An under-sink reverse osmosis system can look finished as soon as the faucet is mounted and the filters are hanging in the cabinet. That is only the visible part of the job. The installation also has to provide a controlled water supply, a dependable drain connection, an accessible storage tank and tubing that will not kink, rub or pull loose.
Before work begins, ask the installer to trace the proposed water path with you. You should be able to identify where untreated cold water enters the system, how product water reaches the storage tank and faucet, and where reject water enters the drain. This short review can expose a poor valve location, an unusable electrical outlet for an optional pump, or a tank position that blocks routine cabinet use.
Confirm what the system will serve
Decide whether the system will supply only its own faucet or also connect to a refrigerator, ice maker or another fixture. An additional run changes the tubing route and may affect how quickly water reaches each outlet. It also creates another connection that must be flushed and checked for leaks.
Ask whether every appliance on the route is suitable for the treated water and expected pressure. If a refrigerator connection is included, the proposal should identify who supplies the tubing, shutoff and fittings. Do not leave that branch described only as an informal extra.
Empty the cabinet and inspect the work area
The installer needs enough room to drill, fasten components and inspect connections under pressure. Remove stored items before the visit and look for damp cabinet material, corrosion, an active drain leak or a cold-water shutoff that does not operate. Existing plumbing defects should be separated from the new installation rather than hidden behind equipment.
Check the planned service clearance before anything is mounted. Filter housings may need room below them to drop or swing away. Cartridges, membranes and batteries may need to be reached from the front or side. The storage tank and shutoff valves should remain removable without dismantling the cabinet plumbing.
Test the cold-water shutoff before adding the feed connection
The system normally takes its supply from the cold-water line. The installer should close the existing shutoff and verify that it actually stops the flow before opening the plumbing. A valve that will not close reliably is a separate problem that should be resolved before the treatment connection is added.
Ask the installer to show you the dedicated feed valve for the reverse osmosis system. You should be able to shut off the treatment system without disabling the entire kitchen sink. The valve should be reachable after the cabinet is restocked, and its tubing should leave the fitting without a sharp bend.
Set the faucet and protect the mounting surface
The faucet location has to work above and below the counter. Above the counter, the spout should discharge into the sink and leave room to fill the containers you actually use. Below the counter, the stem and tubing must clear the sink basin, mounting clips, drawers and other hardware.
Before drilling, confirm that the installer has identified the countertop or sink material and the correct opening location. If an existing accessory hole will be used, check that the faucet base covers it fully. After mounting, the faucet should sit firmly and turn as intended without twisting the tubing underneath.
Mount the treatment assembly where it can be serviced
The filter assembly should be supported as its instructions require rather than left hanging from tubing. Ask the installer to show how each cartridge or housing will be removed. A neat-looking installation can still be difficult to maintain if a sump cannot clear the cabinet floor or a membrane housing faces the wall.
Labels should remain visible where practical. If the inlet, tank, faucet and drain ports are not obvious, ask for them to be marked. Clear identification makes later filter service less likely to disturb the wrong connection.
Place the storage tank on a stable surface
The tank becomes heavier as it fills, so it needs stable support and a position where it will not roll, tip or bear against a fragile drain fitting. Its shutoff valve should remain accessible. The tubing should have enough length for service but not so much that it forms tight coils or catches on stored items.
Ask whether the tank air charge was checked according to the system instructions before the first fill. Tank pressure is evaluated with the water side empty, not after the tank has filled. You do not need to accept a guessed setting or a test performed under the wrong condition.
Put the drain connection in a workable location
Reject water needs an open path into the sink drain. The connection should be placed where waste can flow away and where debris from the sink is less likely to collect at the opening. The exact acceptable location depends on the drain arrangement and the system design, so ask the installer to show you the opening before the cabinet is closed up.
The drain tubing should run without a kink, pinch point or sag that traps material. If the system uses an air gap faucet or another backflow protection arrangement, confirm that every related tube is connected to the correct port. A crossed or restricted drain line may show up later as noise, leakage near the faucet or water that does not stop when expected.
Route and secure every tube
Follow each tube by hand and eye before the water is turned on. It should not rub against a sharp cabinet edge, touch a hot-water pipe, pass through a hinge path or bend tightly at a fitting. Tubing that passes through a cabinet wall should be protected from the cut edge.
Push-fit connections must be made with clean, square tubing ends inserted fully according to the fitting instructions. Ask the installer to tug-test each connection gently after assembly. Clips or retainers supplied for the fittings should be installed rather than left in the parts bag.
Open the water slowly and inspect under pressure
The installer should open the feed valve in a controlled way, allow the housings and tubing to pressurize, and inspect every new connection. A dry paper towel passed around fittings can reveal moisture that is hard to see against plastic or chrome.
This inspection should include the feed adapter, filter housings, membrane housing, tank valve, faucet stem and drain connection. The installer should also operate the regular kitchen faucet and drain the sink to make sure nearby plumbing was not loosened during the work.
Flush the system before anyone drinks from it
New cartridges, a membrane and a storage tank may require a specific flushing sequence. The installer should follow the system instructions, not substitute a brief faucet run for the full procedure. Ask how many fill-and-discard cycles or how much continuous flushing the installed components require, and ask which outlets must be flushed if the system also serves a refrigerator.
During filling, confirm that reject water is reaching the drain. After the storage tank fills, open the treatment faucet and observe the flow. The first tank may fill slowly, so the handoff should account for the actual filling and flushing process rather than ending immediately after the feed valve is opened.
Verify that production and drain flow stop
A tank-style system should eventually stop producing water when the tank is full and no treated-water outlet is open. Ask the installer to verify the automatic shutoff behavior after the system has had time to fill. Continuous drain flow should be investigated before the job is considered complete.
If a pump is part of the system, have the installer demonstrate when it starts and stops. Confirm that its power connection stays dry, remains accessible and does not depend on an extension cord routed through the cabinet.
Complete the handoff at the cabinet
Before the installer leaves, have them point out the feed shutoff, tank valve, filter stages, membrane housing and drain connection. Practice isolating the system without disturbing the sink supply. Ask what normal filling, faucet flow and drain noise should be like so you can recognize a change later.
Get the installed filter and membrane part numbers, the required replacement sequence and the flushing instructions in writing. Record any settings or optional components, including pumps, remineralization cartridges or refrigerator branches. Keep the system manual where the next service person can find it.
Ask what final water check is included and what that check can establish. A dissolved-solids reading may help confirm membrane performance, but it does not answer every water-quality question. If treatment was selected for a specific contaminant, the verification method should match that concern.
Use a start-to-finish scope when comparing installers
Two proposals may name the same general type of system while including different installation work. Compare whether each proposal covers the feed valve, faucet drilling, drain connection, tank placement, refrigerator tubing, initial flushing, leak testing, shutoff verification and removal of packaging or old equipment. Put exclusions in writing before choosing.
Provider ratings can help you build a shortlist, but the installation scope still needs to be compared line by line. Midwest homeowners can start with the regional provider index. Readers in the flagship market can also review the St. Louis water treatment ratings.
The final test is simple: you should know where the water enters, where every tube goes, how to shut the system off and what remains to be flushed before use. If those questions are unanswered, the installation is not ready for handoff.