Installing a reverse osmosis system under a sink is within reach of most people with basic tools, and the kits are designed for it. Three parts of the job account for nearly all the problems: making the hole for the faucet, connecting the drain, and the connections themselves. Get those right and the rest is straightforward.
Before you start
Check three things. Line pressure, because a conventional system needs reasonable pressure to work at all and a low pressure supply needs a booster pump. Cabinet space, since the tank, the manifold and the tubing take more room than the box suggests and the tank must stand upright. And local code on the drain connection, because some jurisdictions require an air gap arrangement and that changes which faucet you buy. Photograph the under sink area before you begin.
The faucet hole
Use an existing spare knockout if the sink has one, which saves the hardest part of the job. Otherwise the material decides the method: stainless steel takes a step drill with cutting fluid, granite and quartz need a diamond hole saw with water cooling, and ceramic and enamel crack readily and are best left to someone who does this regularly. Check clearance underneath for the mounting hardware, and sit the faucet in place before committing, so the spout clears the tallest bottle you fill.
The drain connection
The saddle goes above the trap and on the vertical tailpiece, never below the trap or on a horizontal arm, because those hold standing water and produce the smell owners later blame on the system. Keep it clear of a garbage disposal outlet. Deburr the drilled hole thoroughly on both faces, since the swarf left behind starts the first blockage. Tighten the two screws evenly and only until the gasket compresses. A branch tailpiece with a moulded connection avoids the whole failure mode.
Connections and commissioning
Cut tubing square with a proper tube cutter, never scissors, because push fittings seal against the tube's outer surface and an oval or scratched tube will not seal. Push each connection past the collet until it seats, then pull back to confirm it has gripped. Open the feed slowly, check every joint by hand and again an hour later. Then discard the first two or three tankfuls, which carry carbon fines. Our reverse osmosis guide covers the commissioning checks in full.
Questions people ask about diy reverse osmosis water filter
Can I fit a reverse osmosis system myself?
For most people yes. The faucet hole, the drain saddle and the push fit connections are where the problems concentrate.
Where should the drain saddle go?
Above the trap on the vertical tailpiece, clear of any garbage disposal, with the drilled hole thoroughly deburred.
Why do my push fit connections leak?
Almost always the tube: cut out of square, crushed or scratched where the O ring seals against its outer surface.
What should I check before buying?
Line pressure, cabinet space for the tank standing upright, and whether local code requires an air gap faucet.