Seattle Home Inspector's Blog


Is that gas I smell?

The disSTINKtive odor of natural gas or propane is hard to miss–and for good reason. Since the gases have no natural odor of their own, if we did not put stuff in the gas to alert us to when it was leaking, we would have exploding structures all over the place.

One of the most common gas leaks I find on inspections is at the furnace gas valve.

It is not so much that these valves leak more than they ever did in the past, it is just that modern furnace designs allow us to notice the leaks more often.

So what is an inspector to do? Say nothing? Well obviously we cannot say “nothing”–but we can provide information as to what is going on.

There is probably no way any of the parties involved in the inspection process is going to be willing to accept that “any” amount of gas odor is “normal”–even if it is.

From Honeywell, a major valve manufacturer: “Although the permeability values for gases through rubber diaphragms is extremely small (approximately 0.56 cc/hr) and almost negligible when compared to the 200 cc/hr maximum allowed by ANSI (ANSI allows 200 cc/hr for outerwall gas leakage and 235 cc/hr for valve gas leakage), it appears this is enough to explain an instantaneous indication of gas from a sensitive electronic gas sniffer detector.”

Before the advent of induced draft furnaces, any residual gas fumes would be drawn up the chimney and never noticed. Now these vapors build up inside the furnace compartment after the fan shuts off. The fumes sometimes find their way outside the furnace compartment where the odor can be smelled.

At least one manufacturer suggests that when leaks are noted, the valve should be tested, by a qualified HVAC technician, with a reliable combustible gas detector and then be tested with a flow gauge to see whether the gas is merely residual or actually leaking.

So when the technician shows up to fix the leak and cannot find it, that may be normal. However if they find “leaking” as well, make sure they do a flow test before going to the more expensive solution of changing the valve. All too often there is simply nothing wrong with the valve, and the new one may behave the same way.

Normal gas leaks at gas pipe joints and fittings can be easily detected, located and soaped for clarification.

Leaking furnace valves are considerably more difficult to assess. This testing involves taking things apart, so your friendly home inspector is not likely going to be able to do this at the time of inspection.


By Charles Buell, Real Estate Inspections in Seattle

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Comment balloon 7 commentsCharles Buell • March 25 2016 07:59AM


Charles Buell I have often sensed the smell of the mercaptan, fortunately when visiting homes with buyers. This is just one of the cursory tests I conduct when visiting homes before moving forward with a purchase offer. If the buyer still wishes to move forward, then a professional is engaged during the home inspection process. Glad to see you are also on top of this issue.

Posted by Sandy Padula and Norm Padula, JD, GRI, Presence, Persistence & Perseverance (HomeSmart Realty West & Geneva Financial, Llc.) about 3 years ago

Good post that may save a life or two Charles Buell . I find the problem most often with old gas stoves where the pilot has gone out.

Posted by Bob "RealMan" Timm, Owner of Ward Co. Notary Services, retired Realtor (Ward County Notary Services) about 3 years ago

Charles I didn't know anything about this subject....once again you are a wealth of information.  Excellent post.

Posted by Anna Banana Kruchten CRB, CRS 602-380-4886, Arizona's Top Banana! (Phoenix Property Shoppe) about 3 years ago

This is a common issue in home inspections.  Better to be safe than sorry.

Posted by Belinda Spillman, Colorado Living! (Aspen Lane Real Estate Colorful Colorado) about 3 years ago

Until now I didn't know gas was a fuzzy foam.  Interesting!  Wow, you learn something every day!

So, you really didn't tighten up that Gastite fitting?

What do you mean you don't know "how to bring it up..."?  You had a whole post on what goes down must come up.  That post had me worried about dinner.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) about 3 years ago

Gas leaks are another important reason home inspectors are so vital to the home buying experience.

Posted by Myrl Jeffcoat, Greater Sacramento Real Estate Agent (GreatWest Realty) about 3 years ago

You are my go to guy for inspections in the North end of Seattle Charles Buell 

Posted by Paul S. Henderson, REALTOR®, CRS, Tacoma Washington Agent/Broker & Market Authority! (RE/MAX Northwest.) about 3 years ago