How to Install a Reverse Osmosis System
Updated February 20, 2026
Install an under-sink reverse osmosis system for the cleanest drinking water at home -- feed water connection, filter mounting, storage tank, drain saddle, dedicated faucet, and the critical initial flush.
Overview
An RO system gives you the purest drinking water you can get at home. It forces water through a membrane that strips out 95-99% of dissolved contaminants -- lead, fluoride, arsenic, nitrates, chlorine, the works. The system goes under your kitchen sink with a dedicated faucet for filtered water. Water flows through a sediment pre-filter, carbon pre-filter, RO membrane, and carbon post-filter before being stored in a pressurized tank. Installation takes 2-4 hours, needs basic plumbing skills, and you will need to drill one hole for the dedicated faucet.
What You'll Need
Safety First
- Drilling the countertop for the faucet? Use the right bit: diamond-tipped hole saw for granite or quartz ($15-25), standard hole saw for laminate, step drill bit for stainless steel. Wrong bit on stone can crack the countertop. Go slow and use water as lubricant on stone.
- RO systems produce waste water -- concentrated contaminants rejected by the membrane go down the drain. Typical ratio is 3-4 gallons of waste per 1 gallon of purified water. That is normal, not a malfunction.
- Cold water supply only. The RO membrane is damaged by water above 100-113 degrees (depending on the membrane). Never connect to a hot water line.
Step-by-Step Instructions
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Plan the Layout and Mount the Filter Assembly
Open the cabinet and figure out where everything goes. You need room for the filter/membrane housing assembly (mounts on the cabinet wall or sits on the floor), the storage tank (about the size of a small propane tank), and all the tubing connecting them. Filters need to be accessible for future changes. Most kits include a wall bracket -- screw it in, hang the housings. Position the tank on the cabinet floor where it does not interfere with drain plumbing.
Tip: Before you mount anything, lay out all components and dry-fit the tubing to make sure everything reaches. RO systems have 4-6 tubing connections that all need clean routing without kinks. Plan first, mount second. -
Install the Dedicated Faucet
Check your sink for an extra hole -- usually covered by a blank cap or used by a soap dispenser you can relocate. No extra hole? You need to drill one (typically 1/2 inch). Stainless steel: step drill bit or hole saw with cutting oil, go slow. Granite or quartz: diamond-tipped hole saw with water as lubricant. Insert the RO faucet stem from above, secure from below with the washer, lock washer, and nut. Hand-tight plus a quarter turn. Connect the faucet tubing to the stem fitting.
Tip: Got an unused soap dispenser hole? Repurpose it for the RO faucet -- no drilling needed. Soap dispenser holes are typically 1-1/4 inches, and most RO faucets include a base plate or adapter that covers the larger opening. -
Connect the Feed Water Supply
The system taps into the cold water supply under the sink. Most kits include a feed water adapter -- a tee fitting with a built-in shut-off that goes between the cold shut-off valve and the faucet supply line. Turn off the cold valve. Disconnect the supply line from the valve. Install the adapter onto the valve outlet. Reconnect the faucet supply line to the adapter. The adapter's second outlet (1/4-inch push-fit or compression) connects to the RO system's first pre-filter stage via color-coded tubing. Turn the cold water back on and check for leaks.
Tip: Push-fit tubing connections are simple: cut the tubing square, push it into the fitting until it stops (about 3/4 inch), tug to confirm it is locked. To disconnect, push the collet (the ring at the fitting) inward while pulling the tubing out. -
Install the Drain Saddle for Waste Water
The membrane's waste water needs a path to the drain. The kit includes a drain saddle -- a clamp-on fitting for the drain pipe. Pick a spot above the P-trap (never below it -- that lets sewer gas into the system). Drill a 1/4-inch hole through the drain pipe. Position the saddle over the hole with the rubber gasket in between. Tighten the clamp bolts evenly until secure. Connect the waste water tubing from the membrane housing to the saddle fitting.
Warning: Mount the drain saddle as high on the pipe as possible -- above the disposal outlet if you have one, and always above the P-trap. Too low or below the trap allows drain water to back-siphon into the RO system. Some codes require an air gap faucet to prevent this -- check locally. -
Connect All Tubing and the Storage Tank
Follow the manufacturer's diagram and connect the color-coded tubing between all components. Typical flow: feed adapter to stage 1 (sediment pre-filter), to stage 2 (carbon pre-filter), to RO membrane, membrane pure water out to storage tank (with a tee to the post-filter), membrane waste out to drain saddle, and post-filter to the dedicated faucet. The storage tank has a single connection with a valve on top -- connect the tubing and make sure the valve is ON (counterclockwise). Double-check every connection against the diagram.
Tip: Follow the color-coded tubing exactly. If you mix up the input and output on the RO membrane, the system will not produce clean water and the membrane gets damaged. The membrane housing is directional -- look for the flow arrow. -
Flush the System and Test
Do not drink the water until you flush the system. Open the tank valve, turn on the feed water, and open the RO faucet. Water will trickle through -- first tank fill takes 2-4 hours. Once full (faucet flow picks up to a steady stream), drain the entire tank by leaving the faucet open. Refill and drain two more times -- three total cycles. After the third flush, the water is safe to drink. Check all connections for leaks. Test with a TDS meter ($10-15) -- the reading should be 90-95% lower than your tap water.
Tip: Do not skip the flush. The membrane ships in a preservative solution that has to be flushed out before the water is drinkable. Three full tank cycles clears the preservative and the carbon fines from new filters. Takes 6-12 hours total but you just open the faucet and wait -- no active involvement.
Pro Tips
- Buy a TDS meter ($10-15) and use it. Test tap water versus RO water. If tap is 300 TDS and RO is 15, the membrane is removing 95%. When rejection drops below 80% (RO water TDS climbs to 60+ in that example), the membrane needs replacing -- typically every 2-3 years.
- RO strips out beneficial minerals along with the bad stuff. Want them back? Add a remineralization filter as the final stage ($15-25, replace annually). It adds calcium and magnesium back in controlled amounts, raises pH slightly, and improves the taste.
- Pre-filters (sediment and carbon) every 6-12 months. Membrane every 2-3 years. The pre-filters protect the membrane from sediment and chlorine -- skip pre-filter changes and the membrane's life drops from 3 years to as little as 1.
- Flow rate dropped to a trickle? Three common causes: clogged pre-filters (replace them), low tank pressure (should be 7-8 PSI when empty -- check with a tire gauge on the tank's air valve), or a fouled membrane (replace it).
- Want purified ice? Run a 1/4-inch tubing line from the storage tank tee to your refrigerator. Most kits include enough tubing, or pick up extra at any hardware store.
When to Call a Pro
This is a solid DIY project. Call a pro if you need a hole drilled in granite or quartz and are not comfortable doing it (countertop fabricators charge $50-100), if your under-sink plumbing is unusual and the adapter will not fit, if your code requires an air gap faucet (slightly more complex install), or if you want the RO line run to the fridge through the wall instead of exposed.