Pressure testing pex heating loop, extremely slow leak

Jells

I finished a loop of 3/4 ring clamped pex, and realized all I had to do to pressure test it was close the shutoffs at either end, unscrew the purge valve below the circulator and screw in a 1/2" tee with a gauge and a ball valve to a compressor connection. I've never pressure tested a water job before, just gas. So I bring it up to 60psi, and over several hours it drops to 40. Do I have an actual leak?

I can't hear anything anywhere. Do I need to soapy water test every fitting? A few are fished and out of sight already. Could it be the O-ring on the pump isn't designed to seal dry? Maybe I should open the valve above it and get a little water on the ring. Ideas?

PC7060

Pressure test with air?  Or water?  

ChrisJ

I'd check it in the morning and see if it kept dropping.

Right now I'd guess a mixture of stretching and the area cooling down some.

mattmia2

60 psig to 40 psig is definitely a leak. put some water in it and you'll see where it is leaking

hot_rod

Check the gauge block, the schrader valve if it has one, a common small leak spot

HomerJSmith

My Dad always said, "If the rate of evaporation exceed the rate of drip, you don't have a leak".

Jells

HomerJSmith said:

My Dad always said, "If the rate of evaporation exceed the rate of drip, you don't have a leak".

And that's how you get stalactites hanging from your pipes!

Jells

Well, it's held at 40 psi for 15 hrs. I guess there's not really a leak! Probably @ChrisJ had it right.

mattmia2

where did that firs 20 psi go? There isn't that much expansion and contraction with temp of air or of the tubing.

ChrisJ

mattmia2 said:

where did that firs 20 psi go? There isn't that much expansion and contraction with temp of air or of the tubing.

Are you sure?

I've never pressure tested pex but that being the case I also have no way of knowing how much it expands or what effect it would have on the issue l pressure test.

I do recall my pressure going up and down due to temperature on a short run of black iron pipe.


Keep in mind, 20 psi isn't a volume of compressed air.

ethicalpaul

Plus Chris also mentioned expansion of the pipes I believe. Pex definitely can deform over time, easing the pressure (to a point).

Jells

ChrisJ said:

where did that firs 20 psi go? There isn't that much expansion and contraction with temp of air or of the tubing.

Are you sure? I've never pressure tested pex but that being the case I also have no way of knowing how much it expands or what effect it would have on the issue l pressure test. I do recall my pressure going up and down due to temperature on a short run of black iron pipe. Keep in mind, 20 psi isn't a volume of cozffxfxmpressed air.

I would not be surprised that the PEX tubing expanded under pressure.  The pressure was also pushing against the check valve in the feed szide, but there was only like 36 in between the check valve and the shut off to fill with leaked air. I guess there's not a one-to-one correspondence between pressure and volume, like 40 PSI has twice the volume of air of 20 psi.

I'm glad I did this anyhow, all this run is under the subfloor which is going to get laminate on top of it. So not very accessible! Plus, since it's running directly from the domestic water heater it's at street pressure not the usual low hydronic pressure.

After I first posted I did remember previously air pressure testing another system, and discovering a sweat joint between baseboard sections that I missed. It's not a bad idea...

ChrisJ

@Jells I don't know what your timeline looks like, but if you let it sit for several days I think you'll be able to judge if there's a leak or not.

You'll likely see it move up and down a little with temperature but it should remain about the same overall.

Jells

ChrisJ said:

@Jells I don't know what your timeline looks like, but if you let it sit for several days I think you'll be able to judge if there's a leak or not. You'll likely see it move up and down a little with temperature but it should remain about the same overall.

This apartment is a disaster so I'm sure there's things I can do for a few days! But I don't expect that it'll start leaking after 15 hours of holding. Leaks tend to leak. Thanks for the hand holding though.

EBEBRATT-Ed

Compressed air is hot as @ChrisJ so of course when it cools the pressure will drop. Nitrogen (or C02 which is cheaper) would be better.

Air will work just let it sit.

mattmia2

So it is up against a check valve, not a ball/gate/globe valve and on the other side of the check valve is about 40 psi?

When I pressure tested around 100ft of a combination of copper and pex I could mark the gauge on a piece of tape such that i had a mark lining up with the needle and another with one of the markings on the gauge so I could detect very small movement in the needle and none was detected.

Unless you have tens of degrees f temp swings I wouldn't expect much pressure change.(but I would not be at all surprised if a check valve isn't a perfect seal)

Jells

mattmia2 said:

So it is up against a check valve, not a ball/gate/globe valve and on the other side of the check valve is about 40 psi? 

The loop has ball valves on either end, but there is a check valve 3 ft from one, so a slow leak past the check could account for some of the pressure drop.

EBEBRATT-Ed

In that case the "leak" will eventually stop

rick in Alaska

We used to see our 200 psi pressure tests on fire sprinkler systems change as much as 40 psi in the two hours we tested depending if we tested in the morning or the evening, due to the sun coming out. So, we made sure to do our tests in the morning so the pressure wouldn't drop off. As long as it didn't drop below 200, we were good to go.
So, as long as it is still holding 40, I would let it go.
Rick

109A_5

Jells said:

it's at street pressure not the usual low hydronic pressure.

@Jells, Am I missing something here ? Isn't 40 at or near the minimum for "Street Pressure" I would put 60 (or the appropriate test pressure) back in it and see if it holds. I've seen car tires hold 20 ish for months, but won't hold 35 for more than a day or so due to the characteristics of the leak. Once the laminate is on top its a lot harder and more costly to repair, I'd be real sure there is no leak.