Quick verdict
The first rain off a roof carries dust, bird droppings and pollen. A first flush diverter catches that initial runoff in a chamber and lets only the cleaner water that follows reach the tank, then slowly drains itself ready for the next storm. Rain Harvesting's kit for 3 inch round pipe includes the tee, ball, screen and slow-release valve; you add a length of pipe sized to your roof. It is the professional way to protect a tank.
Strengths
- Diverts the dirty first runoff so cleaner water reaches the tank
- Slow-release valve drains the chamber automatically between storms
- Chamber volume is set by the pipe length you add, so it scales to any roof
- Catch All tee and fittings included; just add 3 inch pipe
- Keeps organic and inorganic contaminants out of stored water
Weaknesses
- Needs vertical space below the tee for the chamber pipe
- Slow-release valve requires occasional cleaning
- Sized for 3 inch round pipe only; rectangular downspouts need adapters
The overall YardToolLab Score is out of 100. Section scores are out of 10.
Why the First Rain Should Not Reach the Tank
Between storms a roof collects dust, pollen, bird droppings and whatever the wind brings. The first few minutes of rain rinse all of it into the downspout. A first flush diverter captures that initial runoff in a chamber and only passes the cleaner water that follows. Rain Harvesting, an Australian company that has specialised in rainwater equipment for decades, makes the kit most installers use. This version fits 3 inch round pipe and includes everything except the pipe that forms the chamber.
This review is based on Rain Harvesting’s specifications and on how first flush diverters work in rainwater systems generally. We have not installed this kit ourselves.
How It Works
Water from the downspout enters the Catch All tee and drops into a vertical chamber of 3 inch pipe below. As the chamber fills, a ball floats up and seals the chamber inlet, and all further water passes through the tee toward the tank. After the rain stops, a slow-release valve at the bottom drains the chamber over several hours so it is empty and ready for the next storm. No power, no moving parts beyond the ball.
Sizing the Chamber
You supply the pipe, which is the clever part: chamber volume scales to your roof. A common guideline is about 10 gallons of first flush per 1,000 square feet of roof, more under trees or in dusty areas. Three inch pipe holds roughly 0.37 gallons per foot, so a 1,500 square foot roof wants around 40 feet of pipe, usually arranged as a long vertical run or a coil.
Installation
Cut the downspout, fit the tee, run the chamber pipe down and cap it with the valve assembly, and continue the outlet to the tank. The main requirement is vertical space below the tee. Rectangular downspouts need an adapter to 3 inch round, which Rain Harvesting sells.
Maintenance
Clean the screen and slow-release valve a few times a year, more under heavy leaf fall. A blocked valve stops the chamber draining and defeats the purpose.
Build Quality
UV-stabilised plastic designed for Australian sun, with a long record in the field. Owners report decades of service with routine cleaning.
Who Should Buy It
- Anyone storing rainwater for more than a few days, where quality matters.
- Households using tank water for washing or irrigation of edibles.
- Roofs under trees or in dusty areas that shed heavy first runoff.
Common Owner Complaints
Finding vertical space for the chamber and remembering to clean the valve. Both are inherent to the design rather than faults.
Alternatives to Consider
Rain Harvesting sells the same kit for other pipe sizes. A simple downspout filter removes leaves but not the first flush. For a single barrel used only on ornamentals, a mesh inlet screen alone may be enough.
Verdict
If you store rainwater seriously, a first flush diverter is the single biggest improvement to its quality, and this kit is the established way to do it. Size the pipe to your roof, keep the valve clean and your tank water will stay clear.
Full specifications
Rain Harvesting First Flush Downspout Diverter Kit for 3 Inch Round Pipe FAQs
A common rule is about 10 gallons per 1,000 square feet of roof, more in dusty or tree-covered areas. A 3 inch pipe holds about 0.37 gallons per foot, so size the length accordingly.
A ball floats up and seals the chamber once it fills, sending water on to the tank. After the rain, a slow-release valve drains the chamber over hours so it is empty for the next storm.
No. Keep a mesh screen at the tank inlet for leaves and mosquitoes. The first flush handles fine contaminants and the first dirty rinse.
Out through the slow-release valve at the bottom. Direct it to a bed or gravel, not against the foundation.
