New to plumbing here, I was trying to do a DIY and add an expansion tank to my water heater. I noticed it was dripping from the fitting, so I tried to take it off to apply more tapes and pipe dope, but when I took it off almost completely from the adapter, the water started to shoot out from the expansion tank like crazy. I was all wet through, and about 1 gallon of pressurized water shot out from the expansion tank.

I did pressurize the tank to 68psi, which is the same as my faucet. Is there something I missed? I did forget to turn off the main water valve or the cold water valve. Could it be the only reason that the expansion tank shot out water to half the room?

Any advice is appreciated.

  • Water expanded. The tank did what it was designed to do and absorbed the expansion. You then removed the tank, and the water it had taken up was ejected by the 68PSI you pumped into it. What's surprising?
    – keshlam
    Apr 2 at 3:18
  • Thanks for the input, the thing is that I have not fired up the water heater yet. How come the expansion tank starts to accumulate that much water in less than 10 mins from the input cold water line? Excuse me if my questions may seem a bit too dumb. Apr 2 at 3:22
  • What pressure is your water supply at?
    – keshlam
    Apr 2 at 3:23
  • 68psi, it is a new water heater, is it that I should empty the air from the hot or cold water line first? Apr 2 at 3:25

1 Answer 1


Earth air pressure is about 14 psi. Your expansion tank has a balloon in it, and you filled it with 68 psi air. So the balloon probably took up nearly ALL the space in the tank. Then you hooked it up to water at 68 psi, which would have squished the balloon until it was taking up only half the tank.

So then you remove the tank, and now you have 68 psi balloon pushing against water that now only has 14 psi of earth air holding it in.

So you get wet.

  • 1
    Thanks, longneck, just wondering, if my expansion tank is 68 and the water pressure is 68psi, isn't that water should not get in the tank at all at this point since the pressure is equal? I thought the expansion tank only gets pumped water when the water heater tank expands to a high than the water pressure level Apr 2 at 3:40
  • 1
    ok, I think i know the explanation now, so pumping the 68 psi to the balloon didn't probably fill the whole exp tank, so that water still got in. and I was basically taking off a pressurized water tank with 68psi water, once it got in contact with 14 psi air, all the water shot out. I should have emptied the water heater tank or used the pressure valve first to release the pressure. Apr 2 at 3:57
  • @ShirleyGuo good comments, why is the expansion tank called "expansion"
    – asinine
    Apr 2 at 4:03
  • @Ruskes thanks for being so kind. I guess the water pressure being 68 psi is the reason we could still get hot/cold water on the 3rd floor or even higher floors? And the water heater tank once filled up it should be 68psi inside as well? what I meant is that if the whole water system works correctly, every water pipe or water tank should be at roughly the same as the main water supply? Apr 2 at 4:13

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