Max Water Pressure from a gravity fed water tank

[abc]

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

[LLigetfa]

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

[jadnashua]

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.

[cacher_chick]

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.

[LLigetfa]

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.

[cacher_chick]

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.

[abc]

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

[toolaholic]

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 !

[LLigetfa]

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 1.81×10−5 kg/(m·s), 18.1 μPa.s or 1.81×10−5 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 Pa·s or 8.90 × 10−3 dyn·s/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): (Pa·s) = A × 10B/(T−C)
where A=2.414 × 10−5 Pa·s ; B = 247.8 K ; and C = 140 K.

[jadnashua]

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.

[abc]

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

[jadnashua]

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

[hj]

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.