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Had a bit of a snafu at home today that resulted in an empty 75 gallon tank getting refilled without opening a faucet. Faucet was opened only after the tank was full, which resulted in a long burst of pressurized air coming out. I'm worried this pressurized air may have caused damage to the pipes, fittings or fixtures. Could this be the case? These are copper pipes, some 3/4'', some 1/2'', in a 4,500 sq ft home.

UPDATE: For what it's worth, few weeks have passed and no leaks to be seen.

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    Interesting point of fact - water is almost incompressible, while air can be compressed quite a bit. Not sure what it means for your water system, but, it is a thing.
    – Wayne Werner
    Oct 1, 2016 at 21:41

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I wouldn't worry about it at all. A gallon of water is roughly 213 cubic inches. Assuming that you filled the tank completely (which is unlikely - see below), you would have displaced 213 * 75, or 15,975 cubic inches of air inside your plumbing. One atmosphere of pressure (14.7 psi) is defined as the pressure exerted by a 33.8 ft column of water. Assuming that all of the pipe in question is 3/4", you would have increased the pressure by 14.7 psi if the total pipe length was 472 feet.

But what actually happened is that the tank stopped filling when the air pressure reached equilibrium with the water pressure. The water that you filled the tank with was coming in at whatever pressure that your household water supply has (typically this is somewhere around 40psi). When the total pressure in the plumbing system reaches 40psi, the tank simply stops filling because there isn't a pressure differential. So basically, once the air you were displacing reached the pressure of your water supply, it would have simply stopped compressing it because the tank wouldn't be filling any more.

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  • Thanks a LOT for this very complete answer. Actually it makes perfect sense that once the air pressure reaches equilibrium with the water pressure, it just stops filling. So the maximum pressure that was reached was my water pressure. Obviously this can't cause damage. For what it's worth, my water pressure is 70 psi, was 80 psi before I adjusted my pressure down a little. I heard 80 psi is at the top of the acceptable range. What is the maximum pressure a house piping system can safely sustain before incurring issues? I know it's a different question...
    – sw1337
    Sep 30, 2016 at 16:35
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    @MelLeet - Pretty much anything will be fine if it's regulated to 80psi. If the pipe is copper, you'd be fine up to around 300psi at the worst case. If it's PEX, IIR the maximum sustained pressure would be around 150psi. I can't speak for the fixtures at those pressure though. Really high water pressures are much more likely to be dangerous to the occupant than the plumbing - a shower at 150psi would not be pleasant in the least.
    – Comintern
    Sep 30, 2016 at 16:44
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    @jpaugh - It doesn't matter - it's measured per square inch across the plane at the bottom of the column. A wider column will have more square inches, but the pressure on each one would be the same.
    – Comintern
    Sep 30, 2016 at 20:44
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    Your first paragraph doesn't make any sense to me... what does 472 feet of pipe have to do with anything? And are you assuming all the pipes are filled with air or water in this hypothetical? The real (and only) answer is your second paragraph: the air pressure will increase only enough to balance the incoming water main pressure.
    – Hank
    Oct 1, 2016 at 3:42
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    I'm wondering, since watertight and airtight are different things, is there any chance that some part of the plumbing started leaking air and became damaged? Could a constant flow of high-pressure air bypassing something like a seal have any measurable effect on it?
    – JDT
    Oct 1, 2016 at 12:58
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The pressure of the air trapped in the system is exactly the same as the water pressure. There is no area for concern pressure-wise.

However, air is much less dense than water, so these problems could occur:

  • tiny leaks which were clogged by debris might be cleared (unclogged) by air and might introduce a new drip
  • water surge hammering might shake pipes and loosen joints, remove scale and oxidation from pipes and cause a valve to not work as well as before (debris in the washer seal)
  • having air in the pipe tends to accelerate rust and corrosion (whereas water has much less oxygen in it and tends to slow oxidation)
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  • Yup, we have had faucet damage from the blast of pressurized air that inevitably escapes after plumbing work.
    – Loren Pechtel
    Sep 30, 2016 at 21:45
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The concern is actually about the top heating element burning out if you forget to purge the air before energizing the circuit. Be wary of being distracted by an impatient family in haste to use hot water again.

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