Downspout First Flush Water Diverter Kit 3"

Downspout First Flush Water Diverter Kit — 3" Pipe

Note: We only recommend first flush diverters in warm climates. In freezing temperatures, the standpipe must be fully drained to prevent cracking. For winter climates, consider our Wisy Vortex Filters or Monjolin Filters instead.

Downspout first flush diverters improve water quality and reduce tank maintenance by preventing the initial flow of water — which carries roof contaminants — from entering your cistern. They protect rainwater pumps and household appliances such as washing machines, toilets, and hot water systems.

Features and Benefits

• Prevents sediment, bird droppings, insects, mosquito eggs, and debris from entering the tank
• Improves water quality and protects pumps and appliances
• Works well alongside our aluminum downspout debris filters
• Easy installation — just add pipe and glue
• No mechanical parts; low maintenance

How It Works

When it rains, the first flush of water from the roof carries bacteria from decomposed insects, bird droppings, concentrated tannic acid, sediments, heavy metals, and chemical residues — all undesirable in a water storage system.

Instead of flowing to the tank, these pollutants are diverted into the diverter chamber. The system uses a dependable ball-and-seat design — no mechanical parts, no manual intervention. As water rises in the chamber, the ball floats; once full, it rests on the seat and redirects subsequent flow to the tank. A slow-release valve automatically drains and resets the chamber after rain. The drain line can also feed a drip irrigation system.

How Much Water to Divert?

Divert a minimum of 5 gallons per 1,000 square feet of roof area. The more water diverted, the better the quality entering your tank. Use this First Flush Calculator to size your system.

Installation Instructions

1. Determine the length of the Diverter Chamber (#4). Each 3-foot section of 3" Schedule 40 PVC holds approx. 1.5 gallons; each 3-foot section of 4" holds approx. 2 gallons. Fit the longest length possible, keeping the Screw Cap (#10) at least 6" from the ground for cleaning access.
2. Place Ball Seat (#9) into Tee Junction (#6) with the narrow end pointing down. Glue Diverter Chamber (#4) against the Ball Seat and hold until set. Glue Socket (#14) to the bottom of the chamber. (Optional: use a 45° bend adaptor for easier Screw Cap removal.)
3. Fix the assembled chamber to the wall using wall brackets.
4. Connect M&F Elbow (#8b) to Chamber Outlet (#8) and connect the downspout (#3). Fit an elbow to the InFeed pipe (#2) if required and connect to the rain head.
5. Select the appropriate Flow Control Valve (#11), insert into Hose Connector (#16) with "TOP" facing up. Start with the smallest gauge washer; use a larger one if blockages occur. Place Filter Screen (#13) into the Screw Cap (#10) and attach Hose Connector (#16). Optional: insert Keeper Ring (#1a) and Stainless Steel Primary Screen (WDAC02) for added filtration.
6. Place Sealing Ball (#5) into the Diverter Chamber and attach the assembled Screw Cap (#10) to Socket (#14).

First Flush Diverter — Installation Overview