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Rainwater harvesting - Technical Tips when considering rainwater harvesting

TECHNICAL TIPS

Surprising as it may seem, England is short of water, and there is now official encouragement to reduce mains water consumption both by maximising the use of rainwater and using water-efficient appliances. You may have seen recent ads by DEFRA in national newspapers exhorting us to save water. As yet there are no grants or tax incentives for the homeowner to install rainwater harvesting systems. However, building regs. are changing in 2010 to make a 120 litre maximum per person per day the norm and planners now often require rainwater harvesting.

Marcus Bicknell at RainWaterHarvesting.co.uk lists key points to be considered when installing a system.

1. Retrofit or New-build
Installing a rainwater harvesting system is more cheaply and easily done in the context of a new-build or major renovation project. Installation is not complicated and can be done by a builder or plumber without specialist knowledge. Cost of equipment for a 3 to 4 bedroomed house will typically range between £2,000 and £3,000 depending on the tank size and mains back up accessory chosen. In a retrofit, installation costs could be another £1-2,000 (digging the hole and fitting separate rainwater pipes to appliances), whereas in a new-build the digger and plumber would be on site already.

2. Size of tank
This depends on several factors, the most important of which are:
1) roof area (how much rain you can potentially collect)
2) rainfall (much higher in Scotland, for example, than south east England)
3) what the rainwater will be used for (number of WCs and washing machines, large garden, car-washing, etc)
4) the drought protection period desired (e.g. two or 3 weeks of rainwater during a dry spell).
In terms of cost effectiveness, it is recommended to put in as large a tank as possible (6500 litres in the Carat range). For the relative small difference in price between the tank sizes, and the installation cost which will not vary so much, it is worthwhile putting in a large tank.

3. Installation of tank
Many people are surprised when they see the “transit van” size of even a 2700 litre tank in comparison to the 200 litre water butts they are used to for their garden. However, the Carat range has made delivery and installation much easier by manufacturing the tanks in two halves which are put together on site. They can, therefore, be installed in small back gardens. The tank should be located in the most suitable place for capturing rainwater from the whole roof. Guttering and downpipes may have to be altered.

To protect the foundation of the building, no part of the tank can come within a line drawn at 45 degrees downwards and outwards from the point where the house wall meets ground. Usually this is 2 to 3 metres away from the house.

The Carat tank comes with a turret which is extendable to allow the tank to be buried deeper depending on the land layout, and particularly if it will lie under a hard-standing where vehicles will pass. A concrete base is recommended and the tank should gradually be filled while back-filling with gravel. The tank can be installed where there is a high water table if concreted up half way to the shoulders.

4. Filters and Pumps
Filters are either installed in the downpipe or in the tank entrance. In the case of the Carat range, the self-cleaning filter is in the turret. To ensure that the filter self cleans efficiently, the pipe bringing rainwater in should be at a sufficient angle so that the water pressure pushes debris out through the siphon.

The pressure-sensitive pump normally sits in the bottom of the tank and provides water when it is needed. The pump is better submersed as it works more effectively pushing rather than sucking water. A floating pipe intake ensures the cleanest water enters the pump. A service pipe from the tank turret carries the pump power lead and the pipe carrying pumped water to where it is needed both inside and outside the building.


5.Main Drain or Soakaway
Excess water and debris can either go if convenient back to the main drain or to a soakaway. The soakaway should ideally be double the volume of the tank, placed 2 metres away, and the outlet pipe from the tank siphon to the soakaway should be at a 2.5% angle to allow a good flow. If the roofs collecting rainwater are surrounded by trees, a leaf-collection chamber should be put between the tank and soakaway.

6.Pipework
Once the separate supply network to WCs and washing machines has been completed, it is essential to flush the tank out to ensure no grit is left which might stop any solenoid valves in mains back systems working effectively.

7.Mains Back up
A mains back-up facility is essential when using rainwater to supply WCs and the washing machine. There are currently three options.

1) A float switch in the tank connected to a solenoid valve, with a tundish to provide the required airbreak so rainwater can’t enter the mains system. This is the cheapest solution. RainWaterHarvesting.co.uk now supplies the Rain Back Up in a Box which contains all the components in a neat and easily installed box. If the level drops too low, the float switch activates the valve which allows just enough mains water into the tank to feed the appliances.

2) A rain manager has the pump, valve and a small tank in a unit usually placed in the utility room or garage. This system allows the small tank to fill with mains water when the main rainwater tank is empty, and this small tank feeds the appliances. This allows easy access to the pump, but the pump will be activated every time a toilet is flushed. This can be noisy as the unit will usually be installed in the house.

3) The Rain Director® is the smart header tank and controller which not only provides fail-safe mains backup when rainwater runs out, but also has other advantages:
a) Less pump cycles, saving 8 times normal pump energy use, and prolonging pump life
b) Avoids discoloured water
c) Assures water supply during power cuts
d) Automatic and user-controlled functions e.g. holiday and flush modes.
The product has three components, a roof space header tank, the electronic control unit which activates mains water and rainwater valves and the pump installed in the main storage tank.


It’s not rocket science to install a rainwaterharvesting system. For an average investment of £2000 to £3000, it is about the easiest and cheapest of the many green options around to take up and benefit from.

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