Downpipe First Flush Diverters



Downpipe First Flush Water Diverters improve water quality and reduce tank maintenance by preventing the first flush of water, which may contain roof contaminants, from entering the tank. They help ensure cleaner water is available for use, which protects rainwater pumps and internal household appliances such as clothes washing machines, toilets, hot water systems, etc.

A simple practical approach- Diverting as little as 25 litres of water will significantly improve water quality divert as much water as the system can afford to maximize water quality.

Downpipe First Flush Diverters to fit 65mm, 80mm, 90mm and 100mm can be installed at each downpipe that supplies water to the tank or as a single unit at the tank. They utilise a dependable ball and seat system – a simple automatic system that does not rely on mechanical parts or manual intervention. As the water level rises in the diverter chamber, the ball floats, and once the chamber is full, the ball rests on a seat inside the diverter chamber preventing any further water entering the diverter. The subsequent flow of water is then automatically directed along the pipe system to the tank.

The first flush units have been redesigned to incorporate New First Flush Tee, Advanced Release Valve and Rapid Exit Release Funnel.

Downpipe First Flush System

65 mm 100 mm RLWDDP20C ADVANCED
80 mm 100 mm RLWDDP20B ADVANCED
90 mm 100 mm RLWDDP20A ADVANCED
100 mm 100 mm RLWDDP20 ADVANCED
100 mm 100 mm RLWDDP11 ORIGINAL

Water release options are

  •  Advanced Release Valve which is the new battery power digital release valve where you can set release frequency in days and release duration in minutes including open and closed.
  •  Original Release Valve (Plus type) using the current slow release filter option.

Features and Benefits

  • Prevents sediment, bird droppings, spiders, insects, mosquito eggs and debris from entering the rainwater tank.
  • Improves water quality, protects pumps and internal appliances.
  • Ideal to use in conjunction with a rain head.
  • Easy installation, just add pipe and glue.
  • No mechanical parts.
  • Low maintenance requirements.

How do they work?

Fitting an appropriately sized First Flush Water Diverter is critical to achieve good quality water. Water Diverters improve water quality, reduce tank maintenance and protect pumps by preventing the first flush of water, which may contain contaminants from the roof, from entering the tank.

When it rains, water slowly builds up in the roof guttering system before it exits through the downpipe. The first flush of water from the roof can contain amounts of bacteria from decomposed insects, skinks, bird and animal droppings and concentrated tannic acid. It may also contain sediments, water borne heavy metals and chemical residues, all of which are undesirable elements to have in a water storage system.

How much water to divert

It is important to prevent heavy sediments and other roof pollutants from entering the rainwater tank. The amount of water diverted should be a minimum of 50 litres per 100 square metres of roof area (or 0.5L per m2). In calculating the amount of water to divert, consideration can be given to (1) the surface area of the roof, and (2) the amount of pollutants on the roof and gutters.


  • This product consists of 3 way tee and first flush kit only.
  • First Flush Chamber pipe is not included and this pipe length needs to be determined by roof size and pollution factor.
  • 65mm, 80mm, 90mm and 100mm unit requires a 100mm First Flush Chamber pipe

Read more about the importance of using First Flush Diverters in a report put together by Stan Abbott from the Roof Water Research Centre at Massey University Wellington.

Additional information


65mm, 80mm, 90mm, 100mm

Release Valve

Advanced, Original