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.

If cabinet space is the obstacle, compare a model-approved remote interior RO layout with a compatible faucet-fed countertop arrangement. Each needs its own approved feed, reject-water route and commissioning; neither is a generic hose relocation.

Before adding the optional refrigerator branch, check the appliance's RO-feed and approved filter-bypass requirements. The RO unit and refrigerator instructions both need to support the intended connection.

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. Low membrane feed pressure and backpressure from a filling storage tank are different conditions to assess. For Aquatec’s named RO systems, an electric booster pump raises feed pressure to the membrane, while a permeate pump uses drain-stream hydraulic energy to move treated water into storage and reduce membrane backpressure. They are not interchangeable fixes for low household pressure. Have the installer identify the actual restriction or pressure problem and confirm the RO manufacturer’s approved pump, controls and plumbing arrangement before adding equipment.

See an RO permeate pump versus an electric booster for the product-specific decision.

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.

When selecting storage, check whether the exact RO system allows a larger or second tank. The installer should confirm usable capacity, the complete approved connection arrangement and commissioning rather than choosing only by a gallon label or available cabinet space.

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.

For an output concern, compare membrane GPD with the complete system’s daily-production rating. A cartridge label is not a universal tank-fill time; the actual model’s source conditions and startup instructions govern that assessment.

Follow the actual RO manufacturer’s first-use sanitizing, flushing or purging, and leak-check instructions before drinking from the system. One discarded tank is not a universal startup procedure or safety clearance. Whirlpool’s WHER25 manual, for example, has its own sanitizing and purge sequence and restrictions on unsafe or unknown-quality feed water; another tank or tankless model can have different requirements. Follow applicable utility or health-authority advisories too. Clear appearance and a completed flush do not independently establish that every drinking-water concern has been resolved.

Testing helps assess performance. An installer can compare TDS before and after as a field check of dissolved-solids reduction, but that result alone does not establish complete water safety. NSF/ANSI 58 is a voluntary reverse-osmosis treatment standard; an RO label or sale for drinking-water use does not prove certification. Verify the actual model’s certification listing, performance data sheet and the specific contaminant-reduction claims needed for your water. NSF cautions that certification does not mean a system reduces every possible contaminant, and CDC recommends matching the filter to the substances of concern. Phoenix tap water runs hard, roughly 10 to 17.6 grains per gallon (City of Phoenix, 2025 Water Quality Report) with the 2025 report listing TDS at 464 to 716 ppm, so a before-and-after TDS reading can be a useful performance comparison. It is not a complete test for harmful germs or every chemical, or permission to disregard a water advisory.

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 and storage conditions. Have the installer distinguish low membrane feed pressure from storage-tank backpressure and confirm the actual RO configuration. A compatible electric booster and a permeate pump address different functions; neither is an automatic remedy for a restricted filter or another existing fault.

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.

To add the remineralization stage mentioned above, see how to remineralize RO water for better taste.

For the most common leak points on an RO unit, see why an RO tank or system leaks.

The drain connection has a code rule of its own: what an air gap is on a faucet or RO system explains it.

If your sink has no free hole, why an RO faucet can mean drilling the sink or countertop covers your options and the materials that crack.