An under-sink reverse osmosis unit is one of the more approachable water treatment projects for a homeowner. The filter housings mount to the cabinet, the tubing runs to the faucet and the drain, and the storage tank sits in the cabinet. The parts that can go wrong are the ones that connect to the water supply and the drain, so those are the steps to size up before you start.
Check the sink before you buy
Measure the cabinet space for the filter housings and the storage tank, and check for clearance around the drain connection and the tubing bends. Find the cold water supply line and its shutoff valve, and confirm the valve turns freely. Look at the sink and countertop for an existing faucet hole, because a new one must be cut through stone or composite material, which is usually a job for a professional.
Tasks a homeowner can usually handle
Mounting the filter bracket, installing the filters, connecting the tank, and running the tubing are typically within reach for someone comfortable with basic hand tools and compression fittings. Take your time with each fitting, make sure the tubing is fully seated, and label the lines before you reconnect anything. Follow the manufacturer's flushing steps before you drink the water.
Tasks that often call for a plumber
Drilling a new faucet hole, replacing a supply line that is old or brittle, and connecting to a drain when your local rules require a specific method are tasks many homeowners hand off. Ask a licensed plumber to confirm the connection type, since some local plumbing rules set the approved method for a drain connection. Check with your city or county building department about permit requirements if you are unsure.
Test the install before you trust it
After installation, check every fitting for drips over several hours, then again the next morning. Pay particular attention to the cabinet floor and the tank connection. Once the system is flushed, check the faucet for steady flow and confirm the tank is filling. If a fitting weeps, fix it before you move on, because small leaks under a sink can go unnoticed for a long time.
Decide who does what
Ask yourself three questions: can I shut off the water and isolate the cold line safely, can I cut and fit tubing cleanly, and am I willing to leave the system running overnight to check for leaks? If you answer no to any of them, a professional should handle that part. If you are choosing an installer, the Top 7 Water Treatment Companies in Missouri page lists providers you can compare. Ask any installer who will handle the drain connection and the permit before you agree to the job.