Reverse osmosis, for one tap, done properly

What is reverse osmosis installation?

Reverse osmosis pushes water through a semi-permeable membrane that lets water molecules pass and rejects most of what is dissolved in it, sending that concentrate down the drain. In a house it almost always lives under the kitchen sink, feeding a dedicated faucet and often the fridge, with a small pressurised tank storing treated water so you are not waiting on the membrane. It is point-of-use by nature: excellent for what you drink and cook with, irrelevant to everything else in the building.

Signs you need reverse osmosis installation

• You are buying bottled water for drinking and cooking and would rather not
• Your tap water tastes or smells wrong even though the utility report is unremarkable
• A lab result flagged something you want addressed specifically in your drinking water
• You have a softener and want an unsoftened-tasting drinking supply back at one tap
• The ice maker and the water dispenser in the fridge are producing ice you can taste
• You are on a private well with a treatment train handling the house but nothing polishing the drinking supply
• You want an aquarium, a coffee setup or a steam oven fed with consistently low-mineral water

Seeing something not on this list? Browse water treatment problems by symptom →

How reverse osmosis installation actually works

1. 1

   Confirm RO is aimed at the right problem

   RO is a drinking-water technology. If your issue is scale on the heating elements or soap that will not lather, RO under one sink will not touch it. Match the technology to where the complaint physically occurs before anything else.

   Watch forBeing sold RO as a whole-house answer. Whole-house RO exists but is an entirely different scale of equipment and cost, and it is very rarely the right domestic answer.

2. 2

   Check incoming pressure - the membrane depends on it

   Reverse osmosis is driven by pressure across the membrane. Low supply pressure means slow production, a tank that never fills properly, and poorer performance. Where pressure is genuinely low, a booster pump is the fix and it should be specified up front.

   Watch forNobody measuring pressure before quoting, especially on well systems where the pressure tank's cut-in point governs the low end of the range. This is the most common cause of a new RO system that "does not seem to work".

3. 3

   Understand the stages, and that the membrane is not the only consumable

   A typical unit runs sediment and carbon pre-filters, then the membrane, then a post-filter polishing the taste on the way to the tap. The pre-filters exist mainly to protect the membrane - chlorine in particular damages some membrane types, which is why the carbon stage is not optional.

   Watch forA quote that prices the system but is vague about the replacement schedule. Pre-filters are changed far more often than the membrane, and skipping them is how an expensive membrane dies early.

4. 4

   Accept the wastewater, and ask what the ratio is

   Every RO system sends concentrate to the drain - that is how it works, not a defect. The ratio of drain water to product water varies substantially between designs, and newer units are markedly better than older ones. Ask for the figure for the specific model.

   Watch forA seller who will not state the ratio, or who claims there is no wastewater at all. On a septic system or a metered supply, this figure is worth asking about specifically.

5. 5

   Site the tank, and understand what it does to flow

   The membrane produces slowly, so a small pressurised storage tank holds treated water ready for use. That tank is why the RO faucet delivers a usable stream rather than a trickle, and it is also why the flow is gentler than your main tap and slows as the tank empties.

   Watch forA tank with lost air pre-charge. It is the most common cause of an RO system that gives you half a glass and then dribbles, it is cheap to re-pressurise, and it is frequently misdiagnosed as a dead membrane.

6. 6

   Drill the faucet hole - the irreversible step

   RO needs its own small faucet, usually in a spare sink knockout or a new hole in the sink deck or worktop. Stainless is drillable with care; granite, quartz and composite need the right bit and a steady hand; some materials are genuinely risky to drill.

   Watch forSomeone drilling an engineered stone worktop without saying out loud that a mistake here is not repairable. If there is no spare knockout, agree the exact position and the method before the drill comes out, and consider an air-gap faucet variant if your local rules call for one.

7. 7

   Make the drain connection properly

   The concentrate line ties into the sink drain, traditionally via a saddle clamp on the waste pipe above the trap. It must go in above the trap and be arranged so nothing can siphon back, and in some jurisdictions an air-gap faucet is required rather than optional.

   Watch forA drain saddle clamped below the trap, or fitted to a pipe material it was not made for. This is the part of an RO install that DIY guides gloss over and that inspectors look at.

8. 8

   Decide on the fridge line and remineralisation

   Running RO water to the fridge is popular and straightforward, but the long run means slower recovery and it is worth checking the appliance is happy with the supply. Some people add a remineralisation stage to put a little mineral content back, mainly for taste - RO water strikes many drinkers as flat.

   Watch forAssuming remineralisation is a health necessity. It is largely a taste and, for some, a corrosivity consideration. Sellers on both sides overstate their case; if the flat taste bothers you, add it, and if it does not, do not.

What does reverse osmosis installation cost?

Under-sink reverse osmosis typically runs from a couple of hundred dollars for a self-installed unit to roughly a thousand or more professionally installed with a fridge tie-in, a booster pump or an air-gap faucet. Ongoing cost is pre-filters changed regularly and the membrane much less often. Ask for the replacement schedule and prices.

See the full reverse osmosis installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →