Hello, I had the gas company over to check a pronounced gas smell in the garage. They could not locate the source within the garage or the adjacent HVAC room. But on the outside they did identify a significant reading coming out of the furnace exhaust pipe even if the furnace was off. (Believe the reading corresponded to 1/10 of a percent).

I've shut off the gas supply to the furnace and will be leaving the HVAC off till professional help arrives. But in the meantime I wanted to ask whether a faulty gas control valve is the likely culprit under similar situations? Perhaps there are other possibilities to rule out first?

Thanks for any insights!

 

It's not surprising to smell gas out the flue since that's where the exhaust is supposed to go...but if you are concerned, yes, it would be best to have it checked out by a pro.

 

I'm a little confused. The gas company determined excessive gas coming out of the exhaust and didn't internally check the furnace ? If they determined it was the furnace they should have "red tagged" it.

Depending on the type of furnace..... the gas should have been strong right at the unit based on how the burners are set up.

 

Thanks for your responses.

@XSleeper: the smell also occurs when the furnace is not in operation, so has little to do with incomplete combustion. Someone from the gas utility identified the issue and told me it is likely that the gas control valve is leaky.

@PJmax: the gas company didn't detect issues at the level of the furnace. He did confirm gas coming out of the vent even if furnace is not operating. He indeed red tagged the furnace. It's a high-efficiency unit with closed burner chamber. I was a little puzzled how this worked (hence to the question to this forum): I thought the chamber would be hermetically sealed so when the gas control valve is leaky when closed, the gas escapes through the vent to the outside. But given that we got gas accumulating in the garage, I suspect that the chamber is not sealed and because of differences in pressures/temperature inside and outside the gas didn't go out. We've only experienced the issue once and were thinking that the combination of weather and a leaky gas control valve is the most likely explanation.

Thanks again.

 

If you are getting a gas odor with the unit turned off then it's most likely the valve at fault.
Gas valves are extremely reliable.... so that is not a common problem.

Do you have two lines from the furnace to outside ?
If yes..... the intake line may not have been glued and the gas can leak out there.
If it's not two lines.... the gas will come out the intake opening.

 

Thank you, Pete. Yes, there are two lines: an intake line that supplies oxygen (and bends downward on the outside of the house) and an exhaust vent (that points upward). The gas smell comes out of the exhaust vent.

I didn't quite understand your point that "the intake line may not have been glued and the gas can leak out there". Did you mean that you would have expected the gas from a leaky gas valve to escape through the oxygen supply line?

Thanks much.