An under-sink reverse osmosis system is a short list of connections, and knowing what each one is for turns a confusing tangle of tubing into a system you can service. This page maps the fittings in the order water meets them, from the cold feed under the sink to the faucet on the counter.
The feed side: adapter, valve and prefilter ports
Water leaves the cold angle stop through a feed adapter tee that carries a small shutoff valve, so you can isolate the system without turning off the sink. From there a quarter inch line runs to the first prefilter housing. The feed adapter is the fitting most often fitted wrong: the sealing washer belongs on the flat face, tape belongs on tapered thread only, and overtightening a plastic adapter into a brass stop is how a slow drip starts behind the cabinet.
The membrane housing, drain saddle and check valve
The membrane housing has three ports: feed in, concentrate to drain, and permeate out. The concentrate line carries a flow restrictor sized to the membrane, then reaches a drain saddle clamped to the sink tail piece above the trap and away from the disposal inlet, so the system never siphons waste back. A check valve on the permeate line stops tank water pushing backwards through the membrane. Fit these the wrong way round and the system either runs constantly or will not fill at all.
Tank, post-filter and faucet connections
The storage tank carries a threaded ball valve at its top port, which is your isolation point for changing a post-filter or diagnosing a soft tank. From there the line runs through the carbon post-filter, which usually has quick-connect stems moulded into each end, and on to the faucet shank under the counter. The faucet connection is often a compression or push-fit adapter on a longer tube; leave slack so the tube is not under tension when the faucet is turned.
Sizes, tubing and what to keep as spares
Most residential systems run quarter inch tubing throughout, with three-eighths inch on the faucet and tank lines of higher flow designs. Buy tubing that matches the fitting size exactly, since a slightly undersized tube will hold briefly then creep out under pressure. Keep a handful of spare push-fit elbows, a couple of tee fittings, a few feet of tubing and a spare tank valve in a bag with the filters. Our reverse osmosis system installation guide covers the physical layout and clearances these fittings assume.
Questions people ask about ro system fittings
Which fitting leaks most often on an RO system?
The feed adapter at the cold angle stop, usually from an overtightened plastic thread or a missing washer, followed by the drain saddle gasket.
Can I use plumbing tape on push-fit fittings?
No. Push-fit joints seal on the tube surface with an o-ring, so tape does nothing there. Tape belongs on tapered pipe threads only.
How high above the trap should the drain saddle sit?
Above the trap weir and on the vertical or upper horizontal tail piece, so waste water never sits against the line and cannot be drawn back.
Do I need a check valve if my system already has one?
No, and doubling them can stop the system filling. Confirm the location of the existing valve before adding anything to the permeate line.