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I am trying to think about how many times I could flush a toilet in my house if I have to shut off the water to do a DIY project or if some disaster disrupts our water service.

Since atmospheric pressure can push water up to a height of 10 meters (33 feet) and my house is not that tall, it seems that the atmospheric pressure would push the water up and prevent me from using most of it. I think typically the only water in the system I could use would be the water that gets displaced with air bubbles going into a faucet or pipe as I draw water from it, and that only works for a body of water that is in contact with some air and is above that air, so the shape of your pipes determines how much water you can draw with that method.

To allow for more toilet flushes while the water is off, would it be possible to "shotgun" the plumbing system in the house, just like people shotgun beers? I am thinking I would turn on the highest component of the system (e.g. a shower on the 2nd story) in order to let air flow into the top of the system. Then I should be able to use a lot of the water from the pipes in order to flush toilets on the ground floor.

Would this plan work? Could it damage my plumbing?

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    The thinking of your question seems confused to me. Atmospheric pressure only pushes water up to 10 meters when it's in a pipe with perfect vacuum at the top. When you crack the top of the system, that lets the atmosphere into the top, breaking the vacuum, and the only remaining force in play is gravity. The net effect IS the shotgunning you describe but not for the reasons you give. – The Evil Greebo Oct 3 '16 at 15:15
  • Many apartment buildings achieve water pressure in their entire unit by filling a storage tank on the roof and then gravity feeding to apts below. Some buildings use pumps to fill the tank, but some rely on utility pressure. – bib Oct 3 '16 at 16:42
  • This question seems a bit off topic to me. – Tester101 Oct 3 '16 at 17:48
  • The toilet tank should store just enough water for a single flush. – Tester101 Oct 3 '16 at 17:52
  • Greebo: I am sorry I confused you, but I think we are in agreement on the facts. – David Grayson Oct 3 '16 at 18:24
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You are correct that you should be able to get a dribble of water out of your pipes this way. But you are probably overestimating the usefulness; my guess is you have less than a gallon in your entire household plumbing. (Of course it depends on your home and you can do your own estimation based on pipe size and total length.) One toilet flush uses about 1.5 gallons (or more, depending on the design).

If you want to have access to water in an emergency I would suggest storing some yourself. You can buy large water jugs at most supermarkets. You can refill a toilet tank yourself by just pouring water into the tank until the float lifts up. And/or if you are expecting a service outage you can fill a bathtub with water.

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    Well, a gallon of hot and a gallon of cold maybe ;) – The Evil Greebo Oct 3 '16 at 15:19
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    @TheEvilGreebo I try to keep about 50 gallons of hot water stored in my basement. – Tester101 Oct 3 '16 at 17:43
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    But not in pipes, I'll bet. :p – The Evil Greebo Oct 3 '16 at 19:20
  • I tried shotgunning my water system tonight. I couldn't actually get any noticeable water to flow into the toilet tanks. Maybe they require a certain minimum pressure in order to let water in, or maybe there is backflow prevention in the system. I am accepting this answer because yes, in practice, my plan doesn't really seem to work. – David Grayson Oct 4 '16 at 4:50
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If you didn't have the chance to cache any in the tub in advance and don't care about hot water, you could drain quite a bit from your heater to flush. I'd turn off the gas or breaker to it so you don't burn it up if you were going to do this though. You'd only have a few gallons at most within your pipes but you should be able to drain all but 5-10 gallons from your tank into some kind of container (shorter the better).

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Yes, venting the system from a high point would release the vacuum and give you access to more water more quickly. This is a common tactic when draining pipes for solder work.

Effectiveness depends a lot on the vertical layout of the system. Opening several high faucets would typically release more water. That said, pressure is now dependent on gravity and height, and will be much lower than a municipal service typically provides.

A more reliable and longer term approach would be to keep a few gallons of water on hand. Fill some old milk jugs or buckets and flush with that, or borrow some from a neighbor.

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    Seriously you're not gonna have enough water stored in the pipes to matter. 7.48 gallons per cubic foot, 1/2 inch pipe, 10 feet high, if my math is right, about half a gallon. You might get a gallon out of the pipes in a tall house. – The Evil Greebo Oct 3 '16 at 15:19

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