Gravity feed or pump: which suits your site?

Water runs downhill for free. If you can put a tank high enough above the tap, you may not need a pump or any power at all. The catch is working out what "high enough" means for your site.

How gravity feed works

Gravity feed is simply using gravity to move a liquid from one place to another. Wikipedia calls it a simple means of moving liquid without a pump¹. Ancient Roman aqueducts were gravity-fed, and so are some village water supplies today. The flow comes from the hydraulic head, the vertical distance from the intake at the source to the outflow¹.

Head is a height, as the page on flow, lift and head explains. Fresh water pushes with about 9.8 kPa for every metre of height². So a tank 3 metres above the tap gives roughly 29 kPa at the tap before friction, using that figure². That's a gentle pressure.

Early hydraulic dock machinery used the same idea. Water was pumped to a tank at the top of a tower, and the height of the water above ground provided the pressure³.

What friction does

Height is only half of the story. The flow through the pipe is opposed by friction, which depends mainly on the pipe's length and diameter, as well as its age and material¹.

The hydraulic head page gives a clear way to think about it. Take the energy of the water in a raised tank and subtract what the pipe walls take away. If the result is negative, the water can't flow at the rate you want, and the tank must be raised².

So a short, wide pipe from a high tank works well. A long, thin pipe from a low tank may only trickle.

When gravity is enough

• You have a slope or a stand that lets the tank sit well above the highest tap.
• The taps you want to run need only a gentle flow.
• You'd like water to be there without needing power, say in a remote shed.

When you need a pump

• The only place for a tank is level with the taps, or below them.
• You want stronger flow than the height can give.
• The water source is lower than where you use it.

Using both

You can combine the two ideas. A university guide describes pumping into a closed storage tank that then gravity feeds into the stock tank⁴. The pump moves water up when there's power, and gravity delivers it.

Another guide suggests a storage tank big enough for at least three days of water demand, to cover nights and cloudy days⁵. That figure is for livestock, so size yours to your own use.

For this mix, a small pump driven by a panel can fill a header tank during the day, and gravity does the rest overnight.

Mistakes to avoid

• Measuring the wrong height. What counts is the drop from the water level in the tank to the outlet, not the height of the stand.
• Using narrow pipe. Friction depends on length and diameter¹, so a thin pipe over a long run can choke the flow.
• Skipping the test. Fill the tank, open the tap and watch the flow before you build anything permanent.

If you do add a pump, the tank, switch and tap layout is the next page to read.