Rainwater harvesting is one of the few sustainability measures that pays back through a simple mechanism: free water falls on your roof, and you avoid buying mains water to flush toilets, irrigate landscaping or top up cooling systems. The concept is easy to sell to a client. The engineering, however, lives entirely in the details — and a system that is sized, filtered and protected badly will underperform, grow biofilm, or quietly cross-contaminate the potable supply.
Two references frame good practice and are worth naming at the start: **BS EN 16941-1:2018** (On-site non-potable water systems — Part 1: systems for the use of rainwater) and, in the UK, **BS 8515:2009+A1:2013** (Rainwater harvesting systems — code of practice). This guide walks facilities engineers, specifiers and importers through the decisions those standards imply: storage sizing, first-flush diversion, filtration, pumps and, crucially, valve selection.
The classic mistake is to size a tank from annual rainfall alone. What matters is the balance between what you collect and what you draw, week by week, through the driest stretch of the year. A workable method:
**Catchment yield** — multiply roof area (m²) by local rainfall (mm) by a runoff coefficient of roughly 0.8–0.9 for a smooth roof, then by filter efficiency (typically about 0.9). Convert the result to litres per month.
**Demand** — add up non-potable draw: toilet flushing, irrigation, vehicle or bin washing, and any cooling make-up. Irrigation peaks in summer, exactly when rainfall is lowest, so seasonal timing matters.
**Balance and buffer** — plot inflow against demand month by month; the tank must bridge the longest deficit. A reasonable default is a storage volume equal to roughly two to three weeks of average daily non-potable demand, refined by a daily water-balance calculation.
Do not oversize. A tank that never turns over keeps water for months, letting sediment accumulate and quality decay; a tank that is too small overflows constantly and delivers little. The sweet spot is the smallest volume that covers the dry spell with an acceptable mains top-up.
The first rain off a roof carries the worst of it: dust, pollen, bird droppings and roof debris. Divert the first 0.5–2 mm of rainfall — or a fixed volume proportional to roof area — to waste before any water enters the tank. A simple first-flush diverter with an automatic reset, backed by gutter guards and a leaf screen, removes most gross contamination and dramatically reduces sediment build-up in the store. It also extends the life of every downstream filter.
Treat filtration as a train, not a single device. Pre-tank, use gutter mesh and a leaf trap to keep large debris out. At the tank inlet, a calmed inlet prevents the incoming flow from stirring up settled sediment and aerating the store. After the tank, a self-cleaning downpipe or inline filter with a rated efficiency in the region of 90 per cent will remove finer solids before the pump. For higher-grade reuse — spray irrigation, or any application where aerosol is generated — add point-of-use filtration and consider disinfection. BS EN 16941-1 distinguishes water-quality categories, and your filter and treatment train should be chosen to meet the category your end use demands, not the cheapest option on the shelf.
Most commercial systems use a submersible pump in or beside the tank. Size the pump against the peak simultaneous demand at the required pressure, not the average flow, and confirm the duty point sits on the pump curve rather than at its limit. Variable-speed control keeps pressure steady as taps open and close; dry-run protection is essential, because a harvesting pump will eventually run the tank low during a dry spell. Fit a float switch or level electrode to stop the pump before it cavitates, and specify a pump class suitable for the water temperature and any treatment used.
This is where rainwater systems most often get into trouble, and it is the part Xindong's customers ask about most. Two rules dominate.
First, the mains top-up must never be able to back-siphon into the potable supply. Provide a physical air gap where possible; where that is not feasible, use a verifiable mechanical backflow prevention arrangement designed in line with **EN 1717** and the local water-fittings regulations. A mains top-up float valve in a rainwater tank is a classic cross-connection risk, so keep the potable fill above the tank's maximum water level or use a break-tank arrangement.
Second, rainwater contacting valves carries sediment that mains water does not. Choose valves rated for the water quality and debris load — full-bore isolation valves, robust float valves and strainers ahead of control valves — rather than domestic brass fittings that will erode or jam. In systems where the primary feed is genuinely non-potable, only the mains make-up and any mixing point need potable-water approval; specify WRAS-approved or equivalent fittings on that potable leg.
Non-potable pipework must be clearly and permanently identified as rainwater wherever it could be mistaken for drinking water, and outlets should be labelled. Provide a sediment trap and a means to drain and clean the tank, keep tanks dark and sealed against light and ingress, and schedule periodic inspection of filters, first-flush devices and the pump. Where stored water could generate aerosols, apply a site risk assessment in the spirit of the UK's ACOP L8.
Before you buy, ask your supplier for: a stated filter efficiency and the particle size it removes; the valve's pressure rating, bore and suitability for debris-laden water; the potable-water approval and its scope on the make-up leg; material and finish designations; and evidence of pressure/leak testing. For the components that touch the potable leg, insist on traceable approval; for the non-potable valves, insist on durability.
**Zhejiang Xindong Sanitary Ware Co., Ltd.** manufactures taps, thermostatic mixing and shower valves, and brass and stainless-steel fittings for importers, specifiers and OEM programmes from its facility in Yuhuan, Zhejiang. Its engineering and testing capability supports custom valve development and project-specific documentation. Learn more at [cn-xindong.com](https://www.cn-xindong.com).
Ask us for a rainwater-system valve and fitting selection guide, or send your duty point for a matched quotation.
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The company, founded in 1999 and located in Yuhuan,specializes in taps, thermostatic shower valves and shower kits. With over 30,000 sqm of space and 380+ employees, we hold numerous certifications and 65+ patents.
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