Gas Line Static Pressure Loss in New Construction

Question

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?

Answer 1

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.

Answer 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.