Gravity-fed water systems

October 30, 2014 Niki Morrell

Our water comes from a stream flowing through our property. It’s piped to two tanks — a 1,000 litre break tank and a combined break/header tank — and then gravity-fed to our livestock troughs and our house.

The great advantage of a gravity-fed supply is that you’re not reliant on a pump. If you live in a region that often suffers severe weather and the attendant risk of power cuts, not being dependent on a pump is a very good thing.

Gravity-fed system - break tanks

The main disadvantage of our gravity-fed system is the potential vulnerability of the source to drought, logging activities and earthquake damage, but none of these have happened so far and the stream has served us well.

If you’re thinking of installing a gravity-fed system from a dam or a stream, here’s what you should take into account:

1. Head

This is the difference in elevation between the source and the storage point. For us, that’s about 30 metres from the stream to the tanks, with a further 7.8 metres from the header tank to the house. Note that head has nothing to do with the distance between the two points; just the elevation.

2. Pressure

The more head, the higher the pressure. The higher the pressure, the greater the likelihood of exploding pipes unless you can control it. See below.

3. Flow

Flow determines how quickly your tank will re-fill. It’s governed by both pressure and the internal diameter of your pipe.

4. Pipe and fittings

Pipe and fittings must be correctly rated for your pressure and for each other. If the pressure’s too high for them, they’ll burst. If the pressure’s too low, you’ll have wasted money buying piping that was more expensive than you actually needed.

Wherever possible, you should always bury your pipe so it’s not affected by temperature fluctuations. Pipe expands and contracts in heat and cold and this, in turn, affects water pressure and temperature.

5. Filtration

Our water is filtered as it leaves the source (mesh), again as it enters the header tank (mesh) and once more on leaving the tank (mesh plus paper). This is a low-cost option and does the job very well.

A word of advice from someone with lots of experience in this particular area: if your flow starts dribbling down to zero, check the filters first before freaking out at the prospect of burst pipes underground.

Gravity-fed system - click to view larger image

Three ways to control pressure on hillside pipelines

1. Buy pipe rated for high pressure. This may seem an expensive option but excavating and replacing burst pipe isn’t exactly cheap either.

2. Install a break tank with a trough valve (ball-cock) half-way between the source and the destination.

3. Install pressure-regulating valves. These are expensive and very sensitive to silt and debris in the water, so you’ll need a good filtration system in order for them to perform efficiently.

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