Under-Sink RO System Installation
Updated February 20, 2026
An under-sink reverse osmosis system gives you bottled-water quality from the tap. The install looks complicated with all the tubing, but it's really just connecting a few color-coded lines: cold water supply in, drain line to the sink drain, and clean water out to a dedicated faucet. Most RO systems come with everything you need, including the faucet. A confident DIYer can have this running in an afternoon.
Overview
An under-sink reverse osmosis system gives you bottled-water quality from the tap. The install looks complicated with all the tubing, but it's really just connecting a few color-coded lines: cold water supply in, drain line to the sink drain, and clean water out to a dedicated faucet. Most RO systems come with everything you need, including the faucet. A confident DIYer can have this running in an afternoon.
What to Know
Tools & Materials
- RO system kit (includes filters, membrane, tank, faucet, tubing)
- Adjustable wrench
- Drill with 1/2-inch bit (if drilling a faucet hole)
- Teflon tape
- Bucket and towels
- Tubing cutter
Step by Step
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Plan placement
The RO tank, filter housing, and membrane all need to fit under the sink. Lay everything out first to make sure it fits. The tank is typically the size of a small propane cylinder. Some systems mount the filters on the side wall of the cabinet.
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Install the RO faucet
If your sink has a spare hole, remove the cap and install the RO faucet through it. If not, drill a hole in the sink or countertop (stainless steel and granite both work, just use the right bit). Secure the faucet with the included mounting hardware from below.
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Tap into the cold water supply
Most RO kits include a feed water adapter -- either a saddle valve (pierces the pipe) or a tee adapter (inline with the shutoff valve). The tee adapter is more reliable. Install it between the cold water shutoff valve and the supply line. Connect the RO feed tubing.
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Connect the drain line
Install the drain saddle on the sink tailpiece (the vertical pipe between the sink and the P-trap). Drill a 1/4-inch hole through the tailpiece, then clamp the drain saddle over the hole. Connect the waste tubing from the membrane housing to the drain saddle. Position the drain connection above the P-trap, not below it.
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Connect the system tubing
Follow the color-coded tubing diagram included with your system. Typically: feed water to the first pre-filter, through the stages to the membrane, clean water to the tank and to the faucet, waste water to the drain. The tubing uses push-fit connectors -- just push in until it clicks.
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Flush and test
Turn on the cold water supply. Check every connection for leaks. Open the RO faucet and let the system fill the tank. The first tank of water will taste off -- run it through and discard it. The second fill should taste clean. Most systems take 2-4 hours to fill the tank initially.
Pro Tips
- Skip the saddle valve if possible -- they're prone to clogging and leaking over time. Use a tee adapter on the shutoff valve instead.
- Replace the RO filters on schedule (every 6-12 months for pre/post filters, every 2-3 years for the membrane). Mark the installation date on the filter housing.
- The RO faucet has two positions: a steady stream (filtering from the tank) and a trickle when the tank is empty and the system is filtering in real-time. If flow suddenly drops to a trickle, the tank pressure may need adjusting.
- RO systems waste 2-4 gallons for every gallon of clean water produced. Higher-efficiency models waste less. If water cost is a concern, check the system's waste ratio.
Mistakes to Avoid
- Installing the drain connection below the P-trap, which causes dirty water to siphon back into the RO system.
- Not flushing the initial tank of water -- the first batch contains carbon fines from the new filters.
- Using a saddle valve on PEX or CPVC pipe -- they're designed for copper only.
- Forgetting to turn on the tank valve. The tank has its own shutoff that must be open for the system to work.
When to Call a Pro
RO installation is very DIY-friendly -- the kits are designed for homeowner installation. Call a plumber if you need a new faucet hole drilled in granite (requires a diamond bit and steady hand) or if your under-sink plumbing is old galvanized that crumbles when you touch it.
Bottom Line
An under-sink RO system is a 1-3 hour DIY install that gives you premium drinking water at a fraction of bottled water costs. Follow the color-coded tubing, skip the saddle valve in favor of a tee adapter, and flush the first tank before drinking. It's one of the most impactful kitchen upgrades you can do.