Max Water Pressure from a gravity fed water tank

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Gringo, Nov 17, 2012.

  1. Gringo

    Gringo New Member

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    I have a water tank on the roof to gravity-feed the house. Will I get more pressure coming out of the tank with a 2" line mounted on the side wall of the tank (at the bottom), or a 2" line mounted on the bottom surface of the tank in the middle?

    Thanks
    Paul
    Last edited by a moderator: Nov 19, 2012
  2. LLigetfa

    LLigetfa DIYer, not in the trades

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    The only way to get more pressure is to raise the water level.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Don't confuse volume with pressure. The larger the pipe, the more volume you can get, but it will have no impact on pressure. If you try to get more volume than a pipe can supply, all outlets will slow down. This is where larger pipes or higher pressure make a difference.

    Taking a shower with a gravity fed supply can be less than thrilling...there, you need a pump or height to increase the pressure.
  4. cacher_chick

    cacher_chick Test, Don't Guess!

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    If your water level is 15 feet above grade, you will have 6.5 psi. This will work for showering if you have a lot of volume. It will not work well using one of today's regulated shower heads.
  5. LLigetfa

    LLigetfa DIYer, not in the trades

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    Ja, but the showerhead is not likely to be at grade. If the showerhead is 7.5 feet above grade the pressure will be 3.25 PSI.
  6. cacher_chick

    cacher_chick Test, Don't Guess!

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    Very true.
    This may very well be much of the reason people did not commonly have showers in their homes 100 years ago. :p
  7. Gringo

    Gringo New Member

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    Another Question Regarding Gravity Flow System

    Lets say the tank (on roof) has a capacity of 1200 liters. If I come out of the tank with a 1" line, run this line across the roof (horizontaly) and then reduce to a 3/4" line vertically downwards through the walls to the fixtures, what size of air inlet tube do I need at the top of the tank to maximize pressure throughout the system?

    Thank you
    Paul
  8. toolaholic

    toolaholic General Contractor Carpenter

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    Just add a pump !
  9. LLigetfa

    LLigetfa DIYer, not in the trades

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    The answer is "it depends" on the temperature of the air and the water and the length of the tube. My guess is 1/4" would do it.

    According to Wikipedia, it is just simple math.
    http://en.wikipedia.org/wiki/Viscosity#Viscosity_of_air
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    How is the tank filled? When I lived in the Middle East, they pumped water into the roof-top tank(s) from a tanker truck in one place, but in another, there was municipal water, but it only came on for an hour or so during any 24-hour period. That one had a float valve to shut off the city water once the tank got full. The showers were pretty anemic! In one place, we had a 3-story apartment, and the showers on the lower floors were definately better (but not great) than those on the top floor.

    You could install a vacuum breaker on the tank, and it would allow air in as the tank emptied. You'd need another valve to let air out if you pumped water into the tank through a fitting, rather than a door.

    To maximize house pressure, you could install a pump similar to what is used on a well along with a bladder storage tank. You'd need an automatic safety shutoff (float switch) to disable the pump if the tank got too low since running the pump dry tends to ruin them quickly.
  11. Gringo

    Gringo New Member

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    13
    Gravity Fed System

    If the tank on the roof has sufficient venting at it's top, are additional air vent tubes necessary at various points in the sytem?

    Paul
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    No need for additional vents in the supply system...not to be confused with vents for the drainage system which is totally separate.
  13. hj

    hj Moderator & Master Plumber Staff Member

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    You probably have an "open tank" system, so there is no need for vents of any kind. The pressure delivered to the toilet, shower, or sink depends on how high the "top of the water" is from the fixture. The lower the faucet, the more pressure you will have. A shower head, because it is "high" above the floor will be closer to the "water level" and thus have less pressure. It has NOTHING to do with the pipe sizes.
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