Pressure Test with Air during construction - reliable?

[DCFan9]

The plumbers installed all of the plumbing lines last week and before they left, they pressurized the lines to 30 PSI. This held the pressure of 30 PSI for about 1/2 day then it dropped to 15 PSI the next day. The PB said they do not use water to pressure test the lines (at this point) in the event they would freeze (yea, we hit 30 degrees this week). How long would you expect to maintain any pressure in the plumbing lines for an air test? Should i be concerned that it dropped to 0 PSI after a 3-4 days?

thank, MJ

 

[diyindux]

In theory, they should hold pressure indefinitely but with hard lines it doesn't require much volume loss to see the pressure drop. It's not like a rubber bladder or tire where the pressure expands what is holding it. That tends to slow down the pressure drop.

What did they use to plug the lines? If they sprayed soapy water on the threaded plugs and there were no bubbles then it must have leaked somewhere else. But if the plumbing had a leak it should have been evident as a pressure drop once the line was charged. Air will leak out much easier than water so it is actually a better way to test lines. The advantage to using water that is then pressurized with air is that if there is a leak it is much easier to find.

Since it held constant for 1/2 a day, I wonder if something let go. What did your PB say? Did they pressurize it again? If not, it might be worth a second try. By the way, I pressure tested my lines with air and only left it for about an hour. Most plumbing only gets tested for that long so you maybe seeing impacts from temperature fluctuations.

 

[jblizzle]

As the temp drops at night the pressure will also drop. When were your 2 measurements? If one was in the afternoon, then the second was first thing in the morning, then the pressure will be lower. You should check it as the same time of day when the temps are more likely to be the same.

That said, I do not think the temp would account for 50% loss.

 

[mgarf33]

If it dropped to 0 you have a leak. Whether it be at the test plugs or elsewhere.
Pressure testing is more accurate with use of Nitrogen for less pressure temperature fluctuation. It's likely the fittings they used to plug were leaking, but it doesn't hurt to retest. PVC glue doesn't always seal 100%, and there are times when a human may make a mistake and not glue a fitting. Unglued fittings may hold pressure for a short time and then loosen under pressure. Screwed fittings may have been over tightened and the additional pressure may cause the female fitting to crack.
At least your PB pressure tested, I had to perform my own test.