Rainwater Harvesting & Use

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Metered water users & well users may be able to save money or provide more total supply by using rainwater for flushing.

Calculations need to be done to compare expected annual to proposed system cost.


  • Save money for metered customers
  • Ensure enough water supply for well users

Why not

Some countries are short of life sustaining water, and conservation is important. But Britain is not one of them. A typical 1/3 acre plot receiving an annual 56" of rainfall collects 1900 tonnes of water per annum, or 5.2 tonnes per day, averaged through the year. Water isn't in remotely short supply here, and pipeline costs are too high to pump it to any other country.

We have such an excess of water that we can afford to waste enormous quantities. Thames Water, with just 8 million customers, was reported to lose over 300 billion litres per annum through leaking pipes. That's almost a billion litres a day, and from just one company. More figures

Numbers show hosepipe bans reduce water use by less than 1% of leaky pipe losses.

Britain is not a dry country with a desert climate.

Rainwater Uses

Untreated rainwater can be used for

  • Toilet flushing
  • Garden watering

If suitably treated it can then be used for many other purposes.

How to Harvest

  • Rainwater downpipe
  • Divertor
  • Filter
  • Tank/barrel
  • Overflow device

A filter on the rain tank inlet will keep detritus out and reduce decomposition in the tank. This is normally enough precaution against decomposition, since the tank water is being rinsed through on an ongoing basis.

Commercially available divertors have filtering and overflow devices built in.

Adding More Storage

Sometimes there isn't enough space to replace the tank with a larger one. If so, 2 tanks can be connected together.

Tanks same size

  • the water outlet should be taken from the 2nd tank, not the water inlet tank. This avoids creating a dead leg where water can stagnate.
  • Pipe connecting the tanks should be near the bottom of the tanks
  • Short wide connecting pipe allows tank 2 to fill quickly. Long thin pipe may result in less water captured.

Different size

As above, plus

  • The tops of the 2 tanks should be level with each other. Tank bottoms don't have to be at the same level.
  • If the 2 tanks are different height, the taller one should be tank 2, otherwise one could only empty partly, which can cause stagnation.

How to Use Rainwater

System Designs

There are 2 main ways to do it.

Pumped direct to cistern

This method uses no header tank. The toilet cistern has a float switch, and when water is wanted, the pump moves water from outdoor storage tank to cistern directly.

  • Simpler cheaper design than a loft header tank
  • Cistern fill speed dependant on pump specs, an inadequate pump would mean slow fill.
  • Less to go wrong
  • Less cost
  • Easier access to the cistern float switch
  • Lower pump specs needed
  • Less pump power used
  • Good for 1 toilet

Vented loft header tank

A loft header tank provides the necessary water pressure for the loo cisterns, and this header tank has a float switch to operate the rainwater pump. When the header tank needs a topup, the pump delivers water to the header tank from the storage tank.

A 2nd float switch low down on the header tank detects low water level, and delivers mains water when the header water level gets low.

  • Loo cistern is unmodified and fed from this header tank, which means replumbing the cistern feed
  • Pump needs a higher head rating
  • Header tank gives a little additional water storage
  • Suited to multi-use setups, eg buildings with multiple flush cisterns.
  • Pump intake requires a filter to exlude fine grit, which could otherwise cause the cistern valve to let water dribble through continuously

Automatic mains water backup

Automatic mains backup for loo cistern fill can be provided in 2 main ways

1. With a headerless system, have the mains toilet cistern valve set to a slow fill. The cistern will then fill with rain and a little mains each time, and if rain runs out it will fill more slowly with mains.

2. For a header tank system, have a 2nd float valve in the rain header tank. Adjust it so that if the header tank water falls to 20% then mains water tops it up to that 20% mark. This system runs on all rain until rain is nearly run out, then mains is used. Fill times are not affected when running on mains.

Pump Specs

Whatever pump is used, check its head spec. Some pumps will have insufficient head to deliver any tanked water to a loft header tank.

Be aware that real world flow rate is generally far below the manufacturers claimed flow rate. Flow claims are normally based on there being no pipe connected to the pump outlet. In the real world:

  1. The pump operates against a column of water (or head) which provides back pressure
  2. Pipes have resistance to water flow

A better idea of real life flow can be gained by plotting head versus flow rate on x,y axes, and drawing a straight line from max flow to max head. This enables flow rates to be read off for any given head. This does not take piping resistance into account, which will lower flow rates further.


There are 2 Safety related issues with rain water:


  • Use a filter on the entrance to the storage tank to minimise muck
  • have a close fitting lid on the tank to keep insects out
  • only use rainwater for purposes safe for non-sterile water, eg loo flushing and garden watering.


  • Float switches and fill valves do fail at times.
  • To reduce switch failure, use an RC snubber across the switch contacts.
  • Check your cistern overflow can handle the flow from the pump by holding down the float switch so the pump continues. If it can't, either improve the overflow or reduce pumping rate until it can cope.

Also Relevant

  • Greywater use
  • Rainwater and bathing
  • A 15 minute shower uses more water than a bath
  • Water charges are based on an assumed relationship between water used and waste water, and both are charged for. Rain harvesting will change this relationship.
  • Water treatment
  • Legal issues
  • Toilets rarely flushed may suffer cistern water stagnation, and are better supplied with treated water.

See Also