Natural Gas Odor from Furnace Exhaust

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Old 12-11-11, 07:43 PM
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Natural Gas Odor from Furnace Exhaust

My mom has a high efficiency natural gas furnace with PVC intake and exhaust pipes going out the back of her house. She thought she smelled natural gas a couple of times in the back yard and had the utility company come out. They didn't find any leaks outside the house, and we have never smelled gas inside the house. I've checked around the furnace area and couldn't smell anything, and soaped the threads that I could access.

Today I caught a whiff of what smelled like natural gas in her backyard, and when I stuck my nose next to the exhaust, it was the source.

Could what I smelled have been a byproduct of the combustion? Is it normal for furnaces to vent a little gas occasionally?

Thanks.
 
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Old 12-11-11, 07:54 PM
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If the furnace was not running at the time it's quite possible the gas valve is leaking thru when it should be off.
 
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Old 12-11-11, 08:14 PM
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I believe the furnace was running. The exhaust was blowing at the time that I caught the odor.
 
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Old 12-11-11, 08:20 PM
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It's still possible the problem is the gas valve. The venter runs a little while before the burner lights & stays on a little while after the burner shuts down. If the burner was on & you smell gas, something is bad wrong. In either case I suggest calling the gas company if they do heater work or a reputable local service company to check things out.
 
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Old 12-11-11, 11:17 PM
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I'd check out what is happening in detail.

IS the gas odor noticeable when the furnace is off? I'd check that several times in between when the furnace is on.


How long is it before the odor is noticable when the burner turns on? Is it noticable every time the burner turn on, or intermittently?
 
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Old 12-19-11, 07:25 PM
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I've checked the exhaust pipe intermittently when the the furnace is off, and can't smell gas. I've also checked the exhaust pipe intermittently when the furnace is on, and almost always can smell natural gas. Still no smell in the basement or elsewhere in the house.

Today I was out in the yard when the exhaust kicked on. I was standing next to the pipe and could smell gas just a few seconds after it started blowing. After a couple of minutes, I could no longer smell gas.
 
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Old 12-19-11, 07:53 PM
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S/P, I'm going to bow out of this one while I learn from your experience & expertise. It's a new one on me.
 
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Old 12-19-11, 10:11 PM
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No problem, Grady.

You probably have some variety of delayed ignition problem. One or more of the burners aren't lighting promptly and reliably.

Please post the make and model of the furnace. If there were a hot surface ignitor and flame sensor, I would expect that would shut off the burner if it weren't lighting.

If you have a Carrier AAW type three wire pilot switch, that wouldn't catch that kind of delayed ignition problem.

Is there a port where you can observe the burners in the burner compartment of the furnace? If so, watch the ignition process carefully and describe it in detail. You might observe one or more of the burners failing to light.
 
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