Rainwater Harvesting

On average a UK citizen consumes around 150 litres of mains water per day. Installing water saving devices such as aerated taps; shower heads; and dual flush toilets can reduce this consumption level to 120 litres a day. To achieve Code for Sustainable Homes level 3 & 4 (mandatory for Housing Associations) you need consumptions levels as low as 105 litres per person per day. CSH Level 5 & 6 is now mandatory for Housing Associations and to achieve this consumption levels must fall to 80 litres per person per day.

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Rainwater Harvesting can supply around 50% of domestic Mains Water supplies and nearly 85% of Commercial demands. For domestics, rainwater can be used for toilets, clothes washing, dishwashing, car washing & gardens. For Commercials they can be used for the majority of water demands except canteen washing and potable processes. we stock the full range of rainwater harvesting systems and work with Freerain (www.freerain.co.uk), one of the leading UK suppliers of rainwater harvesting systems for the UK domestic and commercial markets.
We can price match all listed Freerain prices and provide full consultation and guidance support. We believe in our strong customer care and technical distinction. Our free design service will consider factors like usage of water and collector space so as to ensure we help you choose the right system. We also offer full installation services for both the domestic and commercial sectors.

The following information will provide you with more details about Rain Water Harvesting. If you would like to find out more about purchasing a system and the packages we offer or any other a query give us a call on 01223852267 drop us a line on our Ask the Experts Tab and contact us

Rainwater Harvesting Systems

Types of Rainwater Harvesting Systems:

1. Garden Filter / Above Ground Systems:

These are not just your average water butt tanks, but they use a filter to clean the debris out of the rain water and a pump to feed garden taps etc. This allows the water to be used in hoses and sprinklers. You can also pump the cleaned water into separate storage tanks allowing a greater amount of stored water to be collected and saved.


Installing Rainwater Harvesting Systems on existing properties that is used for toilets or dishwashers can be complex as the tank should be underground and significant external and internal pipework will be required. To get around this you can have the tank at ground level, although they become susceptible to freezing in the extreme depths of winter.

The size of the system required depends on 2 factors:

–          The amount of water required by the property

–          The surface collection area. In the cases of houses, this is the roof area as you cannot collect more water than that which runs off the roof (this will disrupt other grey water issues).


2. Gravity Fed System:

In this system the rainwater is collected from the roof of the property and passed through a filter which removes large objects such as leaves. The water is then pumped to an underground storage tank. As the water settles in the water tank, any heavy particles will fall to the bottom of the tank. The water is then pumped to a Header tank located in the loft. The Header tank then uses a gravity system to feed the toilet system (similar to a vented heating system). The pressure provided by a Gravity system is generally not enough to supply washing machines or dishwashers.

The advantage of this system is that it is easier to install and requires less pumps, giving them a longer life expectancy. However, with it only being suitable for toilet flushing, it does not save as much water as you may like.



3. Direct Systems:

Direct systems are easy to install and provides a pressurised clean water supply to your toilets for flushing; washing machines and dishwashers for cleaning; and garden taps for watering etc as the are connected directly to the Mains supply.

The rainwater is collected through the drainpipe system and goes through an initial filter process that gets rid of larger particles such as leaves and twigs. From here it enters the storage tank where, as it settles, any large particles will float to the bottom of the tank. As water is needed from the tank it goes through a smaller filter before it heads to the house.


There is not enough annual rainfall in the UK to provide all water needs for a house. Thus the use of rainwater is limited to washing machines, toilets, gardens and dishwashers. Also, the rainwater would have to go through a much stricter (and expensive) cleaning process if it was to be used for bathing or heating systems.

The Storage tank is connected to the Mains Water supply in case it ever runs low of rain water it can then be topped up and continues to supply the necessary appliances.


4. Commercial Systems:

Commercial systems work in exactly the same principle as domestic systems, except with a larger roof space they can collect more rainwater and therefore can supply larger water needs. Commercial systems can be used solely for toilets (gravity fed systems) or for industrial purposes such as vehicle cleaning; cooling systems and manufacturing processes. They are especially useful for those with large water needs, such as nurseries; truck cleaning companies and farms.


The storage tanks can be “made to measure” or come “off the shelf” in pre made sizes.


5. The financial advantages of Rainwater Harvesting:

There is no incentive scheme (similar to the FiT or RHI) that applies to Rainwater Harvesting Systems. The savings are purely from reduced water bills. However, for companies the technology does come under a scheme called the Enhanced Capital Allowances (ECA’s).

Enhanced Capital Allowances allow companies to claim tax relief for investments in energy-efficient products that meet published energy efficient criteria. This helps to significantly reduce the payback period for energy-saving and carbon reducing projects and is designed to incentivise companies to buy energy efficient products. This scheme helps alleviate the initial capital outlay by allowing companies to claim back 100% of the cost in the first year, thereby giving the company a cash flow boost.

There are 3 ECA schemes which provide enhanced tax relief:

–          Energy saving equipment

–          Water efficient equipment

–          Low carbon dioxide emission cars

The ECA is open to ALL businesses that pay UK Corporation or income tax and provides 100% first year capital allowances on investments in energy saving equipment against taxable profits. Capital allowances are available for spending “on the provision of” equipment and thus can include costs associated with the installation of the equipment also. From 1st April 2008 loss making companies can also benefit from ECA’s by surrendering losses attributable to ECA’s in return of a cash payment from the Government.

Capital allowances take the place of depreciation of equipment in capital accounts and allow companies to claim 20% a year on a reducing basis. Thus, if a company spent £10,000 on a new piece of equipment it could claim capital allowances of £2,000 (20% of £10,000) for the year of investment. If the company’s corporation tax rate was 28%, then for the period of investment the company could reduce its tax bill by £560 (£2,000 @ 28%). The balance of £8,000 is carried forward to later years and is written off over a number of preceding years. If the company had bought an energy efficient product they could have claimed an ECA. This would have given the company a reduced tax bill of £2,800 (£10,000 @ 28%) for the first year, giving the company a cash flow advantage of £2,240 in the year of the investment. The 100% capital allowance relieves all the qualifying spending and there is no unrelieved spending to carry forward over the coming years. Thus the energy saving piece of equipment provides a cash flow boost of 22.4p for every pound invested versus a non-energy efficient product.

Loss making companies can also claim ECA’s by surrendering the losses in return for a cash payment. The ECA will be expressed as 19% of the loss that is surrendered. Therefore, if the company surrenders a loss of £10,000 then it will receive a payment of £1,900. Payable ECA’s are capped and are limited by the total of the companies PAYE and national insurance payments  for the year in which the claim is made or, if greater, £250,000.

ECA’s are only relevant for products that meet certain criteria. The scheme is designed to help companies invest in technologies that are energy efficient but may be more expensive than alternative technologies. The Energy Technology Criteria List (ETCL) details the criteria for each type of technology and the Energy Technology Product List (ETPL) lists those products in each category that meet them. The groups currently on the ETPL include:

–         Air to air energy recovery

–         Automatic monitoring and Targeting (AMT)

–         Boiler equipment

–         Combined Heat and Power

–         Compact Heat Exchange

–         Compressed air equipment

–         Heat pumps for space heating

–         Heating ventilation and air conditioning zone controls

–         Lighting

–         Motors and drives

–         Pipework insulation

–         Refrigeration equipment

–         Solar thermal systems

–         Warm air and radiant heaters


6. Rainwater Harvesting for the Agricultural Sector:

Rainwater harvesting can be used for arable and livestock farming (although it must be treated for livestock drinking). It can reduce the amount of water entering the slurry store or dirty water tank and thereby reducing the amount of slurry or dirty water you need to dispose of.

When considering RWH systems you need to be aware of Planning and Building Regulations (Part H and Part G); Water Fitting Regulations;  and Environmental Planning regulations.

Funding and Support Mechanisms are available through ECA’s and the “Rural Development Plan for England 2007-2013” which provides grants for RWH systems (see www.defra.gov.uk/rural/rdpe/index.htm). You will also need to consult an Environmental Health Officer and the Environment Agency before installing any systems.

Systems for Farms can be however small or large they require. The only factors that affect system size here is the collectable roof areas and budgets! For farms, the payback period can be as low as one or two years depending on the amount of water required and its use!


7. Sizing a Rainwater Harvesting System:

The amount of collectable rainwater (RWHsize) is equal to the collectable roof area multiplied the run off coefficient multiplied by the filter efficiency multiplied by annual rainfall. The roof area is calculated by the length times the width. The run-off coefficient is the amount of rainwater that hits the roof that can actually be collected. A coefficient of zero means no rainwater can be collected, whilst a coefficient of 1 means 100% can be collected. Typical coefficients are:

Pitched roof = 0.85

Flat Roof (smooth surface) = 0.55

Flat Roof (non-smooth surface) = 0.45

Filter efficiencies are supplied by the manufacturers. In any absence of a filter efficiency a figure of 0.8 (80%) should be used. Annual (or monthly) rainfall data can be obtained from the Met Office.

Let us assume we have a pitched roof area of 250 meters squared and annual rainfall of 500mm. Thus, the amount of rainfall we can collect is:

250 x 0.85 x 0.80 x 500 = 85,000 litres

85,000 / 1000 = 85 meters cubed of storage space as a maximum should be installed


Here are some of the standard systems that can be purchased directly from Cernunnos:

Wall Tank System:


800L: £1,305.60 (+VAT) Fully Installed & Commissioned

1600L: £1,733.40 (+VAT) Fully Installed & Commissioned

2400L: £2,173.62 (+VAT) Fully Installed & Commissioned

3200L: £2,611.08 (+VAT) Fully Installed & Commissioned

4000L: £2,747.70 (+VAT) Fully Installed & Commissioned

The Rain King 1,000 Garden System (from Freerain):


£1,649.35 (+VAT) Fully Installed & Commissioned

This is a 1,000 litre domestic system intended to be based underground. The system installation includes ground works, connections, plumbing and commissioning. We also offer a 10year workmanship warranty.

Related Documents:

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