An under-sink reverse osmosis system can look straightforward on a proposal: filters, membrane, storage tank, faucet, and a few tubing connections. The real installation involves more decisions. The installer must find safe connection points, fit the equipment into a crowded cabinet, route the drain correctly, and commission the system without leaving leaks or installation debris behind.
Before work begins, ask the installer to walk through the complete layout with you. You should know where every component will go, what will be drilled, which valves will be operated, and how the finished system will be tested. That short review can prevent a faucet in the wrong location, a tank that blocks cabinet storage, or tubing that gets crushed whenever supplies are moved.
Start by confirming what the system will serve
Decide which fixtures and appliances will receive treated water. Many systems serve a dedicated faucet. Some also connect to a refrigerator or ice maker. That additional run affects tubing length, routing, available pressure, and the amount of water the storage tank must supply.
If a refrigerator connection is included, ask where the tubing will travel and how it will be protected. Tubing should not be pinched behind an appliance, laid across a sharp cabinet edge, or routed where a fastener could puncture it. Confirm whether the refrigerator has its own filter and whether that filter is supposed to remain in place.
The installer should also identify the incoming water conditions used to select the system. Reverse osmosis equipment has operating limits, and pretreatment may affect membrane life and performance. If the home has a water softener, iron treatment, or another whole-house device, make sure the installer understands where the under-sink system sits in relation to that equipment.
Empty the cabinet and inspect the work area
Remove cleaners, trash containers, shelf liners, and stored items before installation. The installer needs access to the cold-water supply, sink drain, cabinet walls, and underside of the countertop. An empty cabinet also makes small leaks easier to see.
Check the cabinet floor and plumbing before new equipment is added. Existing moisture, swollen particleboard, corrosion, or mineral deposits may point to an old leak. Photograph questionable areas so there is a record of their condition before work starts.
Confirm that the shutoff valve for the cold-water line moves freely and actually stops the flow. A valve that is seized, corroded, or unable to close is a separate plumbing problem. Decide who will replace it and whether that work is included before the water line is opened.
Lay out the filters, membrane, and storage tank
The filter assembly should be mounted where the cartridges can be changed without removing the entire system. Ask the installer to show the clearance needed below each housing. A housing that fits during installation may still be impossible to unscrew once it is full of water and surrounded by drainpipes.
The storage tank needs a stable position. It should not rest on tubing, lean against a sharp fastener, or block access to the sink shutoffs. If the tank is placed horizontally, confirm that the manufacturer permits that orientation. Also check whether the tank valve and tubing connection remain reachable.
Plan for normal cabinet use. Pull out any sliding shelves or trash bins and open every door fully. Make sure those moving parts cannot strike the filter housings, tank, or tubing. Ask how the equipment will be protected from stored objects that might bend a connection.
Choose the faucet location before drilling
A dedicated faucet needs enough clearance above and below the sink or countertop. Above the surface, check the faucet handle, spout reach, window trim, soap dispenser, and main faucet movement. Below the surface, look for sink clips, reinforcement, plumbing, and cabinet framing.
Ask the installer to mark the proposed hole and show you the position before drilling. Confirm that the countertop or sink material can be drilled with the planned tools. If an unused accessory opening already exists, find out whether it can be used without creating an awkward faucet position.
The finished faucet should discharge into the sink, operate without hitting another fixture, and sit firmly without rotating. After installation, inspect the base from above and the mounting hardware from below.
Connect only to the cold-water supply
The feed connection normally comes from the cold-water line. The installer should shut off the water, relieve pressure, and create a serviceable connection using fittings suited to the existing plumbing. A piercing valve is not the same as a properly installed branch fitting, so ask exactly how the feed will be connected.
Look at the completed connection before the cabinet is refilled. The tubing should enter the fitting straight, without side pressure or a tight bend. The new feed valve should be accessible so the reverse osmosis system can be isolated without shutting off the entire sink.
Once the supply is restored, the installer should inspect both the new connection and the existing shutoff. Operating an older valve can reveal a stem or packing leak that was not visible before.
Put the drain connection in the right place
Reverse osmosis systems send reject water to a drain while producing treated water. The drain saddle or other approved connection must be placed where wastewater can enter the drain without flowing into the system tubing.
Under a typical sink, the connection should be located on an appropriate section of drainpipe and positioned according to the equipment instructions and local plumbing requirements. It should not be placed where a garbage disposal can force debris toward the tubing. The drilled opening and saddle port must align, and the tubing should run without a sag that collects dirty water.
Ask the installer to run the sink and disposal, if present, while checking the connection. Listen for unusual drain noise and look for seepage around the saddle, trap, and any fittings that were moved during the work.
Route and label the tubing
Multiple tubes may connect the feed, filter stages, membrane, tank, faucet, and drain. The installer should follow the equipment diagram rather than relying on tubing color alone. Ask for the lines to be identified so future service does not depend on guesswork.
Each tube should have a smooth path with enough slack for service but not so much that it tangles around stored items. Check for kinks, sharp bends, hot surfaces, and rubbing points. Tubing passing through a cabinet wall should be protected from the raw edge of the opening.
If a refrigerator line is installed, identify any hidden couplings and isolation valves. Fewer inaccessible connections generally make leak tracing easier.
Sanitize, flush, and commission the system
Installation is not complete when the last tube is connected. The installer should follow the equipment instructions for sanitizing components, installing cartridges and the membrane, opening valves in the correct sequence, and flushing manufacturing residue or preservative from the system.
Ask how many complete fill and drain cycles are required before you drink the water. During filling, the drain may run and the faucet flow may change as air leaves the system. The installer should explain which sounds and behaviors are expected and which ones indicate a problem.
Every accessible fitting should be checked under pressure. A dry paper towel passed around connections can reveal moisture that is hard to see. The inspection should include the feed adapter, filter housings, membrane housing, tank valve, faucet connection, drain connection, and refrigerator line.
Test the finished installation as a complete system
Open the dedicated faucet and observe the flow after the storage tank has filled. The installer should confirm that the tank delivers water, the automatic shutoff operates, and drain flow stops when the system reaches its normal shutoff condition. Continuous draining after the tank is full deserves investigation before the job is closed.
If the proposal includes a water quality target, ask the installer to perform the specified test after the required flushing is complete. The contract should identify the test method, sampling point, target, and response if the result is outside the promised range. A handheld reading can be useful for a basic operational check, but it does not identify every substance that may be present.
Run the sink, operate the disposal, and move any cabinet accessories through their full travel. If the system serves a refrigerator, dispense water there and confirm that the line does not leak when the appliance is returned to position.
Get the service information before the installer leaves
Ask for the equipment model numbers, cartridge sequence, replacement instructions, valve positions, and startup procedure after service. Record which direction closes the tank and feed valves. Keep the filter wrench and any system-specific tools where they can be found without unloading the entire cabinet.
You should also know who handles a leak, a faucet that sputters continuously, weak flow after the tank fills, or a system that never stops sending water to the drain. Have the installer show you how to isolate the system quickly while leaving the regular sink faucet available if the plumbing layout permits it.
Before choosing a company, compare more than the equipment name. Ask each bidder to describe the faucet drilling, feed connection, drain location, tubing route, sanitation process, flushing procedure, leak inspection, and final operating test. If you need a place to begin that comparison, review the water treatment companies recognized by Best Water Companies STL.
A sound under-sink reverse osmosis installation ends with more than treated water coming from a new faucet. It leaves accessible valves, serviceable filters, protected tubing, a secure drain connection, a dry cabinet, and a homeowner who knows how to shut the system down if something changes.