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Old 11-29-2015, 09:43 PM   #1
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Water system losing air pressure while blowing out for winterization

Curious if it is normal for the water system to loose pressure while blowing out the pipes?

I was having a slow pressure loss, but chocked it up to a loose water heater drain plug I could hear leaking as I was pressurizing.

Then I went to blow out my other trailer, it wouldn't hold pressure for much time at all.

Both trailers have no water leaks and good running plumbing systems.
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Old 11-29-2015, 10:38 PM   #2
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Yep ...
https://www.physicsforums.com/thread...ghness.142407/
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Old 11-30-2015, 07:07 AM   #3
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Every residential water system in a new house or building is pressure tested with air, so it is confusing to me why an RV system can't hold air.
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Old 11-30-2015, 07:33 AM   #4
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Curious if it is normal for the water system to loose pressure while blowing out the pipes?
Yes, it's entirely normal. When you're blowing out the pipes the idea is to have someplace for the water to go, which means the faucets should be open. An open system will not retain air. Blowing out the pipes is not a pressure test.
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Old 11-30-2015, 07:44 AM   #5
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Yes, it's entirely normal. When you're blowing out the pipes the idea is to have someplace for the water to go, which means the faucets should be open. An open system will not retain air. Blowing out the pipes is not a pressure test.
Seems like 1/2 the people close the entire system, pressurize, then open faucets one by one, the the other 1/2 the folks leave a faucet open while blowing.

It would seam to me closing all of the faucets and pressurizing the system should result in the system holding air pressure.

If I left a faucet open, then I would expect results as you mention.
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Old 11-30-2015, 08:36 AM   #6
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Seems like 1/2 the people close the entire system, pressurize, then open faucets one by one, the the other 1/2 the folks leave a faucet open while blowing.
Since I have a compressor with an air tank, I don't need to build pressure in the pipes. But if you are using a tankless compressor, you should allow pressure to build first, then open a faucet. Basically the pipes take the place of the air tank in that case.

But if you're one of those folks who build air pressure in the pipes, remember that watertight is not the same as airtight. The pipes should hold air pressure up to the rating of your built-in water pressure regulator (typically 40 or 45 psi), but if you build up more air pressure than that, you can expect air leaks.
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Old 11-30-2015, 08:37 AM   #7
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The check valves used in the water system along with pressure relief valves could have slow air leaks but won't allow water thru. There are at least 2 check valves, one at the pump and one at the city water connection.
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Old 11-30-2015, 08:43 AM   #8
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Since I have a compressor with an air tank, I don't need to build pressure in the pipes. But if you are using a tankless compressor, you should allow pressure to build first, then open a faucet. Basically the pipes take the place of the air tank in that case.

But if you're one of those folks who build air pressure in the pipes, remember that watertight is not the same as airtight. The pipes should hold air pressure up to the rating of your built-in water pressure regulator (typically 40 or 45 psi), but if you build up more air pressure than that, you can expect air leaks.
I am building pressure only to 20-25 psi. I have a shop compressor, but I am trying to use my travel compressor so I learn how to winterize on the road.

So you guys think there are slow leaks the air can get through and not water?

I'm not sure if I am believing that just yet, we test all plumbing systems with air.

Curious from those who pressurize their systems at the blow out if they experience pressure loss.
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Old 11-30-2015, 08:44 AM   #9
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Water system loosing air pressure while blowing out for winterization

Again, from the link I posted above, air molecules are smaller than water molecules, so a plumbing system may not leak water, but could leak air.
Very simple ...
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Old 11-30-2015, 09:15 AM   #10
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Again, from the link I posted above, air molecules are smaller than water molecules, so a plumbing system may not leak water, but could leak air.
Very simple ...
So if were talking vapor diffusion vs bulk water, then I believe you and the link.

But since every new building water distribution in the country is tested with air, which I have seen a thousand times, I do not believe it.

Very simple.
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Old 11-30-2015, 09:16 AM   #11
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Again, from the link I posted above, air molecules are smaller than water molecules, so a plumbing system may not leak water, but could leak air.
O2 and N2 and CO2 are smaller molecules than H2O? That's news to me. It's not smaller molecules that allows air to leak from a watertight system, it's viscosity. Liquid water is more viscous than air; liquid water molecules stick together.

Not only is watertight not the same as airtight, it's not the same as steam-tight either. Water vapor can leak where liquid water can't, so it's not molecule size that makes the difference. We're talking about a plumbing system, not a permeable membrane like a reverse-osmosis water purifier. For a permeable membrane molecule size does make a difference, and water molecules are smaller.

But that's not important to the OP, only to engineers and scientists. For the OP's purposes, it's enough to know that a plumbing system that's watertight can still leak air if there's enough pressure.
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Old 11-30-2015, 09:32 AM   #12
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Protagonist

You are a mechanical engineer (even retried engineers are still engineers ) so you know we test building domestic supply with air, so from your experience, shouldn't our systems hold air?

One on the reasons why this is important to me is because it would be a good way to check for leaks as routine maintenance. Especially if I silence the pump with an isolation mount, flex hoses, etc. It just doesn't make sense I keep loosing pressure, correct?
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Old 11-30-2015, 10:10 AM   #13
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You are a mechanical engineer (even retried engineers are still engineers ) so you know we test building domestic supply with air, so from your experience, shouldn't our systems hold air?
As I said in an earlier post, it SHOULD hold air at least up to the pressure setting of the built-in pressure regulator, which is probably set somewhere around 40 to 45 psi. Just because your system will hold water up to 45 psi— and air up to the same pressure— there's no guarantee that it will hold higher pressures.

I don't know what factor of safety Airstream uses for their freshwater systems. If it was me I'd want my 45 psi water system to be airtight to 60 psi or better, but then I'm not Jackson Center. Given the number of people whose Airstream's fresh water systems develop leaks when their pressure regulators fail, I'd say the factor of safety is closer to 1.0 than I'm comfortable with.

I'd suggest that if you use a tankless compressor for blowing out your pipes, always use one that has a built-in pressure gauge, so that you know exactly how much pressure you're putting into your system.
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Old 11-30-2015, 04:03 PM   #14
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Looks like testing RV water systems with compressed air is not uncommon, here is a good article on it. Even more interesting, looks like RV manufacturers actually do it just like in building construction.

The RV Doctor: Properly Testing RV Fresh Water System

I'm going to learn how to do it at some point.
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