A reverse osmosis water system is a compact piece of treatment equipment that lives in your kitchen cabinet and feeds a small faucet of its own. Household water pressure pushes tap water through a membrane. Treated water collects for drinking and cooking. Whatever the membrane holds back goes down the drain. This guide is about the hardware, and what it takes to install it and keep it running. For the science, read how reverse osmosis works.

What is under the sink

Open the cabinet on a typical install and you will find these parts, roughly in the order water passes through them.

  • Feed line. A fitting on the cold water supply sends water to the unit.
  • Pre-filters. A sediment cartridge catches silt and grit. A carbon cartridge removes chlorine, which matters here: the Region of Durham disinfects with chlorine, and thin-film composite membranes, a common type, break down in chlorinated water.
  • Membrane. A tightly rolled sheet in a housing, where the reverse osmosis happens.
  • Flow restrictor and shut-off. The restrictor sets how fast reject water leaves. The automatic shut-off stops the process when the tank is full.
  • Storage tank. A pressurized tank, generally 2 to 5 gallons (about 7.5 to 19 litres), according to Nebraska Extension. It lets treated water flow the moment you open the tap.
  • Post-filter. A final carbon cartridge polishes taste and odour.
  • Dedicated faucet. Treated water gets its own small faucet beside the main tap.
  • Drain saddle and air gap. The waste line often joins the sink drain above the trap through a drain saddle, a clamp that bolts around the pipe over a small hole. The air gap stops drain water from ever reaching the unit. Many installs use an air gap faucet, with the gap built in above the sink. If the drain clogs, waste water spills from a weep hole on the faucet instead of backing up, as the Minnesota Department of Health explains.

Our beginner's guide to reverse osmosis filters covers what each stage removes.

The drain rule in Ontario

The drain is the part a kit cannot get right on its own. Ontario's 2024 Building Code lists a water treatment device among the equipment that must be indirectly connected to a drainage system (Article 7.4.2.1). The connection has to end above the flood level rim of a directly connected fixture, leaving an air break of at least 25 mm (Article 7.3.3.11). That means waste water falls through open air on its way to the drain, and drain water cannot flow back toward your drinking water. The code also requires a drinking water treatment system to conform to CAN/CSA-B483.1 (Article 7.2.10.17). The kit is not the code. We set up every drain connection to meet these rules.

Tank, tankless and permeate pumps

Most under-sink units store treated water in a tank, so a full glass comes out fast. The catch is that a filling tank pushes back against the membrane. The U.S. EPA calls this back pressure, and it slows production and lowers the unit's water efficiency.

A permeate pump is one answer. It is a non-electric device that can be added to a tank unit to cut back pressure and improve efficiency. It runs on the flow of waste water, so it needs no outlet.

Tankless units skip the tank and make water on demand, typically with an electric booster pump. The EPA notes they take up less space but may take a little longer to fill a glass. That pump needs an outlet in or near the cabinet.

Every reverse osmosis water system sends some water down the drain. A typical point-of-use unit sends five gallons or more to the drain for every gallon it treats, according to the EPA's WaterSense program. Units that earn the WaterSense label send 2.3 gallons or less, so ask about the ratio before you buy. On a septic system, that extra water adds to the load.

Check the certification too. Health Canada strongly recommends a treatment device certified to the matching performance standard, which for reverse osmosis is NSF/ANSI 58.

What a reverse osmosis water system needs from your home

Cabinet space

The filter assembly and the tank each need floor space. They share it with the drain trap, the dishwasher lines, maybe a garbage disposal, and everything you keep under the sink. A tankless unit is more compact. At least one manufacturer allows the unit to sit one floor below, in the basement, with tubing run up to the faucet. The faucet needs a hole of its own. A spare one in the sink deck, often left from a sprayer or soap dispenser, can work. If not, one is drilled.

Water pressure

Most units run on household pressure alone, and that pressure has to overcome the membrane's osmotic pressure plus any back pressure from the tank. One manufacturer's reverse osmosis FAQ puts the ideal at 60 psi. It calls anything under 40 psi insufficient and says it should be boosted with a pump. If your taps already feel weak, our guide to low water pressure shows how to check yours. A low-pressure kitchen may be better off with a pump-assisted or tankless unit.

Your water

The Region of Durham lists the hardness of the Lake Ontario supply that serves Oshawa at 125 mg/L as calcium carbonate. Port Perry's water is 274 mg/L. In relatively hard water, a softener ahead of the unit can extend membrane life, according to Nebraska Extension, which is why some homes pair RO with a water softener. Private wells need a different setup. Nebraska Extension warns that a carbon pre-filter can encourage microbial growth on the membrane when the water is not chlorinated, and that only coliform-free water should feed the system. The Region offers free testing of private well water.

What a Hayes installation involves

  1. We test your water before recommending anything.
  2. We recommend the setup that fits your water and household. That might be an under-sink RO unit, whole-home filtration, a softener or a mix, and the price comes in writing. Whole-house reverse osmosis is a bigger question, best discussed after the test.
  3. A registered plumbing technician installs the unit, tank, feed line and faucet, and sets up the drain and air gap to code.
  4. We show you how it works and what to watch for. One example: water running to the drain while nobody is using the faucet can mean the automatic shut-off needs replacing.
  5. We set up your cartridge replacement schedule, and we handle replacements and repairs when they come due.

The cartridge schedule

Your unit's manual sets the intervals. Health Canada advises following the maintenance instructions that come with the device, and your water and how much you draw both affect the timing. As a rough guide, one manufacturer says pre-filters and post-filters need replacing about once a year and that a membrane normally lasts two to five years. For tank units, the EPA's guide to RO systems adds two jobs: drain the tank about every two weeks to keep the water fresh, and sanitize it yearly, as many manufacturers recommend. See how often to change a water filter for more on timing, and maintaining a water filtration system for signs of trouble.

Cost, rental and financing

Our 2026 Oshawa plumbing cost guide puts an under-sink water filtration system at $300 to $800. We do not publish a separate reverse osmosis figure, because the price depends on the unit and on the faucet, drain and pre-treatment work your kitchen needs. We confirm it in writing before we start.

Hayes also offers financing and rental options on water treatment products, so you do not have to pay the full amount up front. See our financing page, or ask about it when we test your water.

Next step

A reverse osmosis water system starts with a water test. Call (905) 576-3043 or request a visit. Our water filtration service page lists the other systems we install and repair. Hayes Plumbing serves Oshawa and all of Durham Region.