How to Install an Under Sink RO System: The Complete Guide
Installing an under-sink RO system is a manageable DIY project for anyone comfortable with basic plumbing. Here's everything you need to know — tools, steps, timeline, and the mistakes that cause most installation failures.
Time estimate2–3 hours for first-time installers, under 1 hour for experienced DIYers
Don't rush. The most common installation problems — leaky fittings, air-locked tanks, slow flow — all trace back to taking shortcuts. Plan for a full afternoon on your first installation.
What you'll need
- Your RO system box — all required hardware should be included
- Adjustable wrench and pliers
- Drill + 1.375" hole saw bit (for faucet hole, if your sink lacks a pre-drilled hole)
- Teflon tape (usually included with the system)
- Bucket and old towels for drips
- Flashlight or headlamp for working under the sink
- Optional: TDS meter to verify performance after installation
Step-by-step installation
- Clear the cabinet. Remove everything stored under the sink. You need room to work.
- Turn off the cold water supply at the shut-off valve under the sink.
- Install the drain saddle on the P-trap or drain pipe. This is where waste water exits the system.
- Install the feed water adapter on the cold water supply line. This taps into your home's water supply.
- Drill the faucet hole (if needed). Most sinks have a pre-drilled knockout hole. If not, drill a 1.375" hole in the sink deck or countertop.
- Install the dedicated faucet through the hole. Tighten the mounting nut — for traditional systems, this is the hardest part (reach-under tightening). iSpring's top-mounted design makes this much easier.
- Mount the filter assembly inside the cabinet using the included bracket.
- Connect all tubing. Push each tube fully into its fitting until it stops, then give a slight tug to confirm it's locked. Color coding in the manual guides each connection.
- Connect the storage tank (tank-based systems only). Ensure the tank valve is open (counterclockwise = open).
- Turn the water supply back on slowly and check every connection for leaks.
- Flush the system. Let the tank fill and drain at least twice before drinking the water. This clears carbon fines and any manufacturing residue from new filters.
The mistakes most first-timers make
- Half-inserted tubing. Push tubing ALL THE WAY into John Guest fittings until it physically stops, then tug. A half-inserted tube will drip slowly and may not be visible until you've replaced the contents of the cabinet.
- Tank valve closed. The tank won't fill if the valve is closed (clockwise = closed). Turn it counterclockwise to open before testing.
- Skipping the flush. The first one or two tank-fills will contain carbon particles and taste slightly off. Always flush before drinking — this is not optional.
- Overtightening filter housings. Hand-tight plus 1/4 turn with the wrench is enough. Overtightening cracks the plastic housing.
- Forgetting the drain saddle clamp. The saddle must be clamped tightly to the drain pipe or waste water will back-flow into the system.
System-specific installation guides
When to call a plumber
Under-sink RO installation doesn't require a plumber for standard installations. Call one if: your home's water pressure is below 45 PSI and you need a booster pump installed, your supply lines use unusual fittings that don't match the included adapters, or you encounter corroded or damaged plumbing while working under the sink.