Downspout First Flush Water Diverter PLUS 4"

First Flush Diversion for Rain Harvesting

NOTE: We only recommend first flush diverters in warm climates. In freezing temperatures, water left in the standpipe can cause the unit to crack. For winter climates, consider our Wisy Vortex Filter or Monjolin Filter instead.

Downspout First Flush Water Diverters improve water quality and reduce tank maintenance by preventing the first flush of rainfall — which carries the highest concentration of roof contaminants — from entering your storage tank. This protects rainwater pumps and household appliances such as washing machines, toilets, and hot water systems.

The First Flush Plus Tee’s specially designed inlet prevents fast-moving dirty water from “skipping” over the first flush chamber during heavy rainfall, ensuring the dirtiest water never reaches your tank.

Features & Benefits

• Diverts water containing organic and inorganic particles into collection chambers on each downpipe
• Patented First Flush Plus tee prevents dirty water from “skipping” over the traditional T-junction gap during heavy rainfall
• Adjustable diverter chamber pipe length
• Automatic reset valve drains after each rainfall event
• Comes as a complete kit
• Transparent, rapid-release exit funnel for easy inspection and maintenance

Installation Instructions

1. Determine the length of 4" pipe required for your first flush diversion chamber. As a rough guide, 3 ft of 4" pipe holds approximately 2.3 gallons. Ensure all cut edges are clean and smooth.
2. The First Flush Plus tee can be installed in horizontal or vertical orientation. The outlet must remain accessible for maintenance and inspection.
3. Select the orientation appropriate for your installation point — this determines which socket attaches to your diversion chamber. If installing beneath a horizontal pipe, orient the flow arrow in the direction of flow so the branch socket connects to the diversion chamber. For horizontal branch installations, orient the flow arrow upward.
4. Orient the ball seat to fit inside the First Flush Plus tee socket that will attach to your diversion chamber. Apply solvent weld glue and press firmly into the socket.
5. If attaching to existing 90mm downpipes, install pipe reducers into the remaining tee sockets. For 100mm downpipes, reducers are not required.
6. Measure your existing downpipe and cut to create space for the First Flush Plus tee. The outlet of your diverter must sit at least 5.9" from the ground when fully assembled. Apply solvent weld glue to the sockets and pipe, then insert the pipe into the sockets at either end of the tee.
7. Apply solvent weld glue to the end coupling socket and one end of your 4" diversion chamber pipe. Insert firmly into the end coupling.
8. Apply solvent weld glue to the ball seat socket and the other end of the 4" diversion chamber pipe. Insert firmly against the ball seat.
9. Attach to the wall using the supplied brackets, supporting the unit until fully secured. The upper bracket should sit directly under the First Flush Plus tee to bear the weight of the unit.
10. Place the ball inside the diversion chamber through the end coupling. Firmly screw the transparent, rapid-release exit funnel with O-ring onto the end coupling to ensure a good seal.
11. Attach the threaded coupling to your diversion chamber pipe using solvent weld glue. Push together and hold until the glue sets.
12. Select the appropriate flow control washer and fit it into the hose connector with the “TOP” side showing. Start with the smallest gauge hole (lowest number) and size up if blockages occur. Insert the plastic filter screen through the base of the exit funnel and secure by attaching the hose connector and flow control washer.

Full installation instructions on Rain Harvesting’s website

First Flush Diverter PLUS — overview and installation

First Flush Diverter PLUS — additional installation guidance