Under-Sink vs. Whole-Home RO and Water Filtration

Reverse osmosis can treat water at a drinking-water faucet or be designed as a whole-home system. Whole-house filtration can also use other treatment methods. The names sound similar, but they do not tell you which water problem the equipment will solve.

The right starting point is the water test and what you want to improve. From there, we can discuss under-sink RO, whole-home RO, or other filtration that fits the house. A.J. LeBlanc Heating installs both under-sink and whole-home reverse-osmosis systems.

What under-sink reverse osmosis does

Reverse osmosis uses pressure to move water through a semipermeable membrane. A point-of-use system is normally installed under a sink and supplies a dedicated drinking-water faucet, refrigerator, or other selected outlet.

RO can reduce a range of dissolved contaminants when the equipment is certified for the specific reduction claim. It also produces a reject-water stream. EPA WaterSense now labels qualifying point-of-use RO systems that meet efficiency and performance criteria, helping homeowners compare water use as well as contaminant-reduction claims.

What whole-home reverse osmosis changes

A whole-home RO installation is designed to serve the home's water supply rather than one drinking-water faucet. It is not simply a larger cartridge added under the sink. Water quality, pretreatment, storage, delivery pressure, household demand, and the reject-water arrangement all need to be evaluated as part of the system.

That additional design work is why whole-home RO should be recommended for an identified need. More treatment is not automatically a better answer. In another home, targeted whole-house filtration and an under-sink drinking-water system may be the more appropriate combination.

We install both scopes. Our reverse-osmosis service page explains the options, and the final selection starts with the laboratory results and the home's requirements.

What whole-house filtration does

Whole-house, or point-of-entry, treatment is installed where water enters the home. The technology depends on the problem. A sediment filter captures particles. Activated carbon can reduce certain tastes, odors, and contaminants. Oxidation and filtration can address iron and manganese. A softener reduces hardness minerals.

No generic whole-house filter removes every possible contaminant. The equipment and media must match the laboratory results, flow requirements, and certification claims.

When whole-house treatment and RO work together

An RO membrane performs better when the incoming water is within the conditions specified by the manufacturer. Heavy sediment, hardness, iron, manganese, or oxidants may shorten component life or reduce performance. Whole-house pretreatment can protect the RO unit while solving problems that affect the rest of the home.

For example, a home may use whole-house treatment for iron and hardness, then use under-sink RO for selected drinking-water concerns. Another home may need only an efficient under-sink system. The laboratory results tell us what needs to be treated, and the homeowner's goals help determine whether that treatment belongs at one faucet or throughout the home.

Arsenic is one example where the laboratory report matters. NHDES explains that the form of arsenic and the rest of the water chemistry affect treatment selection. A certified under-sink RO system may be appropriate for drinking and cooking water in some homes, while other results call for pretreatment or whole-house treatment. We select the approach from the test results and the verified reduction claims of the proposed equipment.

Questions to ask before choosing

• Is the concern at one drinking-water faucet or throughout the home?
• Which contaminants or water characteristics were measured?
• Is the proposed system certified for the claimed reductions?
• What flow rate and household demand must the equipment support?
• What filters, membranes, salt, media, lamps, or service will be required?
• Where will reject water, backwash, or regeneration discharge?

A clear maintenance plan is part of the purchase. Treatment that is not serviced as designed may stop performing long before it looks different from the outside. Our whole-house water-filter maintenance guide explains what to record and why the schedule depends on the equipment.

Water treatment from A.J. LeBlanc Heating

A.J. LeBlanc Heating collects samples for an independent laboratory and uses the results to size treatment for Southern New Hampshire homes. We install under-sink and whole-home reverse osmosis, sediment filters, activated-carbon systems, softeners, iron and manganese filtration, ultraviolet systems, and the plumbing that connects them.

Explore reverse osmosis systems, compare our whole-house filtration options, or request water testing.

Frequently asked questions

Is reverse osmosis a whole-house filter?

Residential RO is most often installed as a point-of-use drinking-water system. Whole-house RO exists, but it has different design, storage, flow, wastewater, and plumbing requirements.

Does whole-house filtration improve drinking water?

It can, depending on the technology and contaminant. A generic sediment filter will not provide the same reductions as activated carbon, RO, or contaminant-specific treatment.

Does reverse osmosis waste water?

RO creates treated water and a reject-water stream. WaterSense labeled point-of-use systems meet EPA efficiency limits and use less reject water than many conventional units.

Should well water be tested before installing RO?

Yes. Testing identifies the target contaminants and any pretreatment needed to protect the membrane and support reliable performance.

Can reverse osmosis reduce arsenic?

Some certified RO systems are designed for specific arsenic-reduction claims, but the arsenic form and other water conditions matter. The system should be selected from laboratory results, and follow-up testing should confirm performance.

Can A.J. LeBlanc Heating service the system after installation?

Yes. We install and service the water-treatment equipment we provide and explain the filter, membrane, media, lamp, salt, and testing schedule that applies.

Sources

• U.S. EPA WaterSense: Point-of-Use Reverse Osmosis Systems
• U.S. EPA: Point-of-Use and Point-of-Entry Treatment Devices
• NHDES: Arsenic in New Hampshire Well Water

Last reviewed: September 2026.