A gravity-fed water system moves water downhill through a pipe with no pump, and it is almost always the right choice when the source sits at least 10 meters above the settlement. Every 10 meters of vertical drop gives about 1 bar of pressure, the tap with the least pressure needs at least 0.5 bar (5 meters of head), and a pipe section should not carry more than 60 meters (about 6 bar).
Part of Community Water System
A gravity-fed system is a water supply whose only energy is the height of the source above the users: water falls through a buried pipe to storage and standpipes with no motor, fuel or electricity.
Gravity Is Free And Never Breaks Down
A pump needs fuel, parts and someone who understands it. Gravity needs none of these. Water in a pipe pushes at the bottom with a pressure set by the height of the water above, and that height is the whole engine. This is why the community water system page calls a gravity-fed design almost always the right choice where a source stands above the settlement.
The numbers to hold in mind are three. Ten meters of vertical drop is about 1 bar. The tap with the least pressure, which is the highest or the farthest one, needs at least 0.5 bar, that is 5 meters of head, to run properly. And a pipe section should hold no more than 60 meters of static head, about 6 bar; beyond that the line is divided with a break pressure tank (Break Pressure Tanks).
Static head is the pressure with the taps closed. Once water flows, friction in the pipe uses up some of the head (Friction Losses), which is why a long thin pipe delivers less than a short wide one from the same height.
What You Need
- A source higher than the highest tap by at least 10 m
- A way to measure height — a water-tube level works, see Water Level
- Pipe rated for the highest static pressure on the line (Pipe Materials)
- A storage tank to hold at least one day’s supply
Method: Check Whether Gravity Is Enough
Step 1: Measure the vertical drop from the water surface at the source to the highest tap you want to serve.
Step 2: Compare. Under about 10 m the supply may still work but leaves little margin for friction; over 60 m you need a break pressure tank.
Step 3: Subtract friction to find what is left at the tap (Head Calculation).
Step 4: Bleed air from high points when the line is first filled; trapped air stops flow (Air Valves).
Worked Example
A spring is 25 m above the highest tap. Static head is 25 m, about 25 ÷ 10 = 2.5 bar with taps closed. The minimum at the tap is 5 m, so the friction allowance is 25 - 5 = 20 m for the whole run. That is generous: a 400 m run of 32 mm pipe carrying 0.33 L/s loses about 3 m (Friction Losses), leaving about 22 m, or 2.2 bar, at the tap.
The same pipe 90 m below its source would carry about 9 bar static, which is over the 6 bar limit, so a tank is needed partway down.
Common Mistakes
- Assuming more height is always better. Over 60 m the pipe and the taps are overstressed.
- Measuring along the slope. Head is the vertical difference, not the length of the pipe.
- Skipping the air vents. An airlocked high point can stop the flow of a line that has plenty of head.
Safety
A pipe under pressure can burst and throw fragments
A gravity line holds its full static head whenever the taps are shut. Use pipe rated above that pressure, divide lines over 60 m with a break pressure tank, and test with water after bleeding the air. Never test plastic pipe with compressed air: it stores far more energy and can shatter.
What’s Next
- Learn next: Head Calculation — turning heights into pressure at each tap
- Combine with: Ram Pump — lifting water when the source is below the settlement