(206) 949-5683, Top Rated Plumber, Seattle
Results 1 to 13 of 13

Thread: Max Water Pressure from a gravity fed water tank

  1. #1
    DIY Junior Member abc's Avatar
    Join Date
    Nov 2012
    Location
    Mexico
    Posts
    8

    Default Max Water Pressure from a gravity fed water tank

    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 Terry; 11-19-2012 at 05:11 PM.

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
    Join Date
    Feb 2011
    Location
    NW Ontario, Canada
    Posts
    3,677

    Default

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

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,390

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Test, Don't Guess! cacher_chick's Avatar
    Join Date
    Jan 2008
    Location
    Land of Cheese
    Posts
    3,142

    Default

    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. #5
    DIYer, not in the trades LLigetfa's Avatar
    Join Date
    Feb 2011
    Location
    NW Ontario, Canada
    Posts
    3,677

    Default

    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. #6
    Test, Don't Guess! cacher_chick's Avatar
    Join Date
    Jan 2008
    Location
    Land of Cheese
    Posts
    3,142

    Default

    Quote Originally Posted by LLigetfa View Post
    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.
    Very true.
    This may very well be much of the reason people did not commonly have showers in their homes 100 years ago.

  7. #7
    DIY Junior Member abc's Avatar
    Join Date
    Nov 2012
    Location
    Mexico
    Posts
    8

    Default 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. #8
    General Contractor Carpenter toolaholic's Avatar
    Join Date
    Jul 2005
    Location
    Marin Co. Ca.
    Posts
    829

    Default

    Quote Originally Posted by abc View Post
    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
    Just add a pump !

  9. #9
    DIYer, not in the trades LLigetfa's Avatar
    Join Date
    Feb 2011
    Location
    NW Ontario, Canada
    Posts
    3,677

    Default

    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
    Viscosity of air
    Pressure dependence of the dynamic viscosity of dry air at the temperatures of 300, 400 and 500 K
    The viscosity of air depends mostly on the temperature. At 15.0 C, the viscosity of air is 1.8110−5 kg/(ms), 18.1 μPa.s or 1.8110−5 Pa.s. One can get the viscosity of air as a function of temperature from the Gas Viscosity Calculator
    Viscosity of water


    Dynamic Viscosity of Water
    The dynamic viscosity of water is 8.90 10−4 Pas or 8.90 10−3 dyns/cm2 or 0.890 cP at about 25 C.
    Water has a viscosity of 0.0091 poise at 25 C, or 1 centipoise at 20 C.
    As a function of temperature T (K): (Pas) = A 10B/(T−C)
    where A=2.414 10−5 Pas ; B = 247.8 K ; and C = 140 K.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,390

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    DIY Junior Member abc's Avatar
    Join Date
    Nov 2012
    Location
    Mexico
    Posts
    8

    Default 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. #12
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,390

    Default

    No need for additional vents in the supply system...not to be confused with vents for the drainage system which is totally separate.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  13. #13
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Location
    Cave Creek, Arizona
    Posts
    25,632

    Default

    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.
    Licensed residential and commercial plumber

Similar Threads

  1. gravity water pressure
    By deform in forum Plumbing Forum, Professional & DIY Advice, Tips & Tricks
    Replies: 2
    Last Post: 04-21-2012, 03:27 PM
  2. Gravity fed water system help
    By extendedpath in forum Pumps and Tanks Well Forum & Blog
    Replies: 26
    Last Post: 03-20-2011, 09:23 AM
  3. Gravity Fed Hot Water
    By ScottinOhio in forum Solar and Geothermal Water Heating Forum
    Replies: 1
    Last Post: 01-04-2010, 05:43 AM
  4. American Standard Cadet: Gravity vs Pressure Assist
    By Remodlr in forum Toilet Forum discussions
    Replies: 3
    Last Post: 10-29-2005, 06:03 PM
  5. Gravity-Flow hot water recirc.
    By chucknewell in forum Plumbing Forum, Professional & DIY Advice, Tips & Tricks
    Replies: 6
    Last Post: 02-23-2005, 05:57 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •