An under-sink reverse osmosis system can look finished while important details remain hidden behind the cabinet door. The faucet may run, but the drain connection could be poorly placed, the storage tank could be inaccessible, or a fitting could begin leaking only after the tank reaches full pressure.

Follow the work in order. At each stage, ask the installer to show you what was checked and what still needs to happen. You do not need to supervise every tool movement. You do need to know where water enters, where reject water drains, how treated water reaches the faucet, and how the completed system will be tested.

Start with the cabinet empty and the route visible

Remove cleaners, bins, shelf liners, and anything that blocks the cabinet floor or plumbing. Look for swollen wood, old stains, corrosion, active drips, loose valves, and electrical equipment that could be exposed to water. Existing moisture should be identified before new tubing makes the source harder to trace.

Ask the installer to place the filter assembly, membrane housing, and storage tank before drilling or cutting. Confirm that filters can be removed without taking apart the drain plumbing or pulling the entire system from the cabinet. The tank valve, feed shutoff, and filter housings should remain reachable by hand.

If the equipment will barely fit, ask what must be moved during routine service. A system that requires disconnecting tubing or unloading a packed cabinet for every filter change creates more chances for damage and skipped maintenance.

Confirm the cold-water feed connection

The RO system should receive water from the cold-water line. Have the installer identify the exact shutoff valve and feed adapter before making the connection. Do not accept a plan that depends on a valve that will not close completely without first deciding how that valve will be repaired or replaced.

After the feed connection is installed, check that the tubing is not kinked, stretched, or rubbing against a sharp cabinet edge. There should be enough slack for normal service, but not so much loose tubing that it can be snagged by stored items.

Ask which valve shuts off only the RO system and which valve shuts off the sink supply. Label the RO feed valve if its purpose is not obvious.

Approve the faucet location before drilling

Stand where you normally fill a glass, bottle, kettle, or cooking pot. Make sure the proposed faucet location is comfortable to reach and allows the spout to discharge fully into the sink. Check for interference with the main faucet handle, soap dispenser, window trim, backsplash, and anything stored above the sink.

Then look under the proposed hole. The installer needs clearance for the faucet stem, mounting hardware, and tubing. A clear spot on top can be blocked underneath by a sink rim, brace, disposal, drawer, or cabinet wall.

Ask what material will be drilled. Stainless steel, composite material, stone, and porcelain-covered surfaces require different methods. If the installer is using an existing accessory hole, confirm that the faucet base will cover it and remain stable when the spout is turned.

Inspect the drain connection before it is hidden

An RO system sends reject water to the sink drain. Ask the installer to show you where that connection will be made. It should not be positioned where standing debris or disposal discharge can easily enter the RO drain tube.

The opening in the drain pipe and the fitting attached over it must line up. Ask the installer to verify that alignment before tightening the fitting. The tubing should run without a low loop that holds dirty water and without a kink that restricts flow.

If the faucet or local installation uses an air-gap drain arrangement, have the installer identify each drain tube and explain its route. A pinched or crossed tube can cause noise, slow drainage, or water at the faucet base.

Trace every tube before startup

Before water is turned on, ask the installer to trace the system with you. Identify the feed-water tube, treated-water tube, tank tube, faucet tube, and drain tube. Labels are useful when several tubes have the same color or disappear behind equipment.

Check each route for tight bends, contact with sharp metal, and pressure from a drawer or waste bin. Tubing should be inserted fully into its fittings according to the fitting design. Locking clips, if required by the system, should be installed rather than left in the packaging.

Confirm that the filter cartridges and membrane match the system documentation and are installed in the correct housings and direction. Keep the model information and replacement schedule where the next person servicing the unit can find them.

Pressurize the system in stages

The first leak check should begin when the feed water is opened, not after the cabinet has been repacked. Watch the feed adapter, housing seals, tank valve, faucet connection, and every tubing fitting. Use a dry paper towel around each connection to reveal moisture that is difficult to see.

Opening the faucet may release air and irregular flow during startup. Follow the system's startup instructions for filling, flushing, and discarding water. Do not treat the first clear glass as proof that startup is complete.

A second leak check is needed after the storage tank fills and the system reaches its normal shutoff condition. Full pressure can expose a slow seep that was absent while the tank was empty. Check the cabinet floor as well as the fittings because water can travel along tubing before it drips.

Verify operation, not just water flow

Have the installer demonstrate that the faucet opens and closes cleanly, the tank valve is open, reject water reaches the drain while the system is producing water, and production stops as intended after the tank is full. Ask about any sound you should expect and what sound would justify a service call.

If performance testing is part of the job, write down what will be measured, when it will be measured, and what result the installer considers acceptable for that exact system. A reading taken before the required flushing and filling sequence may not represent normal operation.

Ask the installer to record the installed filter and membrane identifiers. If the installation included a written performance claim, compare the completed setup with the equipment and conditions named in that claim.

Finish with an owner handoff

Before the installer leaves, operate the RO faucet yourself and locate the RO feed valve, tank valve, filter assembly, and drain connection. Ask how to isolate the system if a leak appears without unnecessarily shutting off water to the entire house.

Collect the equipment instructions, cartridge identifiers, startup record, warranty information, and service contact. Ask which parts require periodic replacement and whether the first maintenance visit depends on time, water use, measured performance, or another condition.

Leave the cabinet mostly empty until the system has completed a full fill and use cycle. Recheck the fittings and cabinet floor with a dry paper towel. When you reload the cabinet, keep heavy bottles and bins away from tubing, valves, and housings.

A complete installation ends with accessible equipment, traceable tubing, a sound drain connection, a fully pressurized leak test, and an owner who can isolate the system. If any of those pieces is missing, the job is not ready to disappear behind the cabinet contents.