Whole house reverse osmosis system: who needs one and what to ask

Reverse Osmosis Company is an independent referral service, not an installer or plumber. When you send the form, your details go to an independent local water treatment pro who can contact you and prepare a quote.

Under-sink RO and whole-house RO

The familiar RO system is what the EPA’s WaterSense program calls point-of-use systems: connected to a single fixture, such as under the kitchen sink. They treat the water you drink and cook with. A whole-house system sits where the water enters the home and treats all of it, including the water you wash, bathe and do laundry with.

	Under-sink RO	Whole-house RO
Where the water is treated	One fixture, such as the kitchen faucet	All the water entering the home
Size of the job	Fits in a sink cabinet	A bigger installation, for example in a garage, basement or utility area
How it is sized	To the faucet it serves	The installer sizes it after a water test
Water to the drain	EPA figures published for point-of-use units	Ask for the figure for the system you are quoted
Upkeep	Filters and membrane on the maker’s schedule	Ask what the system needs and how often

Neither one is better in general. They solve different problems. The question is which problem you have.

Start with a water test, not a product page

Sellers’ pages make the case for whole-house RO in general. Your water makes the case, or does not, in particular. The Texas Commission on Environmental Quality’s advice on choosing home water treatment starts with one question: what contaminant do you want to remove or reduce? A whole-house system only makes sense when the answer is something you want out of all the water in the house, not just the water you drink.

On city water

Your water utility publishes a yearly water quality report. Read it first. It shows what was found in the water and at what levels. If what bothers you is taste at the kitchen tap, the report may point toward a smaller fix. If it is something else, it gives the pro a concrete starting point.

On a private well

The EPA says private well owners are responsible for delivering safe drinking water to their households, and private wells are not regulated by the federal government under the Safe Drinking Water Act nor by most state governments. The EPA advises testing a private well every year for total coliform bacteria, nitrates, total dissolved solids and pH, and using only laboratories certified to do drinking water testing. See reverse osmosis for well water.

Signs that lead people to ask about whole-house RO

The EPA publishes secondary drinking water standards: guidelines for nuisance chemicals that affect taste, color and odor. The EPA does not enforce them, but they are a useful yardstick when you read a test. Several of them describe exactly the problems that make people think about treating the whole house:

Test result	EPA secondary guideline	What the EPA says it causes
Total dissolved solids	500 mg/L	Hardness, deposits, colored water, staining, salty taste
Chloride	250 mg/L	Salty taste
Iron	0.3 mg/L	Rusty color, sediment, metallic taste, reddish or orange staining
Manganese	0.05 mg/L	Black to brown color, black staining, bitter metallic taste

A result above one of those lines is a reason to talk to a pro. It is not, by itself, a reason to buy a whole-house RO system. Some of these problems have other treatments, and a pro who looks at the whole test can tell you which ones apply.

Other treatments to ask about

A straight quote compares options. Ask the pro whether any of these would solve your problem with less equipment:

• An under-sink RO system for drinking water, if what matters is the water you drink and cook with. See RO installation.
• A water softener, if the problem is hardness. Ask which standard the softener is certified to. See reverse osmosis vs a water softener.
• Filters, certified for the problem you have. The EPA points to NSF/ANSI 53 for filters certified to reduce lead or PFAS; ask which contaminants a filter’s listing names.
• A combination, such as whole-house treatment for one problem and under-sink RO at the kitchen.

Whatever you choose, the TCEQ’s advice holds: select a unit certified by NSF, Underwriter’s Laboratory or the Water Quality Association to remove the contaminants you care about, and check that the certification covers them.

The drain question

Every RO system sends part of the water it receives down the drain, carrying away what the membrane held back. The EPA says a typical point-of-use system generates five gallons or more of reject water for every gallon of treated water, some inefficient units up to 10, and WaterSense labeled point-of-use units 2.3 gallons or less. Those are the EPA’s figures for under-sink systems. A whole-house system treats far more water, so ask the pro for the expected drain volume of the system they quote, for your household. The guide reverse osmosis waste water has more.

What to ask before a whole-house quote

1. What in my test result is this system meant to treat?
2. Would an under-sink system or another treatment solve it?
3. Which standard is the system certified to, and for which contaminants?
4. How did you size it, and from which test?
5. Where does it go, and how much space does it need?
6. How much water goes to the drain per day for a household like mine?
7. What upkeep does it need, on what schedule, and what do the parts involve?
8. Does my city need a permit, and who pulls it?
9. What does the written quote include?

Permits and the house

Many cities require a plumbing permit for a new softener or RO drain connection, and the rules vary from city to city. Ask the pro whether your project needs one and who pulls it. If you rent, or live in a building with shared plumbing, the owner or the building’s management has a say first.

How to send a useful request

When you fill in the form, choose ‘Whole-house RO’, and on the contact page use the notes box to say what your test found and when it was done. If you have not tested yet, say so. A pro can tell you what testing would help before talking about equipment.

Living with a whole-house system

A whole-house system becomes part of the plumbing, so think past install day. Ask how the house gets water while the system is being worked on. Ask what happens to the water supply during a filter or membrane change. Ask how much space the pro needs around the equipment to service it, and keep that space clear. And ask for the maker’s manual, so the schedule for every part is in your hands, not only in the pro’s records.

Write down the date of every service and what was changed. If the household grows, a new appliance goes in or the well is worked on, tell the pro: each of those can change what the system is asked to do.

The short version

• Under-sink RO treats one faucet. Whole-house RO treats all the water, and is a bigger installation that the installer sizes after a water test.
• Test first: your utility’s yearly report, or a certified lab test of your well.
• Ask whether a smaller or different treatment would solve the same problem.
• Ask for certification, drain volume, upkeep and the quote in writing.

If what you mostly want is better drinking water, start with RO installation. If you already have a system that needs attention, see RO maintenance or RO repair.

Questions people ask