Rainwater Harvesting Pump Basics and What to Ask an Installer

Collecting rainwater can cut how much mains water you use for jobs that do not need drinking-quality water. The pump is the part that gets stored water to where it is used. This guide explains the basics and gives you questions to put to an installer.

How a system works

A typical system has a catchment surface (usually a roof), a storage tank, filtration, a pump and labelled pipework. Per the Designing Buildings wiki, the system may pump water directly to points of use, or pump it to an elevated cistern before gravity feed. Pumping directly to points of use is viable in domestic buildings because the water travels a short distance.

Typical uses listed there include flushing toilets, supplying washing machines, irrigation and sprinklers, and vehicle washing. Approved Document G describes harvested rainwater as water that, when appropriately treated, may replace wholesome water in WCs, urinals, irrigation or washing machines. It is not for drinking.

Is it legal?

One UK supplier says collecting rainwater is perfectly legal and encouraged by most water companies, and that planning permission is generally not needed under permitted development, though you should check with your local planning office. The same page stresses requirements that apply "at time of writing", so verify them before you start.

The pump: what to understand

The sources we checked do not compare submersible and external pump types, so ask your installer to explain the choice for your setup. Questions to cover:

Where will the pump sit (in the tank or in a separate unit), and how will it be reached for servicing?
What flow and pressure does it provide, and are these enough for every outlet you plan to supply at the same time?
How does it protect itself if the tank runs low?
How noisy is it, and where is it located relative to living spaces?
What are the energy use and replacement costs? The Designing Buildings wiki lists pump energy and replacement costs as factors in the financial analysis. We have not verified any figures.

Mains back-up and backflow

Systems often need a mains top-up for dry spells. The rainharvesting.co.uk page says any mains connection needs a physical air gap to stop non-potable water being siphoned back into the mains, and cites the Water Supply (Water Fittings) Regulations 1999 for England and Wales, with separate rules for Scotland and Northern Ireland. Ask your installer to show you exactly how this is achieved on your system.

Labelling

Harvested rainwater must be labelled as non-potable. Pipes, valves, outlets and appliances should be tagged, with examples including the back of a washing machine and the inside of a WC cistern lid. The wiki also says BS 8515 Annex C sets colour standards to distinguish the pipework from the mains supply.

Standards and notification

BS 8515:2009, "Rainwater Harvesting Systems, Code of Practice", covers non-potable applications. The wiki says it sets standards for installation, testing and maintenance, filtration, tank manufacture and installation, and tank sizing. We could not confirm whether a newer edition replaced it, so ask your installer which version they work to.

A UK water-regulations page lists items that may need notifying to your water supplier, including changing a building's use in a way that adds water systems such as rainwater harvesting, and pumps or boosters delivering over 12 litres per minute. The list is not exhaustive. Your water supplier can advise and grant consent, and the page says consent takes a maximum of ten working days and is free. It also mentions WaterSafe approved plumbers. Ask your installer whether they will handle any notification.

Questions for installers

Which standard do you design and install to, and can I see the design calculations, including tank size?
Which pump do you propose, and why? What happens if it fails?
How is the mains top-up arranged, and how is backflow prevented?
Will you notify my water supplier if needed?
How are pipes and outlets labelled?
What servicing does the system need, and who does it? The wiki suggests cleaning filters every few months, cleaning roofs annually and replacing UV lamps.
What does the guarantee cover, in writing?
Which parts need a registered electrician or plumber, and who is carrying that out? Hire qualified professionals for all electrical and plumbing work.

Realistic expectations

Online claims about savings vary. One source cited via the wiki says up to 50% of domestic mains supply can be replaced, but we have not verified this, so ask installers to estimate based on your roof area and water use rather than relying on headline figures.

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General information, not engineering, legal or plumbing advice. Check current rules with your water supplier and local authority.