An under-sink reverse osmosis job follows a natural order: shut off and tap the cold line, mount the prefilters and membrane, connect the drain and the storage tank, then start up and flush before the water is used. If you know that order before the installer arrives, you can tell when the work is going to plan and when steps are being skipped.
Before the visit
Confirm where the cold supply line runs under the sink and whether the cabinet has room for the filter housings and the storage tank. Ask the provider where the feed will be tapped, where the drain will go, and where the faucet will sit. Confirm that the cold line, not the hot line, will supply the unit. Ask whether a permit is needed for your address, since some local plumbing rules govern drain connections and fittings.
Mounting and connecting the components
The crew should shut off the cold supply, install a feed valve or adapter on the cold line, and run a feed line to the unit. The prefilters and membrane housing should be mounted in the order the manufacturer lists, with each filter oriented the way the manual shows. Fittings should be tightened by hand and then gently with a wrench, not forced. If the crew seems to be working from memory, ask to see the manufacturer's instructions.
Drain connection and air gap
The drain line carries reject water, and its connection should follow local plumbing code and the manufacturer's instructions. Those instructions commonly call for an air gap or an approved drain fitting, which helps prevent backflow into the unit. Ask the installer to point out where the drain connects and whether that meets the code requirement at your address.
Storage tank and faucet
Connect the storage tank with its own valve, and keep that valve closed until the startup checks are done. Install the dedicated faucet or tap connection as planned, and confirm that it sits where you can reach it without bumping other fixtures.
Startup and flushing
Open the feed slowly, check every fitting for leaks, and let the system run through the flush the manual describes. Drain the storage tank and discard the initial water as the manufacturer directs before drinking from the faucet. Ask the installer to show you the flush step and the leak check, not just tell you they were done.
The handoff
Before the crew leaves, ask for a written filter change schedule, the location of the feed shutoff, and the warranty terms for the unit and its parts. Ask who to call for service and how long a response usually takes. A job that ends with those items in hand has followed the sequence. One that ends with a verbal walkthrough and no paperwork has not.