Set aside a morning for this. You don’t need a plumber, and you don’t need special skills. What you need is the right preparation and enough patience to go slowly on each connection.

What Comes in the Box

Standard RO systems from brands like iSpring, Waterdrop, and similar manufacturers include everything you need for a typical install. Before you start, confirm your kit has:

• Pre-filter housing (sediment stage)
• Second pre-filter housing (activated carbon stage)
• RO membrane housing
• Pressure tank with ball valve fitting
• Dedicated faucet
• Saddle drain valve or compression drain fitting
• Color-coded push-fit tubing
• Feed water adapter (saddle valve or compression tee)
• Wall-mount bracket for filter housings

Color-coded tubing is your friend. The colors tell you which line goes where. Keep the manufacturer diagram out the whole time.

Decide Where the Faucet Goes First

This is the one decision that shapes the rest of the install. The dedicated RO faucet needs its own hole, separate from your main faucet and sprayer.

Most kitchen sinks have a knockout hole, usually covered by a decorative plug, meant for a soap dispenser or sprayer. If that hole is empty, use it. Remove the plug and install the faucet there.

If all your knockout holes are used, you have two options: drill a new hole in the sink or drill one in the countertop. A 1-3/8" hole saw in a standard drill handles stainless steel and acrylic sinks without much trouble. Mark the spot with a center punch first so the bit doesn’t wander. Apply steady pressure and go slowly.

Granite and stone are different. Those require a diamond core bit and a different technique. If you’re not experienced with stone work, call a plumber for that step and do the rest yourself.

Tools You’ll Need

• Adjustable wrench
• Drill with a 1-3/8" hole saw bit (only if drilling a new faucet hole)
• Teflon tape
• Bucket and rags
• Permanent marker (for labeling the tubing)

Step-by-Step Installation

1. Turn off the cold water supply valve under the sink. It’s the valve on the right side of the cabinet interior. Turn it fully clockwise until it stops.

2. Put a bucket under the supply valve area. There will be residual water when you disconnect or pierce the line.

3. Install the feed water adapter. You have two options here, and they’re not equal. The needle saddle valve clamps onto the cold supply line and uses a needle to pierce it. It’s included in most kits because it’s simple. The compression tee fitting is more reliable, it inserts inline with the cold supply line and requires a short supply line disconnect. If your supply line uses standard connections, go with the compression tee. It’s less likely to drip over time.

4. Install the drain saddle. Clamp the drain saddle onto the drain pipe above the P-trap. Use the included screws. Then drill a 1/4" hole in the center of the drain pipe where the saddle’s port lines up. This is where the RO system’s wastewater exits.

5. Install the dedicated faucet. Drop it through your chosen hole, secure the mounting nut from below, and connect the faucet supply line. Hand-tight is enough for now.

6. Mount the filter housing bracket. Screw it securely to the cabinet wall. The housings are heavy when full, don’t mount into drywall alone. Hit a stud if you can. Then attach the filter housings: sediment filter first (closest to incoming water), carbon pre-filter second, RO membrane housing third.

7. Install filters and the RO membrane. Follow the manufacturer’s order. The sediment and carbon stages use standard 10" cartridges that drop in and thread closed. The RO membrane slides into its housing with a specific orientation, check the diagram. Don’t force it.

8. Install the pressure tank. Set it on the cabinet floor. Connect the tank ball valve and the provided fitting. The tank stores filtered water so the faucet dispenses at a reasonable flow rate.

9. Connect all the tubing. This is where the color-coding earns its keep. Push-fit connections work by pushing the tubing firmly into the fitting until it seats. Then tug it back, hard. If it pulls out, push again and seat it deeper. A connection that passes the tug test won’t leak.

10. Slowly open the cold supply valve. Open it maybe a quarter turn at first. Watch every connection for drips. Let pressure build gradually, then open the valve fully. Check again.

11. Open the tank ball valve. The system will start filling the pressure tank. This takes 2-4 hours for the first fill.

12. Run the first tank flush. Once the tank is full and water flows from the dedicated faucet, drain the entire tank. This flushes the membrane and pre-filters of any production residue. Refill and drain once more. After the second flush, the water is ready.

What Can Go Wrong

Dripping at a push-fit connection. Pull the tubing out and re-insert it. Push firmly until it’s fully seated, then tug. Almost every leak from push-fit connections is an incompletely seated tube.

Slow flow from the faucet. RO systems need at least 40 psi of incoming water pressure to operate efficiently. Under 40 psi, the flow rate slows dramatically and the membrane doesn’t reject contaminants as effectively. If your pressure is low, a booster pump (sold separately) fixes this. Check pressure with an inexpensive gauge at the supply valve.

Strange taste from the first few fills. Normal. New carbon media releases some fine particles and has a characteristic taste that clears after 2-3 tank flushes. Run the system and drain it.

Tank stays empty or pressurizes slowly. The pre-charge pressure in the tank should be around 7-10 psi when empty. Most new tanks ship in that range. If the tank isn’t filling, check that the ball valve is open and all tubing connections are seated.

When to Call a Plumber

Three situations: stone countertops that need a new hole drilled, non-standard plumbing that doesn’t work with the included fittings, and persistent low water pressure under 40 psi. Everything else is well within DIY range.

Most homeowners who can change a faucet can install an RO system. Go slowly on the connections, check every one before turning on the water, and you’ll finish in under two hours.

The biggest mistake is rushing the push-fit tubing. Every connection needs to be fully seated and tug-tested before water pressure is applied. Do that, and you’re unlikely to have any problems.

Internal links: check out the best under-sink RO systems for model comparisons, and when to replace your water filter for a maintenance schedule once your system is running.