gas heat it might be a good idea to pull the top cover and watch the entire light up cycle to see how it works.

And the wait to purge the air out of the gas line from the valve to the pilot burner…

I had a client from India (buying a very large and expensive home) who had never had a furnace, boiler, ac, or a water heater. That was a long inspection.
When I lived in Belize, I had a Brazilian-made shower head heater that warmed water as it flowed over a 120V live coil. To this day I have a permanent frizz in my hair which is very attractive. :stuck_out_tongue_winking_eye:

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