Intermittent natural gas odor coming from registers

Old 02-05-18, 10:26 AM
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Intermittent natural gas odor coming from registers

Asking this for my sister's house, I have hydronic heating in my house and don't know enough about forced air to truly help her.

Furnace is approximately 30 years old.

About 50% of the time the thermostat calls for heat, the registers on the first floor emit a natural gas odor for the first 30 seconds or so, then dissipates. If you're standing in the basement next to the furnace when the heat kicks on, the odor is stronger.

I suspect heat exchanger or gas valve but you tell me. I'm just worried for my sister and her husband's health living in a home with this issue. They called a local HVAC company but when the technician "diagnosed" the issue he didn't wait long enough for a cycle which emitted the gas and told them "everything is fine."

Any input would be greatly appreciated, thank you!
Old 02-05-18, 05:28 PM
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I am not a pro with HVAC but I am thinking it might be a cracked heat exchanger and the heater is past its normal life. I would call another HVAC guy and replace the heater. Hope the pros will be here soon.
Old 02-05-18, 05:34 PM
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I would agree 100% with a cracked heat exchanger. I would recommend calling your gas supplier for technical assistance. They usually have some excellent technicians and they will tell you like it is.

Don't wait.... this is an urgent problem.
Old 02-05-18, 07:47 PM
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Get someone to come over and do a combustion analysis and evaluation ASAP. If it’s a cracked heat exchanger it’ll show up quick.
DO NOT depend on hardware store CO detectors. They are purposely set to go off at high levels of CO (~70 PPM). Below that they will not go off, and brain damage or death could have already occurred. Low level CO detectors are much safer.
Old 02-05-18, 08:21 PM
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> I suspect heat exchanger or gas valve but you tell me.

obviously you need to take advice and no one can give you "an answer": no one can tell you such a thing for sure unless they are (a mechanic) who is on site and is doing a complete inspection.

#1 A SMELL CAN BE DANGEROUS if it is the smell of UNLIT FUEL GAS (NG, LNG). call the fire department immediately if your unsure and ask what to do.


* leaky fittings. when cool you can spray a soap (spray 9) on the fitting to see if any bubble

* their should be either a forced vent or vent letting gasses up (exhaust gasses, assuming the fuel is properly burning).

* a forced vent should be on before gas flows, and remain on until after gas stops flowing: automatically

* a gravity vent (not forced) will not work well or "right" if the gasses are not hot (if gasses are never lit, do to flow before lighting or lighting never occuring: it may not exit "up the pipe" well

* LESS likely is the "diaphram valve(s)" (there may be two in series for safety). if they fail to completely close they can cause a SMELL

that's not "all" the possibilities, but it's many most or all of them

Last edited by argile; 02-05-18 at 08:24 PM. Reason: clarity
Old 02-05-18, 10:22 PM
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Thank you to all for your input and advice! A second company came by today for a much more thorough analysis. The technician wasn't able to see any cracks in the exchanger with his inspection camera but couldn't quite get to all sides of it. He did say for sure that the gas valve and relay control needed replacing - fan was turning on too early or something like that (I wasn't there, only heard over the phone). Needless to say, he recommended replacing the entire furnace which is what my sister will be doing. Thanks again!

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