A reverse osmosis water faucet is the small dedicated tap that serves treated water at the kitchen sink. It exists because an RO system produces water slowly into a storage tank and cannot supply a main faucet, and choosing between the two designs on offer decides how the system drains and what your local code will accept.
Air gap or standard, and why it matters
An air gap faucet has an extra passage in the body: the reject water from the membrane travels up into the faucet, crosses a gap open to atmosphere, and falls back down to the drain line. That gap makes it physically impossible for drain water to be siphoned back into the RO system, and some jurisdictions require it. It needs three tubes and a larger hole in the sink. A standard faucet carries only treated water, uses a single tube, is quieter, and relies on the drain connection being made correctly instead.
Finding a hole, or making one
Most kitchen sinks have a spare knockout used for a sprayer or soap dispenser, and that is the easiest home for the faucet. If not, you drill: stainless steel takes a step bit or hole saw with cutting oil and steady low speed, and a backing block to stop chatter. Composite and enameled cast iron are riskier, since the coating chips. Granite and quartz worktops need a diamond core bit, water cooling and patience, and are the case where hiring a professional is a genuine saving. Check clearance under the deck before you cut, since the mounting shank and nut need room.
Connections that do not leak
Match the tubing size the faucet expects: RO systems are commonly plumbed in a small diameter tube, and mixing sizes with an unsuitable adapter is a slow leak waiting to happen. Cut tubing square with a proper tube cutter, push it fully home into the push-fit collet, then tug it to confirm it is seated. On compression style fittings, use the insert supplied so the tube cannot collapse under the ferrule. Support the tube where it turns, keep it clear of the disposal, and leave a service loop so the faucet can be lifted without straining a joint.
Diagnosing slow flow
Slow flow at an RO faucet is nearly always the storage tank rather than the faucet. Check the tank's air charge with the tank isolated and empty, using a tire gauge on the schrader valve, and recharge to the maker's figure if it has dropped. Other causes are a clogged prefilter, a fouled membrane, low feed pressure, or a kinked tube. If the faucet itself is the problem you will usually see it as a drip from the spout or a stiff handle, and cartridge or valve stems for common faucet designs are available separately.
Finish, height and living with it
Match the finish to your main tap if that matters to you, and check the spout height against the vessels you fill, since many RO faucets are short. Long-reach and gooseneck designs suit deep sinks. Air gap faucets gurgle during a production cycle, which is normal but surprises people. Keep the drain saddle or drain connection accessible, because a partially blocked drain line is the usual cause of water appearing at the air gap opening. Our reverse osmosis guide covers choosing the system that sits behind the faucet.
Questions people ask about reverse osmosis water faucet
Do I need an air gap faucet?
Some local plumbing codes require one for the backflow protection it provides. Where it is not required, a standard faucet with a correctly made drain connection is quieter and simpler.
Why does water come out of the small hole on my faucet?
That is the air gap opening, and water there means the drain line is partly blocked or kinked downstream. Clear the drain line rather than blocking the opening.
Can I connect an RO system to my existing kitchen tap?
It is possible with a diverter, but not usual. RO produces water slowly from a small tank, so a dedicated faucet keeps the treated supply separate from high-volume tap use.
My RO faucet dribbles and the tank sounds empty. What is wrong?
Most often the storage tank has lost its air charge. Isolate and drain the tank, check the pressure at the schrader valve, and recharge it to the maker's specification.