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A gas run loses about 0.2 PSI per hour when tested with pressurized air (went from 15PSI to ~4PSI over ~51 hours).

The loss seems to level out as the PSI gets lower and I think stops losing altogether at a certain point in the few-PSI range, which makes me wonder if it's fine since our natural gas is 0.25PSI / 7 in. WC.

Another run that's being pressure tested loses absolutely nothing as I can tell. Thoughts on if this is safe? Does anyone know if this is up to code or not too?

EDIT: as an update, I had the plumber find and fix the leak. It ended up being right near the pressure gauge but I would have wondered if it was in the house had it not been found. Thanks everyone for you advice on this.

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    If you believe in this sort of thing ;) any gas leak is also a climate change burden (methane is 34x worse than CO2). Think of the millions of miles of gas piping in the US and all the leaks. Cities cite this as a reason to electrify (read: ban gas).
    – Harper - Reinstate Monica
    Commented Oct 14, 2022 at 3:59
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    Of course it slows down as the pressure drops. The rate of leak will be proportional to the pressure differential between the inside and outside of a pipe. Even at low pressure it's still leaking, just not at a rate you can easily measure with equipment designed to measure significantly higher pressures. Don't be an idiot - get this fixed. Honestly, I can't believe "how bad does a gas leak have to be before it's too bad" is even a question.
    – SiHa
    Commented Oct 14, 2022 at 12:51
  • Where I live, you can call the gas company if you have a leak (i.e.: smell gas) and they will show up at pretty much any hour of the day and fix it at no cost.
    – JimmyJames
    Commented Oct 14, 2022 at 18:22
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    @SiHa: Since there's no such thing as a leak rate being exactly 0.0000, of course that's a question. The answer may be "any leak fast enough to detect is a problem", which is indistinguishable from but doesn't actually mean that there is no threshold below which leaks can be safely ignored.
    – Ben Voigt
    Commented Oct 14, 2022 at 18:57
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    New construction with a pressure test? zero. Old work? I use my dad's 80yo nose. If I can smell it, it should be fixed. If he can smell it, the building is to be evacuated.
    – Mazura
    Commented Oct 15, 2022 at 13:38

2 Answers 2

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The Uniform Mechanical Code section 1314 asks you to test at 10psi or more. It defers to your Authority Having Jurisdiction (AHJ) on what a leak is, though it does specify 'no perceptible loss of pressure' in fifteen minutes. Check with your AHJ. Personally I would call that a leak but I'm picky.

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    Exactly. You test at a pressure where a leak will be easily detected.
    – SiHa
    Commented Oct 14, 2022 at 12:48
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    What is an AHJ, Accelerated Hornet Jousting?
    – MonkeyZeus
    Commented Oct 14, 2022 at 13:50
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    AHJ = Authority Having Juristiction, usually refers to "who's in charge of enforcing the relevant building/gas/electrical/etc code"
    – maples
    Commented Oct 14, 2022 at 13:53
  • Thanks, someone else expanded AHJ for me and I decided to expand UMC as well to avoid confusion.
    – KMJ
    Commented Oct 14, 2022 at 14:57
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The rate of the pressure loss is only one question in the safety equation.

The real question you have to assess is whether or not the gas is pooling up to a dangerous gas/air ratio near a source of combustion. Just because you don't smell mercaptan does not mean the gas has dissipated.

Only your local ordinance can tell you what's acceptable but here is what Home Depot has to say:

Acceptable Gas Line Pressure Drop

In order to pass gas line pressure tests, lines must hold a specific psi for the length of time mandated by local codes. When testing for leaks, there will be some natural rises and drops in the pressure of the gas line.

It is common for standard gas line inspections to require lines hold three times their working pressure load for at least 30 minutes. Typically, if the gas line loses approximately 2 psi from a test of 20 psi on the line, the lines are acceptable. Conversely, if the line loses more than 10% of the pressure, it may have a leak. This can be true after a 30 minute or even a 24 hour test.

Environmental factors also affect gas line fluctuations. Heat will cause line pressures to rise while cold temperatures will create a drop in pressure. As days and seasons progress, these conditions correct. It is normal to see drops overnight, and they do not typically indicate leaks in the line.

https://www.homedepot.com/c/ai/how-to-pressure-test-a-gas-line/9ba683603be9fa5395fab9013ef5e5fa

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