How to Install the Flash Undersink Reverse Osmosis System

The Flash Undersink Reverse Osmosis System is designed for straightforward DIY installation. Most homeowners complete it in about 30–60 minutes. No plumber needed. Here is what to expect.

Installation Video

Watch the full installation walkthrough before getting started:

What Is Included

• Flash RO system (pre-assembled with filters)
• Chrome lead-free drinking water faucet
• Holding tank with shut-off valve
• Feed water adapter (connects to your cold water line)
• Drain saddle (connects to your sink drain pipe)
• Tubing and fittings

Installation Steps (Overview)

1. Mount the faucet — If your sink has a spare hole (soap dispenser hole), use that. Otherwise, you may need to drill a hole in the countertop or sink.
2. Connect the feed water adapter — Shut off the cold water valve under your sink. Disconnect the cold water supply line from the valve, attach the included adapter (tee fitting), then reconnect the supply line.
3. Install the drain saddle — Clamp it onto your sink drain pipe above the P-trap. Drill a small hole through the pipe where marked.
4. Place the system and tank under the sink — The Flash unit and holding tank both fit in a standard under-sink cabinet.
5. Connect the tubing — Color-coded quick-connect fittings make this simple. Push the tubing into the fitting until it clicks. No tools needed.
6. Turn on the water and check for leaks — Open the cold water valve slowly. Inspect every connection. Small drips usually mean a tube is not pushed in all the way.
7. Flush the system — Let the tank fill and drain it completely 2 times. This rinses carbon dust from the new filters. Discard this water.

Tips

• Use thread seal tape (Teflon tape) on any threaded connections.
• The tank takes 2–4 hours to fill completely the first time. This is normal.
• Make sure the flow restrictor is in place on the membrane drain line. Without it, the system will drain continuously.

A detailed step-by-step guide with diagrams is included in the box. You can also download the manual from the Flash product page. If anything is unclear during installation, reach out to our team — we are here to help.