It seems to be the general recommendation to turn off the main water supply when you're away from the house for longer (some even recommend it for overnight absences).

My question is:
Should the water system stay under pressure (= don't open any faucets after closing the main valve),
or should it be left depressurized?

Does either way produce any potential issues? Does it hurt the system to be without pressure for weeks or even months?

  • Pressurizing and depressurizing often(everyday) would probably not do the system any good. Not using water(turned off) for months will let p-traps dry out and let sewer gas in. In case of a leak system will depressurize quite fast, but spray water over larger area than just drip out, amount should be almost the same, depending where the leak is.
    – crip659
    Aug 2, 2021 at 21:51
  • it's probably better for many things to be without pressure; looking at you washer hoses...
    – dandavis
    Aug 3, 2021 at 0:55
  • Unless there is a potential for freezing, I don't think it pays to shut the water supply off for an overnight absence. For a long trip away from home, I'll turn the supply water off as well as the gas line, and keep all faucets fully open in order not to get hammered when turning the water back on.
    – r13
    Sep 2, 2021 at 1:49
  • Turn off the water main if you're gone overnight? Really?? I was gone for 3 weeks and we just made sure that the taps were closed tight (actually, I don't even think we checked water before we left, just ass-u-med that everything was off because we didn't hear dripping) & the lights were off!
    – FreeMan
    Sep 2, 2021 at 12:34

1 Answer 1


Turning the water off can be dangerous if there is a hot water heater in the system. As the water heats it will expand, this will happen when hot water is used for example taking a shower. Normally when this happens the water flows back to the source. With the supply valve closed it has no place to go the pressure rises. It is not much but in a closed system pressure can rise very rapidly. Conversely as you remove pressure the boiling point of water in the tank drops. This is OK, it is normal however if a leak develops and the water level in the heater drops things can become problematic quickly. For a day or so I would not worry about it and and if you're comfortable with that leave the water on. For vacations etc where you will be gone for an extended time if you turn the water off be sure to turn the hot water off.

  • 2
    Don't they prevent backflow into the source around the main, to avoid contaminating the whole city's water?
    – dandavis
    Aug 3, 2021 at 0:54
  • And don't they have pressure relief valves on HW tanks?
    – DJohnM
    Aug 3, 2021 at 6:01
  • @dandavis Not in all cases. It seems more of a recommendation than a requirement. A few questions on here about HWs burning out/stopped working when city did work on their pipes.
    – crip659
    Aug 3, 2021 at 11:44
  • 1
    In Florida, at least, a thermal expansion tank is required to be installed with new water heaters. I imagine this is the case in most jurisdictions.
    – Edwin
    Dec 31, 2021 at 1:34
  • I agree, for some setups you should shut off the water heater if shutting off the water for more than a brief time. My home was built in the 80's with no expansion tank, pressure reducer, or backflow prevention. The water heater does have a pressure relief valve but unless it releases, thermal expansion just causes back feeding to the supply.
    – izzy
    Dec 31, 2021 at 2:34

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