Gas smell from cold air return

[appliancenovice]

Hi,

About a week after turning the furnace on for the year, I kept noticing a gas smell in the hallway in which the return air vent is located.  We did a soapy water check on all the gas appliances and can't find any leak.  We have natural gas heater, with the return air vent in the ceiling of that hallway.  Could there be a leak from within the heater that is creeping back through the return air vent?  We have a new Kidde Gas detector in that hallway, but it doesn't pick up anything. 

We keep the windows open alot when we're home, so that probably contributes to the now-you-smell-it, now-you-don't aspect.

Thanks.

 

 

[hvacdrd]

Not likely that a gas leak would present itself in the return air duct unless gas piping runs thru the duct. If there was a leak at the control valve, gas leaking by the valve, or piping the odor would tend to stay in that area. Natural gas is lighter than air and the odor tends to go up as well.

Do a quick check using your gas meter to perform a shadow test. With all appliances off, turn off any pilots. At the gas meter you will typically see 6 dials. The top four are usually the read, the other two are proving dials. Which ever one has the smallest measurement (ie 1/2 cu. ft.) is the one you want to watch. Mark the dial position and walk away for 10 minutes. No dial movement = no gas leak up to any & all appliance control valves. Your gas provider will also respond to gas odors if you want. The shadow test is the easiest method to determine if in fact there is a leak.

Dead mice &  sewer gases are more likely to be the source of your odor. Any odor will tend to be drawn in the area of the return air due to air flow patterns.

[appliancenovice]

Thanks for the clearly written answer.  We'll try it as soon as my husband gets home this evening.  I'll post what we find out.  Thanks again.

[appliancenovice]

Husband is delayed out of town.  Luckily, we have good weather and don't have to use the furnace much for a few days, and can keep some windows open.  It doesn't smell like a dead animal decaying to me, and we've never had mice that I know of.  If it was sewer, wouldn't  I smell it pretty stongly near the source? 

Oh, by the way, the gas detector we have is a retail Kidde model.  I did test in in front of the fireplace -turned on gas but didn't light it, and it took the detector probably 20 seconds to go off.  By then I could smell the gas strongly (don't worry, I did this with doors and windows open).  I mention this because I wonder whether it simply took the sensor that long to recognize the smell regardless of strength, or whether the sensor only picks up on very intense concentrations. 

Thanks.

 

 

[Pegi]

Not to say you do not have a problem with a gas leak...that said we have had on two occasions here at the house where we knew there was a gas leak from the kitchen area...lasted about a week, no gas leak was ever found, turned out to be an animal that had died under the house both times and the smell went away in about a week....you could be smelling something that has died until it dries out and the odor would be gone...dead animals do smell just like the stuff they put in the gas so you can detect it in the area...

[appliancenovice]

Thanks for the info, Peg.  It sure would be easy, albeit sad, if it ends up being a dead animal! 

[hvacdrd]

The Nighthawk Combo detector is calibrated to alarm at 25% of LEL (Lower explosive limit) - for natural gas LEL is 4% gas. Which means the smell would be very strong by the time the detector alarmed. Most leaks I have responded to with a strong odor present upon entering the home are in the 1-5% LEL range at best.

Sewer gas and dead animals are top on the list of false "gas leak" odor calls. Sewer gas can take on the form of several different odors, I have even had it smell like an electrical burning odor so don't rule anything out.

Most importantly if you are concerned contact your gas provider, as part of public safety they will respond promptly to make sure it is not a dangerous situation.

[appliancenovice]

I should have replied sooner to this.  We did the test just as HVACDRD suggested, and it confirmed that there was no gas leak.  I had accidentally left one of the gas fireplaces on at first, and my husband came back in the house because he could see that gas was still being used.  Once we turned off the fireplace, it quit moving completely.  We waited a total of 30 minutes.

The smell did go away about two weeks later.  I think it may have been a situation similar to that of Pegi, but longer lasting! 

Thanks again for all of your quick, helpful, and accurate responses!

Judy

[Pegi]

Thanks for the update....