An under-sink reverse osmosis system can look finished while still having problems that show up later. A loose drain connection may leak only when the sink is full. A poorly supported tube may kink when cleaning supplies are moved. A storage tank may be left without enough room for filter service.
The best time to catch these issues is before the installer packs up. You do not need to supervise every fitting, but you should understand the installation plan, see the system operate and know which parts you will need to access later.
Start with the water source and treatment goal
Before drilling or connecting anything, the installer should confirm which cold-water line serves the chosen faucet and what the system is expected to address. Reverse osmosis systems are commonly installed for drinking and cooking water, not as a substitute for diagnosing an unknown plumbing or water-quality problem.
Ask whether the proposed filters and membrane match your water source. If the home already has a softener, iron filter or whole-house carbon filter, the installer should understand how that equipment affects the reverse osmosis system. The installation plan should also follow the equipment manufacturer's requirements for pressure, temperature, pretreatment and drain placement.
Choose a location that can actually be serviced
The system needs more than enough space to fit under the sink. Filter housings, cartridges and the storage tank must be removable without dismantling the cabinet.
Before installation begins, check:
- Whether the filter assembly can be reached without removing the disposal or drain piping.
- Whether there is room below filter housings for cartridge replacement.
- Whether the storage tank can be lifted out when necessary.
- Whether tubing will remain clear of drawers, hinges, cleaning supplies and sharp cabinet edges.
- Whether a leak could reach flooring or nearby cabinets before being noticed.
If the cabinet is crowded, ask about placing the storage tank in an adjacent cabinet or another approved location. Longer tubing runs can affect performance, so the installer should confirm that the equipment permits the proposed arrangement.
Confirm where the drinking-water faucet will go
A dedicated faucet requires a suitable opening in the sink or countertop. Do not approve drilling until the installer has checked the material, underside clearance and faucet reach.
Look below the proposed location for sink clips, reinforcing ribs, disposal components and plumbing. Above the sink, make sure the faucet can rotate as intended and dispense into the basin without interfering with the main faucet or window trim.
If you want treated water supplied to a refrigerator or ice maker, say so before work starts. Ask whether that connection is included, where the tubing will run and whether the refrigerator's internal filter should remain installed. The correct answer depends on the appliance instructions and the water system design.
Inspect the feed-water connection
The system should connect to the cold-water supply through a compatible, accessible shutoff arrangement. Ask the installer to show you the valve that isolates the reverse osmosis unit.
The connection should not depend on a valve that is already corroded, difficult to turn or leaking around its stem. If the existing sink shutoff is in poor condition, address that before relying on it as part of the new installation.
After the water is turned back on, inspect the feed connection with a dry hand or paper towel. A slow seep may not form a visible drop immediately.
Pay close attention to the drain connection
Reverse osmosis systems send concentrate water to a drain while producing treated water. The drain connection is therefore a working part of the system, not just a place to tuck a tube.
The installer should place the connection where wastewater can enter the drain properly and where sink waste is unlikely to enter the treatment tubing. The exact location and fitting depend on the sink, disposal, plumbing configuration and equipment instructions.
Ask the installer to explain:
- Where the drain tube connects.
- Whether an air-gap faucet or another approved backflow protection arrangement is used.
- How the tubing is secured against movement.
- What sound during normal production is expected.
- Which gurgling, leaking or continuous-flow conditions should trigger a service call.
Run water through each sink basin and operate the disposal, if present, while checking the connection. A drain fitting should remain dry and stable during ordinary sink use.
Check every tubing route
Color-coded tubing is helpful only if you know what each line does. Ask the installer to identify the feed, product-water, tank and drain lines.
Tubing should have smooth bends rather than sharp folds. It should not rub against a sharp opening, touch a hot-water line or hang where stored items can pull it loose. Connections should be fully seated and secured in the manner specified by the fitting manufacturer.
A neat installation also leaves enough tubing for reasonable service access. Lines stretched tightly between components may pull on fittings when a filter is changed.
Verify the storage tank setup
The storage tank should sit securely and remain accessible. Ask whether the tank orientation and support match the manufacturer's instructions.
The tank valve should be easy to identify and operate. Have the installer show you which position is open and which is closed. Do not adjust the tank's air charge unless you have the correct procedure and pressure specification for that model. Tank pressure is checked under defined conditions, and an uninformed adjustment can reduce usable storage or prevent proper operation.
Make sure startup is completed, not merely started
A new system generally requires flushing and inspection before its water is used. The correct sequence varies by model, so the equipment instructions should control the process.
Ask the installer to explain:
- Which cartridges and membrane were installed.
- Whether any protective solution or manufacturing residue must be flushed.
- How the storage tank is filled and emptied during startup.
- When the water is ready for drinking and cooking.
- Whether cloudy water or small carbon particles are expected during initial flushing.
If the full startup procedure cannot be completed during the appointment, get the remaining steps in writing. You should know which valves stay open, what water to discard and how to recognize normal operation.
Test operation under realistic conditions
Opening the drinking-water faucet for a moment is not a complete final check. The installer should verify the system while it is producing water and while water is flowing from the faucet.
Useful checks include:
- Confirming that the drinking-water faucet operates smoothly.
- Looking for leaks at the feed valve, filter housings, tank valve, faucet connection and drain fitting.
- Making sure the system starts producing water after treated water is drawn.
- Checking that tubing does not move, kink or strike the cabinet when the faucet is used.
- Verifying that any refrigerator connection delivers water without leaking.
- Confirming that the system eventually stops sending water to the drain after the storage tank is full, according to the equipment's normal operating design.
The final shutoff check may require more time than the installer can spend at the sink. If so, ask how you can verify it later and what continued drain flow would indicate a problem.
Get baseline readings when they are relevant
A dissolved-solids meter can help confirm that a membrane is reducing dissolved material, but a single reading does not establish that water is safe or that every unwanted substance has been removed. The result also depends on feed-water conditions, system pressure, membrane condition and whether the unit has been fully flushed.
If the provider takes readings, ask for the untreated-water and treated-water results, the test method and the point where each sample was collected. Keep those results with the system paperwork. They can provide a useful comparison if performance changes later.
Leave with the system identity and service instructions
Before the installer leaves, photograph the installed system and equipment labels. Obtain the model information for the complete unit, cartridges, membrane, faucet and any leak-control device.
You should also receive:
- The owner's manual or a reliable way to access it.
- The required cartridge and membrane specifications.
- The recommended replacement procedure or service schedule.
- Warranty terms and any registration instructions.
- A written description of the work performed.
- Contact information for leaks, loss of flow or continuous drain operation.
Ask whether replacement filters are proprietary or available from more than one source. Also ask whether changing them requires special tools, resetting a monitor or sanitizing the housings.
Know how to isolate the system
Have the installer demonstrate how to shut off the feed water and close the storage tank valve. Then locate the main sink shutoff in case the treatment-system valve cannot stop a leak.
Write the instructions inside the cabinet or store them with a photograph of the valves. In a leak, a clear shutdown sequence is more useful than trying to identify unfamiliar tubing under pressure.
Do one final cabinet check
Remove loose items from the cabinet and place a dry paper towel beneath each connection. Recheck it after the system has produced water and after the sink and disposal have been used. This simple step can reveal a slow seep that was not visible during installation.
Do not accept unexplained moisture, tubing under strain, a drain connection that moves or instructions that amount to waiting for a problem. A complete installation ends with a dry cabinet, demonstrated operation, a clear startup procedure and a homeowner who knows how to shut the system down.