How to install a rainwater harvesting system

Paul Gormley talks through the process of installing a rainwater harvesting system.

A rainwater harvesting system collects water from a house’s roof to be used in the garden or for some domestic appliances. A rainwater harvesting system can be installed to save water, or planners may ask for a system to be installed to help with potential local flooding issues.

Key considerations for installing a rainwater harvesting system:
Presuming the system has been sized correctly, there are a few key elements to consider when installing a rainwater harvesting system:

Drainage
It is important to bring together all downpipes into one pipe, for entry into the tank. They should only carry roof water; surface water should bypass the rainwater system as it can contain dirt and oil. It is important to ensure that pipework is kept as shallow as possible. If it’s too deep (beyond a metre), the tank will have to be installed deep in the ground which will have an impact on the ability to clean the filter.

Installing the tank

In most cases a flat, shallow tank is preferable. This avoids deep excavations and makes future maintenance easier.
The tank will have an inflow and an overflow. The overflow needs to go to a soakaway or surface water drain. It is important not to go too close to the foundations of the property. If you are tight for space, then a good rule of thumb is the tank needs to be at least twice the distance away from the building as it is deep in the ground.
Backfilling around a tank should be done with concrete or a compactable gravel, depending on the tank. We only supply tanks that are strong enough for gravel backfill but there are some cheap tanks that require concrete.

Installing the pump
A pump needs to be installed in the tank to send the water back to the property. It is important that the pump is not carried or lowered by its cable – this is a common problem which can cause problems with water getting into the electrics. Lower the pump with a rope to prevent any damage. The pump pipe connected to the pump must be green and black MDPE pipe (similar to standard water pipe). It is vitally important that fittings are sealed and completely watertight; the use of PTFE tape is recommended for the fitting that goes directly on the pump.

Internal plumbing
The internal pipework must be a dedicated feed to the toilets and washing machine. There can be no physical connection between the mains water supply and the rainwater supply, so your plumber must be aware of this while he is working on the installation. Pipe marking tape must also be used to identify the internal plumbing. Many suppliers do not supply this as part of the kit, therefore you should check when ordering. It is also important that you label the water main to identify that there is a rainwater recovery system installed in the property. You also need to label outside taps and appliances to ensure the water supply is not misidentified. Again, ensure you have labels included with the kit upon ordering.

Other things to consider:

Filtration
Leaf filters are mostly built into the tanks and filters are also installed on the pump to stop particles coming through. If necessary, you can also buy fine particle filters which can be installed within the property to ensure there is no risk of any small particles blocking toilet valves. Our systems include a very fine particle filter with the kit to ensure you get no problems when the system is being used.

Mains back up system
To ensure there is an uninterrupted water supply to the building, a mains water top up is used to feed a small amount of mains water back to the tank in the event that there is no rain and the tank is almost empty. This is normally a magnetic valve and sensor. The valve opens when the sensor in the tank registers there no water.

The sensor should be set very slightly higher than the inlet of the pump.

The mains back up kit, which is sometimes a box unit, needs to be installed in the house. This is normally done in a utility room or under a sink. It is important that the water can flow back to the tank through a dedicated 110mm pipe, or via the drainage from the roof.

There must also be an air gap between your mains water and the pipe that it feeding the tank. This prevents the possibility of cross contamination between mains water and rainwater.

The quality of the mains back up kit will depend of the system you have purchased. Some use heavy duty brass magnetic valves and probe sensors, other cheaper systems use lower quality valves and cheap float switches. With this, it doesn’t pay to scrimp on quality.

Sign off from the water authority
The water authority will want to make sure you have adhered to the regulations; therefore you must be able to prove the following:

Identifying labels are installed at the appropriate locations
The pipe from the pump is regulation green and black pipe
The mains water top up has an air gap to prevent cross contamination
Your internal pipework has identifying taps

It is worth taking photos of the internal pipework to prove that identifying tape has been used, since it can often be covered up following installation.

Summary: 5 key tips
Keep tank as shallow as possible

This is not only good for you as it requires less digging and less excavated earth, it is better for the homeowner as the filter is then easy to access.

Test pump when complete system is installed

Once the system has been installed and all the plumbing has been done, test the pump by flushing a toilet. The pump should turn on to feed the toilet and off again when the cistern is full. The pump should then stay off. If it turns back on again then you have a small leak in the pipework

Always use a particle filter

Whilst standard rainwater filters are good, installing a fine particle filter will ensure it is not possible for any fine particles to get into the toilet valves creating a leak. Leaks will cause the pump to turn on and off so this “belt and braces” measure is highly recommended

Add a pressure vessel

A pressure vessel (or expansion vessel) like on a heating system, helps to maintain the pressure in the pipework. This is very advantageous with a rainwater harvesting system as it can stop a pump from turning on in the even that someone leaves an outside tap dripping. This will can prolong the life of and protect the pump.

Keep things clean

We’ve seen many builders have to return to properties to unblock toilet valves because too much debris has entered the tank during the installation. Try to keep the tank clean and make sure that everyone on site knows there is a rainwater harvester connected to the drainage. Furthermore, once the roof has been constructed, you may get a small amount of grit entering the tank, this should be caught by the filter but we would highly suggest checking the filter and giving it a clean prior to handover to the client.

This guide was written by Paul Gormley, technical sales manager at rainwater harvesting specialists Owls Hall Environmental.