Reduce use without spoiling function, and keep reused water separate from the wholesome supply.
Water efficiency means using less water for the same useful job. A fitting that makes people flush twice, or run the tap much longer, can wipe out the saving.
Follow the water's origin
Greywater is used water from baths, showers, washbasins and the like, but not from WCs. Rainwater harvesting collects rainfall, usually from roofs. Neither becomes drinking water because it looks clear. Softening changes the water's minerals; it does not turn mains water into greywater.
A reuse system must treat and manage the water it collects to suit what it will be used for. WCs can be flushed with alternative water of suitable quality, but the kitchen drinking tap cannot be fed with it. Identify the separate pipes and outlets, prevent cross-connections, and plan for maintenance and for the treatment failing.
A mains top-up still needs separation
A rainwater tank may need a mains top-up in dry weather. Joining the two supplies through a pair of ordinary valves is not acceptable separation. Use a break tank or air gap suited to the assessed risk, and agree the installation with the water undertaker. The alternative water must never cover or touch the protected mains inlet.
Keep the fitting's function
Choose a WC pan and flushing arrangement that work together at the intended flush volume. A reduced-flush button is part of a designed system. Turning down the cistern's inlet valve slows the refill; it does not reliably cut the volume a full cistern flushes. Loose objects in the cistern can jam the mechanism.
For taps and showers, match flow-control fittings to the appliance and supply. Repair leaks as well as reducing unnecessary running time.
Worked example: a tap runs for three minutes at 12 litres/minute: 12 × 3 = 36 litres. At 8 litres/minute for the same time, it uses 24 litres: a saving of 12 litres. If the slower tap then runs for 4½ minutes, it uses 36 litres again. Compare the completed task, not just the advertised flow rate.
Recall check: bath water collected for treatment is greywater; roof runoff is rainwater; WC discharge is neither. Each needs collection, treatment and distribution suited to its own risk.
A flow-limiting outlet cuts the litres per minute, if it suits the supply. Some aerators mix air into the stream without cutting the flow, so check the product’s rating. At a hot outlet, using less water also saves the energy to heat it, and on a metered supply it cuts the water bill too. For rainwater or greywater reuse, count the pumps and treatment as well: saving mains water does not always save energy overall.
How much a flush uses
An older WC cistern commonly used 9 litres or more per flush; a modern low-volume cistern uses 6 litres or less, and a dual-flush suite typically offers something like 6 litres for the full flush and 4 litres for the reduced one.
Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 caps any single WC flush at 6 litres, and where a cistern gives different volumes, the lesser flush must not exceed two-thirds of the largest one.
What the water fittings rules are for
The water fittings rules exist to prevent waste, misuse, undue consumption and contamination of the water supply, not to maximise pressure and flow or to control how a fitting looks. A fitting can be any finish or style, provided it performs correctly and protects the supply.
Shower or bath: which uses less
An efficient shower generally uses less water than filling a bath, even run for several minutes, because the flow is spread over the washing time rather than filling a fixed volume. The exception is a long, high-flow power shower, which can use as much water as a bath, or more.