A reverse osmosis sink filter has a hidden half under the cabinet and a visible half on the countertop, and the visible half is where most installations go wrong. The dedicated faucet needs a hole, that hole may need drilling through stone, and the choice between an air gap and a standard faucet changes the plumbing behind it. Sort the faucet out before you buy the system.
Why RO needs its own faucet
Treated water is produced slowly and stored in a small tank, so it cannot supply a main kitchen faucet at normal flow. It also should not mix with untreated water in a shared spout. That is why every under sink system ships with a dedicated faucet on its own small bore line. Modern versions come in finishes to match common fixtures, and some are combination taps that deliver both filtered and unfiltered water through separate paths in one body, which is useful when there is no spare hole at all.
Finding or making the hole
Most sinks have a spare knockout used by a sprayer or a soap dispenser, and repurposing it is by far the easiest route. If there is none, you drill: stainless steel takes a step bit and cutting oil at low speed, enameled cast iron is fragile and chips, and composite or solid surface counters need a diamond hole saw with water cooling. Granite and quartz should generally be drilled by a stone fabricator, because a cracked countertop costs many times the price of the filter. Check clearance under the deck before you cut anything.
Air gap or non air gap
An air gap faucet routes the membrane drain water up through a channel in the faucet body and back down to the drain line, creating a physical break so drain water can never siphon back into the system. Some local plumbing codes require it. The tradeoffs are real: an air gap faucet needs a larger hole, uses extra tubing, and makes an audible gurgle when the system produces. A non air gap faucet is quieter and simpler and relies on the drain saddle placement instead. Confirm what your local code expects before choosing.
Running the line and the fridge
The faucet connects with small bore tubing to the post filter under the sink, and the run should be gentle, unkinked and long enough that the cabinet door does not pinch it. Many households also want RO water at the refrigerator for ice and the door dispenser. That is a tee off the product line plus a longer tubing run, and because a fridge draws more than a faucet, a larger storage tank or a permeate pump often helps keep up. Our under sink RO comparison guide covers matching tank size to how the household actually draws water.
Countertop alternatives when nothing can be drilled
Renters and owners of stone counters they will not drill have options. Countertop reverse osmosis units sit on the worktop, fill from a jug or a diverter on the existing faucet, and need only a socket, at the cost of taking up space and needing filling. Faucet diverter valves let a small system share the existing spout. Neither matches a plumbed in system for convenience, but both avoid a permanent hole, and both can move with you when the lease ends.
Questions people ask about reverse osmosis sink filter
Can I use my existing kitchen faucet for RO water?
Not normally, because stored RO water cannot supply a full flow faucet and should not mix with untreated water in a shared spout.
Do I need an air gap faucet?
It depends on your local plumbing code, since the air gap exists to prevent drain water siphoning back and some jurisdictions require that protection.
Can a granite countertop be drilled for an RO faucet?
Yes, with a diamond hole saw and water cooling, but it is usually worth paying a stone fabricator because a cracked slab is far more expensive than the system.
Why does my RO faucet gurgle?
Air gap faucets route drain water through the faucet body, and the noise as it falls into the drain line is normal for that design.