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Q&A Spotlight

The Case of the Mystery Gas Leak

A blower-door test uncovers a hard-to-trace odor

Finding air leaks:

A blower-door test is helpful in tracking down the source of air leaks, but for one homeowner it revealed an entirely different kind of problem.
Image Credit: FHB

What began as an attempt to track down the source of air leaks in his one-year-old home has [no-glossary]led[/no-glossary] Kevin Hilton to a deeper mystery — a natural gas odor that is apparent only when energy auditors are running a blower-door test.

As Hilton explains in a Q&A post at GreenBuildingAdvisor, the source of the gas leak has been impossible to track down so far.

“During the blower door test run at 50 Pa, I began smelling natural gas,” Hilton writes. “The test was immediately stopped and the smell dissipated. The natural gas hand-held detectors were unsuccessful in detecting a leak with the blower door turned off. The operators then turned the blower door back to 25 Pa, and behind one of the walls the detectors isolated the area of the leak.

“The problem, however, is that there are no gas lines located behind this particular wall where the leak was detected.”

The plumber who installed the gas lines said he couldn’t detect any leaks with his equipment.

“What do I do now?” Hilton asks. “It seems like the natural gas leak is only detectable with the blower door in place and operating. I am concerned since based on the results of the audit I believe I am going to spray foam the attic and turn it into a conditioned space. I don’t want to create a closed envelope with combustion hazard.”

This puzzle is the topic of this week’s Q&A Spotlight.

Test the gas lines

David Meiland suggests going directly to a gas-line test rather than spending any more time doing spot checks around gas-burning appliances with a gas sniffer.

His two-pronged plan of attack starts with disconnecting all of the appliances from the piping and installing caps in place of connectors. Disconnect the meter, install a test cap with a pressure gauge, he says, and pressurize the piping to see how the gauge behaves.

“It should be possible for the piping to hold pressure with no drop for a long period,” Meiland says.

The second step would be to reconnect the system and fill the lines with gas. Attach a manometer to one of the appliance shutoff valves, he says, and watch for a pressure drop. He recommends a qualified gas plumber perform both tests.

“I don’t see the point of sniffing any more,” Meiland writs. “A possible gas leak needs to be ruled out using pressure. A sniffer (I have one…) is a useful tool for finding leaks, but it isn’t the same thing as pressurizing and making sure that pressure is held. Without doing that you are not going to be sure.”

Maybe it’s not the lines at all

Senior editor Martin Holladay suggests that Hilton may be smelling something other than natural gas. “Many things can smell like natural gas,” he notes, “including a decomposing dead animal.”

J Chesnut agrees with Holladay that the “natural gas leak” may be something else entirely. “My bet is while you are depressurizing the house sewer gas is entering the house through the basement floor drain,” Chesnut writes. “Check to make sure all your plumbing drains have appropriate traps and make sure they are filled with water.”

Michael Chandler agrees, and outlines this strategy: “Drive to the grocery store with an empty coffee can in your car,” Chandler says. “Buy four little bottles of peppermint extract from the bakery section. Place them un-opened in the coffee can and rest it between the truck motor and the hood so they get nice and warm as you drive home.

“Without entering the house climb up on the roof and pour all that peppermint down the sewer vent. You will now smell like a Christmas cookie so have someone else sniff around inside the house for peppermint smell; run blower door or range hood as required. Replace wax gasket at base of toilet that smells like Christmas.

“Been there, done this,” he says, “it’s why my hair is this impressive shade of gray and so thin on top where I pull on it.”

Meiland has seen a similar situation with a client whose house started to smell after the septic tank was pumped. Turns out, removing the solid layer on top of the tank allowed sewer gases to enter the house through an uncapped pipe left in the wall.

To find the source, Meiland used a bath fan to push smoke down through a vent on the roof and traced it from there.

Let’s not forget the original problem

As Hilton scrambles to discover a potential safety hazard in his house, it’s easy to overlook the fact that it all began with an effort to find and remedy air leaks. The solution to the gas leak may turn out to be relatively simple, but correcting “massive” air leaks in a new house won’t be so easy.

The builder tells Hilton that he met all code requirements and was given the green light by the local inspector.

“The cost for me to remove the blown-in insulation and spray foam the underneath side of the roof deck is estimated to cost me anywhere from $25K to $40K depending on the company, the type of foam (open-cell or closed-cell), the amount of drywall that needs to be removed to access certain areas of the attic, etc.” Hilton writes. “The builder has informed me that if I choose to go this route, I’m responsible for all the costs associated with this intervention.”

As GBA senior editor Martin Holladay points out, the 2009 IRC toughened air-sealing requirements. Whether Hilton’s builder actually met code requirements would have depended on what was in effect at the time locally.

Our expert’s opinion

Here’s how GBA technical director Peter Yost viewed this issue:

It seems as though others, particularly our own GBAdvisor master builder and plumber, Michael Chandler, have given great guidance on the mysterious gas odor and leak.

On the air tightness of a new home, I just wanted to add these two perspectives, one on the qualitative side of blower-door test results, and one on the quantitative side.

Qualitative blower door testing: I have seen blower door tests done where there is a rush to get the CFM50 number and be done, missing the opportunity to dial back to -25 Pa, and go around and look for the big leaks with a smoke stick and/or an infrared camera. The how and where of the leaks is just as important as — if not more important than — the actual -50 Pa result.

Quantitative blower door testing: On the other hand, I have seen the rush to the CFM50 single-point result miss the information you can get with the multi-point regression curve. That little exponent can have quite a bit of information packed into it.

Quoted from the US Army Corps of Engineers Air Leakage Test Protocol For Measuring Air Leakage In Buildings, page 13: “In general, an n value closer to 0.5 indicates large holes that are much shorter in length than they are wide, where an n value above 0.65 indicates the hole characteristics that are smaller cracks or holes that are much longer than they are wide. Most ‘tight’ residential homes exhibit an n value of 0.60 to 0.65, where larger buildings will likely have an n value slightly less.”


  1. William Moriarty | | #1

    Gas smell
    I would call the local utility to check the ground outside. There could be a leak in the main & when the blower is on it sucks it through a void or crack in the foundation.

  2. Rock Termini | | #2

    Gas Smell
    Not to throw a wrench in the pile, but 'natural' gas (methane) is odorless. Utility companies add mercaptans (sulfur compounds) to give it a noticeable universal odor. One question is whether the detectors were specific for methane or for mercaptans. Both varieties exist. There are many sources of sulfur odors, and it could be one of those rather than fuel gas.Sewer gas is one, sulfur in ground water another. Does the house have public utility water or is it on a well? Is it on a septic system or connected to a POTW? New construction - has settling cracked a sewer lateral from the house to the street, or has the back flow preventer or lateral trap been damaged? Lots of possibilities. (Interesting mix of required talents - you either need a builder who was a chemist, or a chemist who was a builder . . .)

  3. L H | | #3

    Gas Smell
    I am hoping you will follow this up later with the answer as to where the gas smell was coming from. I'm really curious now.

  4. Matt Peters | | #4

    Smell under negative pressure
    Senior editor Martin Holladay has my vote! Although I have been out of the business for almost 30 years, back in the late 1970s and early 1980s I ran an operation with 5 "blower door crews" and as many as 15 techs doing remediation. We ran into sewer gas several times. I think seldom used dishwashers, showers or laundry drains were the most common culprits. We are in a dry climate and traps on unused drains dry out and become ineffective. A bad wax ring seal on a toilet could have the same effect -- leakage of sewer gas, which can smell like natural gas. The peppermint in the drain pipe is a great idea. We never thought of that!

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