Had LP gas lines installed in new construction build. Everything is capped. At the outside of the house where a regulator will go in the future they installed a pressure gauge and charged the gas lines.

This was done on 9/22, the air temperature that day was 92F and the gauge read 15 PSI. I have checked the gauge a few times and it has been slowly going down. Today the high temperature was 53F and the gauge is reading 9 PSI.

I communicated this to the gas line company today, asking if it should be checked before insulation and drywall go in, and they said the pressure would change as the temperature drops and it is normal.

So the question is what is a normal pressure drop due to temperature change in a statically charged gas line? If pressure fluctuation is normal then how does installing a gauge verify anything one way or the other? What basis do I use to either insist on further inspection or take their word for it?

  • Pressure drop with temp drop is normal. If it is suppose be close to 1 to 1 is the question. I do not think it should be that much.
    – crip659
    Oct 30, 2023 at 23:00
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    Expecting any normal gas piping system to hold that pressure for over a month is ridiculous. Get some soap solution in a spray bottle and check each joint for bubbles. Any leak worth bothering about will be detected almost immediately.
    – kreemoweet
    Oct 30, 2023 at 23:49
  • @kreemoweet are you aware of anything in code that specifies what an acceptable loss rate is? I don't mean that argumentatively. Just trying to figure out how to approach this.
    – FM2020RI
    Oct 30, 2023 at 23:52
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    Just to re-emphasize: you're testing at 30x working pressure for a month. Ask them to bring it back to 15psi and check it in 24 hours. Assuming the day to day temperature is similar, I really can't imagine you're going to see any difference on the gauge. Oct 31, 2023 at 1:22

2 Answers 2


Air is basically an ideal gas. Given fixed volume, then, your temperature change would have changed the pressure to

(15.0 psi)(52°F-(-460.°F)) / (92°F-(-460.°F)) = 13.9 psi.

The pressure test duration for single family dwellings is 10 minutes and no longer than 24 hours and the 15 psi pressure is something like 3000% of the LP's operating pressure, so they might realize that there's a bit of a leak. Nobody wants to hear, "yeah, uh, that leak is in spec," so the temperature claim is a good excuse.

Insist enough and most contractors will humor you, but nobody is obligated to come fix this theoretic leak a month after the pressure test. Assuming that this theoretic leak bleeds air at a constant rate regardless of pressure (this isn't the place for calculus), during the 10 minute test your month old leak would have registered as a pressure loss of

(13.9 psi - 9 psi)(10m) / [(30d)(24h/d)(60m/h)] = 0.0011 psi.

Nobody reading a pressure gauge used for these tests will notice this tiny pressure loss. It legitimately passed inspection.

More Excuses:

Iron pipes absorb odorant smell and off gas it for a while after removal from service. Maybe your pipes absorbed the air. Maybe the air diffused out of the lines. I understand that 2 liter soda bottles get overcharged with gas because the gas diffuses through the plastic over time.

  • Thank you. This answer is along the lines of what I was looking for. So regardless of up and down daily temperature change the equation you provided suggests a maximum pressure drop to 13.9 PSI? So I have 4.9 PSI pressure drop outside of what is explainable by temperature variation? Are you suggesting there ACTUALLY IS an acceptable leak rate?
    – FM2020RI
    Oct 30, 2023 at 23:31
  • There actually is, though I certainly wouldn't accept it.
    – KMJ
    Oct 30, 2023 at 23:34
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    @FM2020RI, I'm not a gas person or anything, but my understanding is that the code limits the use of the more leaky fittings. You're not allowed to use a union, for instance, in a confined space where a leak's gas could accumulate. As long as I still had access, I might check fittings inside gas-confining spaces like wall cavities. You're about to lose access and a reasonable contractor would fix those. But yeah, there is an acceptable leak rate. In a pissing contest, your contractor could come out and execute a 10 minute test instead of searching for leaks.
    – popham
    Oct 30, 2023 at 23:38

If this were temperature related, it wouldn't be slowly going down. It would be going down when it gets cooler, then up when it gets warmer. The reading it had at 52F last week overnight would be the same as the reading it has at 52F this week.

It appears there is a small leak in your freshly installed plumbing. If the contractor isn't interested in coming out and looking at it, time to make up some very soapy water and go after all the joints. I bet you find one that is making very tiny bubbles. It won't be quick to notice, as it's taken a month to leak down this amount.

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    A leak this small is likely to go unnoticed in practical terms once the building is constructed, as it would get ventilated out through normal activity. I wouldn't want to be the one to find out the hard way that it actually was a dangerous leak though.
    – KMJ
    Oct 30, 2023 at 23:04

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