RO systems under sink are compact reverse osmosis units that treat drinking and cooking water at one fixture rather than the whole house. Each one is a small assembly: prefilters, a membrane, a storage tank, a dedicated faucet and a drain connection. Knowing what goes in the cabinet, and what the plumbing needs, is what separates a smooth install from a returned box.
What is in the box
A conventional under-sink unit has a manifold carrying two or three prefilter cartridges, a horizontal membrane housing, a small pressurized storage tank, a postfilter that polishes taste on the way out, a dedicated faucet for the treated water and a saddle or insert that ties the concentrate line into the sink drain. There is also a feed valve that taps the cold supply and an automatic shutoff that stops production when the tank is full. Tankless designs replace the tank with a booster pump that makes water on demand, trading cabinet space for an electrical outlet.
Space, and where it actually goes
Measure before you buy. The filter manifold needs clearance below it to drop cartridges out, typically a hand span, and the tank stands upright taking a footprint comparable to a small bucket. Garbage disposals, deep sink bowls and pull-out waste bins routinely eat the space these units assume. If the cabinet is genuinely full, the tank can be sited in an adjacent cabinet or a basement below with a longer tube run, and some households mount the manifold on the cabinet wall to keep the floor clear. Plan the layout on paper first, including where you will reach to change cartridges.
Pressure, drain and the faucet hole
Membranes are driven by incoming pressure. Low house pressure means slow production, a tank that never quite fills and noticeably reduced flow at the dedicated faucet, and it is the single most common complaint after installation. A booster or permeate pump solves it and needs a nearby outlet. The drain connection has to sit above the trap weir on the vertical tailpiece so waste water cannot siphon back. And the dedicated faucet needs a hole in the countertop or sink deck: many sinks have a spare accessory hole under a blank, and if yours does not, drilling granite or quartz is a job for a fabricator.
Living with one
Prefilters are changed on a schedule and the membrane far less often, and the carbon prefilter is the one that matters most, because chlorine reaching the membrane destroys it. Sanitize the housings and the tank whenever the system has stood unused for a long period. Expect the first tank of water after any filter change to be flushed and discarded. Check under the cabinet for drips after every service, since quick-connect fittings that were disturbed are where leaks start. Our best RO system under sink guide compares the current units against these practical constraints.
Questions people ask about ro systems under sink
What does RO stand for?
Reverse osmosis. Water is pushed against a semipermeable membrane that lets water molecules through and rejects most dissolved contaminants to a drain line.
Do I need a plumber to fit an under-sink RO system?
Many are homeowner-fittable with basic tools. A plumber is worth it if you need a new hole drilled in a stone countertop, or if the drain and supply connections are awkward.
Can I connect an RO system to my existing kitchen faucet?
Not usually. Treated water goes to its own dedicated faucet, which keeps the untreated supply available for washing and prevents the system from being drained by ordinary use.
Will an under-sink RO unit slow down my kitchen tap?
No. It only feeds its own faucet. The main tap is untouched, apart from the small tee taken off the cold supply line.