What is involved in installing a reverse osmosis system?

A standard under-sink RO job follows the same sequence in most homes. Knowing the order helps you understand why it takes a few hours and where the plumber spends the most care.

1. 1Mount the unit and tank. The filter bank, with its sediment and carbon pre-filters and the RO membrane housing, is fixed to the cabinet wall or set on the floor of the cabinet. The pressurized storage tank sits beside it. The tank holds treated water so you get a steady pour instead of waiting on the membrane.
2. 2Install the dedicated faucet. RO water comes out of its own small faucet, separate from the main one. If the sink or counter has a spare hole (often a covered sprayer or soap-dispenser hole), the faucet drops in there. If not, the installer drills the counter or sink deck.
3. 3Tap the cold supply. A feed valve connects to the cold-water line under the sink and sends water into the system. Hot water would damage the membrane, so the feed always comes off the cold side.
4. 4Connect the drain. RO separates each batch into treated water (the permeate) and rejected water (the concentrate, or brine). A drain saddle clamps onto the sink drain pipe above the P-trap to carry the reject water away. Some local codes call for an air-gap faucet to keep drain water from siphoning back.
5. 5Run optional lines. Many installs add a line to a refrigerator or ice maker so the fridge dispenses filtered water and makes clearer ice.
6. 6Flush and sanitize. The new filters and membrane are flushed to clear carbon fines and any manufacturing residue, and the system is sanitized before first use.
7. 7Test the water. The installer checks the treated water for total dissolved solids (TDS) to confirm the membrane is working as it should.

The EPA describes the basic layout plainly: a point-of-use RO system "connects to a single water faucet (e.g., under the kitchen sink)" and splits incoming water into treated and rejected streams. That single-tap design is what keeps an under-sink unit compact and affordable.

Plan on a few hours for a typical under-sink install. Drilling a faucet hole through stone or running a fridge line adds time, while a sink with an open hole and easy cabinet access goes faster.

Before mounting anything, a good installer checks your incoming water pressure. RO membranes push water through a very fine barrier, and they need adequate pressure to work well. The CDC notes that an RO membrane has pores around 0.0001 micron, fine enough to reduce calcium, magnesium, fluoride, arsenic, and nitrate, which is also why feed pressure matters so much. If household pressure is on the low side, the system fills its tank slowly and wastes more water per glass. The fix is a permeate pump or a booster pump, which raises the working pressure across the membrane and improves both fill speed and efficiency.

The installer also confirms there is room in the cabinet, that the cold-supply stub and drain pipe are sound, and that the drain connection sits above the trap. These checks take only a few minutes but head off the most common callbacks.

Once the system is plumbed in, the storage tank needs to fill before you draw a full glass. That first fill usually takes a couple of hours.

Here is the part people skip: discard the first full tank of water. The initial batch clears any remaining carbon dust and lets the membrane settle into normal operation. Fill the tank, drain it completely, and let it refill. After that the water is ready to drink.

Testing closes the loop. An installer measures TDS before and after to show the drop, which is the quickest field check that the membrane is rejecting dissolved minerals. RO units sold for drinking water are tested and certified to NSF/ANSI 58, the standard written specifically for residential RO systems and the one that verifies a unit's TDS-reduction claim. Phoenix tap water runs hard, roughly 10 to 16 grains per gallon with TDS often in the 360 to 766 ppm range per the city's water-quality report, so a clear before-and-after TDS reading is a satisfying confirmation that the system is doing its job.

Most RO systems are point-of-use: one unit, one tap. A whole-house RO setup treats every drop entering the home, which means large membranes, a big pressurized storage tank, a pump, and pretreatment ahead of it. It wastes far more water, strips minerals from water you only bathe or flush with, and costs much more to install and run. You generally do not need RO on your shower or toilet water, which is why the under-sink approach is the common, lower-cost choice. The trade-offs between the two setups are covered in our guide to choosing under-sink versus whole-house reverse osmosis.

Three parts of the install are worth a professional's hands:

• The faucet hole. Drilling a clean hole through a granite, quartz, or porcelain deck without chipping or cracking takes the right bit and technique. A mistake here is expensive.
• The drain connection. The drain saddle has to sit above the P-trap and seal tight, and an air-gap setup has to be plumbed correctly. A sloppy drain tie-in leaks or lets drain water back-siphon toward your treated water.
• Pressure issues. Diagnosing low feed pressure and sizing a permeate pump is judgment work. Done right, it is the difference between a fast, efficient system and one that trickles and wastes water.

RO efficiency varies widely. The EPA reports that many conventional units waste 5 or more gallons for every gallon of treated water, and inefficient models can run as high as 10 to 1. WaterSense-labeled units waste 2.3 gallons or less per gallon treated and can save a household about 3,100 gallons a year. Getting the install and pressure right keeps your unit on the efficient end of that range.

The day the system goes in, the maintenance clock starts. RO is not set-and-forget. As a rule of thumb, the sediment and carbon pre-filters get changed every 6 to 12 months, the RO membrane lasts about 2 to 3 years, and the system is sanitized once a year. Slow tank fill or an off taste are the usual signs a change is overdue. Full intervals and replacement steps live in our reverse osmosis filter replacement and maintenance guide.

This is where Phoenix water earns special mention. Hard, mineral-rich water scales up pre-filters and the membrane faster than soft water does, so the pre-filters do real work here and need timely changes to protect the membrane behind them. The WQA recommends that an RO system "should be preceded by some form of pretreatment if the water contains hardness," because softened or pretreated water meaningfully extends membrane life. Many local homes run a water softener upstream of the RO for exactly this reason, pairing whole-house scale control with point-of-use drinking-water treatment. The softener and the RO unit do different jobs, and the cost factors that drive an RO install, such as filter stages, a pump, or a remineralization stage, are broken down in our reverse osmosis system cost overview.