First Flush

The Marley First Flush Diverter reduces the pollution of tank water by diverting the first load of contaminated water away from the storage tank.

  • About

  • Components

  • Technical

img-flush1After a period without rainfall, dirt and debris builds up on your roof and also collects in your spouting system. Without additional measures, when the next rainfall arrives, this dirt and debris will then be washed straight into your water collection system. The Marley First Flush Diverter is designed to reduce the pollution of tank water by diverting the first load of contaminated rainwater away from the storage tank.


Improves the quality of water entering your collection system thereby making it safer for drinking and other use.


The Marley First Flush Diverter is available with a 90mm or 300mm diameter chamber and in a variety of lengths to suit your particular roof size. The larger the roof area the greater the initial amount of water that needs to be flushed before directing into your tank system.


If you clean the water before it enters your tank then you will reduce the pressure on other more expensive filter and pump systems in place. This should then help to increase the lifespan of your rain harvesting system and reduce maintenance costs.


img-flusht1img-flusht1 90MM STORMWATER PIPE 6m (to be ordered separately from kit) 700.90.6 img-flusht2img-flusht2 300MM STORMWATER PIPE (Available in std 1.5m or 2m lengths*, to be ordered separately from kit) *other lengths available on request FFD.300.1.5
(300mm X 1.5m)FFD.300.2
(300mm X 2m)
Tee JunctionTee Junction TEE JUNCTION
K0019 300mm End CapK0019 300mm End Cap 300MM END CAP X2
Wall BracketsWall Brackets WALL BRACKET X2
Wall Bracket Wall Bracket WALL BRACKET
K0036 Filtter (plastic)K0036 Filtter (plastic) FILTER (Plastic)
Mounting Post Galvanised SteelMounting Post Galvanised Steel MOUNTING POST
(galvanised steel)
K0014 BallK0014 Ball BALLL
Primary Filter (stainless steel)Primary Filter (stainless steel) PRIMARY FILTER
(stainless steel)
K0006 SeatK0006 Seat SEAT
K0036 Filtter (plastic)K0036 Filtter (plastic) SECONDARY FILTER
K0012 90mm Screw End CapK0012 90mm Screw End Cap 90MM SCREW END CAP
PVC Pipe End Cap 90x90mmPVC Pipe End Cap 90x90mm PVC PIPE (end cap to threaded socket adaptor) – 90mm diameter X 90mm length
K0005 NutK0005 Nut NUT
PVC Pipe Filter Spacer 90x8mmPVC Pipe Filter Spacer 90x8mm PVC PIPE (filter spacer) – 90mm diameter X 8mm length
K0004 TailK0004 Tail TAIL
PVC Pipe 90x22mmPVC Pipe 90x22mm PVC PIPE (ball seat keeper ring) – 90mm diameter X 22mm length
K0009 Threaded CouplingK0009 Threaded Coupling THREADED COUPLING
K0006 SeatK0006 Seat SEAT
K0002 Red Rubber WashersK0002 Red Rubber Washers RED RUBBER WASHERS X4

(1.25mm – 2.0mm aperture)

K0014 BallK0014 Ball BALL
K036 Black Rubber WashersK036 Black Rubber Washers BLACK RUBBER WASHERS X4

(0.25mm – 1.0mm aperture)

K0012 90mm Screw End CapK0012 90mm Screw End Cap 90MM SCREW END CAP
K0005 NutK0005 Nut NUT
K0004 TailK0004 Tail TAIL
K0009 Threaded Coupling K0009 Threaded Coupling THREADED COUPLING
K0002 Red Rubber WashersK0002 Red Rubber Washers RED RUBBER WASHERS X4

(1.25mm – 2.0mm aperture)

K036 Black Rubber WashersK036 Black Rubber Washers BLACK RUBBER WASHERS X4

(0.25mm – 1.0mm aperture)

Material: uPVC (unplasticised PVC)

Certified for use with potable water? Yes (certified to AS/NZS 4020)

Compatible Downpipe Systems: RP80® 80mm round downpipe or 90mm/100mm Stormline® stormwater pipe. Adaptors available to enable use with RP65® 65mm round downpipe.


Step 1 – Calculate the amount of polluted water to be diverted

Wet System
(Roof Area x Pollution Factor) + (Length of wet pipe x pipe cross-section factor) = litres to be diverted

Dry System
Roof Area x Pollution Factor = litres to be diverted

Roof Area = length x width in m2 of actual roof area draining into the water tank system.

NB. This is not the floor area of a house. Need to calculate the area of each roof section draining into the water tank and then add them together to get the total roof area

Pollution Factor = the level of water pollution likely in the particular environment.

This differs depending on the surrounding environment i.e. flora and fauna, but preliminary testing indicates there are two approximate pollution factors that can be applied as follows*:

  • Minimal Pollution (open field, no trees, no bird droppings, clean environment): use pollution factor of 0.5
  • Substantial Pollution (leaves and debris, bird droppings, various insect matter): use pollution factor of 2

*For more accurate assessment individual site testing and analysis would be required.

Length of Wet Pipe = in a wet system water is held in the pipes and will grow stagnant over time. Therefore this water also needs to be flushed before it enters the tank and so the volume contained in the pipes needs to be allowed for in the calculations.

Pipe cross-section factor = as per above the volume of water in the pipes of a wet system needs to be allowed for in the calculations. To calculate this for different diameter pipes, the Length of Wet Pipe is multiplied by the Pipe Cross Section factor.

Marley RP65®  65mm round downpipe 3.30
Marley RP80®  80mm round downpipe 4.40
Marley Stormline® 90mm stormwater pipe 5.75
Marley Stormline® 100mm stormwater pipe 8.60

Step 2 – Choose the appropriate First Flush Diverter and Chamber length required

90mm First Flush Diverter 1 metre 5.7 litres
2 metres 11.4 litres
3 metres 17.1 litres
300mm First Flush Diverter 1 metre 80 litres
1.5 metres 112 litres
2 metres 147 litres
3 metres 218 litres

NB. The 90mm First Flush Diverter is usually installed on each downpipe that feeds into a tank whereas the 300mm First Flush Diverter has a much larger capacity and can handle feeds from either single or multiple downpipes.

Slow Release Valve

The slow release valve ensures the first flush chamber empties itself after rain and resets automatically.

The slow release valve is 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. The Ball prevents diverted water from escaping back into the main flow.

The Slow Release valve begins draining water the moment water enters the diverter chamber and is continuous.

Negative pressure created by the continuous release prevents contaminated water from escaping between the ball and seat for the duration of the rain event.

The optimum drain rate is between 4 and 15 litres per hour.

The Downpipe First Flush Diverter is supplied with eight Flow Control Washers. Each of the eight washers incorporate different sized drain holes ranging from 0.25mm to 2.0mm. Washers are exchanged to achieve optimum flow from the chamber.

Optimum flow is defined as, the slowest reliable flow. As a guide, small drain hole washers are installed in low pollutions environments; larger drain hole washers are exchanged in higher pollution environments to prevent blockages between maintenance.

The following table details flow rates under controlled conditions; clean water. The flow rate for each washer will vary in environments with increased pollution.

flow control







The Marley First Flush Diverter system includes technical elements, to ensure optimal performance Marley recommends you use a trade professional for installation.

For further information please consult the Marley Rain Harvesting Systems brochure.


To ensure continuing function, unscrew the cap on a regular basis to allow debris to fall out. Hose or wash the filter screen if needed and check and clean the flow control valve.