How quickly should thermocouple cut off gas after pilot goes out?


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Old 01-11-11, 09:16 PM
J
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How quickly should thermocouple cut off gas after pilot goes out?

Hi all,

I have a gas fireplace which has always been a pain. In the middle of enjoying a roaring fire, the gas would cut off and the fire would disappear. Sometimes we could light it again, other times the pilot wouldn't stay lit. Prior to this fireplace I have never had any gas appliance, so I didn't know what the problem was. I finally discovered that this behavior is the result of a problematic thermocouple so I replaced it and the fire now stays lit.

However, when I was testing out the new thermocouple's functionality, I discovered that blowing out the pilot light does NOT result in the gas cutting off immediately. This panicked me at first, but it seems that my new thermocouple takes a little while to cool down after the fire is gone and THEN it will cut off the gas. Is this normal?

My prior thermocouple obviously didn't have this problem because it was ready to turn off at the drop of a hat. So I don't know if this is normal behavior or not.

Is this how it should work? How long should it take before a thermocouple in a fireplace cools down enough to cut the gas?

Thanks!
 
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Old 01-11-11, 10:06 PM
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What is a little while??? Yes they are still hot and I would think less then 15 seconds. And if the pilot is not lit the burner will not lite again..... Gas off saftey!!!! Just like your other issue. Dont dwell on it. Thermal couplers have been around many years..

Plus your by-passing the saftey feature by blowing out the pilot. If the t coupler is bad the pilot wont stay lit there for the burner wont light...

Mike NJ
 
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Old 01-12-11, 12:10 AM
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Thanks for the reply, Mike. If I run the fireplace for about 5 minutes and then turn it off (which leaves the pilot light on) and the pilot light gets blown out, it continues to emit gas into the air for at least 90seconds to 2minutes.

I expected something more like the 15 seconds that you suggest, and it seemed odd to me that once the fire was out that the gas would continue for so long.

I may not have been clear enough earlier. I wasn't blowing out the pilot in an effort to bypass the safety feature. I was only doing it to test the safety feature. It seems plausible that a pilot light accidentally gets blown out and my understanding is that the thermocoupler would then shut off the gas immediately. That's why I was surprised that it continued after the light was out.

Thanks again for your response.
 
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Old 01-12-11, 01:15 AM
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What you are experiencing is normal.


A standard thermocouple will allow the gas to stay on 60-90 seconds. A more modern thermocouple design shuts off after 30 seconds or so and has a shorter length and different size.

As a practical matter, either one is fine.
 
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Old 01-12-11, 05:50 AM
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You are probably okay.

Realize you need to build to a minimum fuel/air concentration before it is flash-able. Also during that time you likely still have a draft induced that is both sucking the gas up the chimney while further dispersing the gas.
 
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Old 01-12-11, 06:44 PM
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Pilot lights usually go out when the main burner is off. The thermocuple drops out at it's leisure ---no harm done.


Or if the gas supply is interrupted while the main burner is on, both the main burner and pilot are extinguished. If the gas comes back on, you will have gas coming out the main burner and pilot burner but no ignition source. Again the gas will shut off after the thermocouple cools off.


As a practical matter it's not much of a problem.
 
 

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