An under sink reverse osmosis filter system is the standard way to get membrane-treated water at the kitchen tap. It lives out of sight in the cabinet, taps into the cold supply, sends its concentrate to the drain, and delivers treated water through its own small faucet on the sink deck. Whether it fits comes down to three things: cabinet space, a place for the faucet, and enough incoming pressure to drive the membrane.
What actually goes in the cabinet
A typical system is a manifold of cartridges hanging from a bracket, plus a pressurized storage tank. Water enters through a sediment prefilter, passes one or more carbon stages that protect the membrane from chlorine, crosses the membrane itself, and the treated stream fills the tank while the concentrate leaves via a flow restrictor to the drain. When you open the dedicated faucet, water leaves the tank through a final carbon post filter that polishes taste. The connections are a feed adapter on the cold angle stop, a drain saddle or a proper drain connection on the sink tailpiece, and tubing between the tank, the manifold and the faucet.
What your sink and cabinet must have
Check three things before ordering. First, space: the storage tank is the bulky part, and while most tanks can lie on their side or be remoted to an adjacent cabinet, that has to be planned. Second, a home for the faucet, which means either a spare hole in the sink deck, a hole occupied by a soap dispenser or sprayer you can give up, or a willingness to drill, which is straightforward in stainless steel and a job for someone with the right bit in granite or quartz. Third, pressure: membranes need decent incoming pressure to produce properly, and a well household should check the pressure switch settings before blaming the system.
Tank versus tankless
A tank system stores treated water so you get a normal flow when the faucet opens, and it refills quietly afterwards. It is the simpler, cheaper and more common design, and its weakness is the space the tank takes and the fact that stored water sits between uses, which is why the post filter exists. A tankless system uses an internal pump to produce on demand at a rated delivery rate, which frees the cabinet and eliminates storage, at the cost of needing power under the sink and making some noise while it runs. Neither is better in the abstract: the deciding factors are cabinet space, whether you have an outlet, and how much water you draw at once.
Living with the system
Prefilters and the post filter are consumables on the manufacturer's schedule, and the discipline of changing them on time is what makes the membrane last. Sanitize the housings and the tank when you change cartridges, and keep the tank pressure checked with a tire gauge on the schedule in the manual, since a flat tank bladder is the usual reason a system that produces fine delivers a trickle. Slow flow at the faucet is nearly always a tank pressure problem, a clogged prefilter or a spent post filter, in that order. Many people add a tee to the fridge and ice maker line, which is easy, but expect a slower ice fill. Our best reverse osmosis system under sink comparison covers how the models differ.
Questions people ask about under sink reverse osmosis filter system
How much space does an under sink reverse osmosis system need?
The filter manifold is slim, but the storage tank is the bulky part. Most tanks can lie down or be relocated to a neighboring cabinet, which is the usual solution in a crowded sink base.
Do I have to drill a hole for the reverse osmosis faucet?
Only if the sink has no spare hole. Many people repurpose the hole from a sprayer or soap dispenser, and drilling stainless steel is simple while stone and composite need the right bit and care.
Why is my reverse osmosis faucet flow so weak?
Check the storage tank air pressure first with a tire gauge, then the prefilters, then the post filter. A flat tank bladder is the most common cause of a system that produces but will not deliver.
Can I connect the system to my refrigerator ice maker?
Yes, with a tee off the treated line. Expect slower ice production, since the fridge is now drawing from a system that produces gradually rather than from full house pressure.