Under-sink water filters

Types of under-sink filters

• Inline filters connect to the cold supply and feed the main faucet or a separate faucet.
• Multi-stage filters use two or three cartridges for different contaminants.
• Reverse osmosis systems push water through a membrane, store filtered water in a tank and send waste water to the drain.

Look for products certified to NSF/ANSI standards for the contaminants you care about, and check the certification listing.

What we install

1. Add a tee or adapter on the cold supply with its own shutoff.
2. Mount the filter housing to the cabinet wall at a height that leaves room to change cartridges.
3. Drill the sink or counter for a dedicated faucet, or use an existing hole such as a sprayer or soap dispenser hole.
4. For RO systems, install the drain saddle on the drain line above the trap and place the tank.
5. Flush the system as the maker directs and check for leaks.

Drilling for a faucet

Stainless sinks drill easily. Stone counters need a diamond core bit and care. Porcelain or cast iron sinks can chip, so an existing hole is better when possible.

Space under the sink

Disposals, dishwasher lines and storage compete for space. We plan the layout so cartridges are reachable and nothing blocks the shutoffs.

Maintenance

Change cartridges on the maker's schedule. Old cartridges can grow bacteria and lose effectiveness. Write the change date on the housing.

What is outside our scope

Whole-house filtration on the main line and lab testing for specific contaminants are outside our scope.

Picking a system

Start with what you want to address: taste and odor, specific contaminants, or general peace of mind. Your water provider publishes a yearly Consumer Confidence Report describing what is in the water supply, which helps choose. Then check product certification listings to confirm the filter is certified for those items.

Activated carbon filters improve taste and odor. Reverse osmosis removes a wider range but wastes some water and needs a drain connection and space for a tank.

Counter and sink holes

A dedicated filter faucet needs a hole. Many sinks have an unused hole covered by a plate, or a sprayer or soap dispenser that can be swapped. When none exists, we drill stainless sinks or counters with the right bit.

Quartz and granite can be drilled with diamond bits and water cooling. We avoid drilling porcelain sinks when another option exists.

Tying into the fridge

Some homeowners run filtered water to the fridge too. That needs a tee off the filter output and a line to the fridge. If the fridge has its own filter, you may want to bypass one or the other.

Leak protection

Filter housings and RO tanks add connections under the sink. A water sensor on the cabinet floor and a drip tray give early warning. Check connections after cartridge changes, since those disturb the fittings.

What to send

Photos of the sink top, the cabinet underneath and the filter model you plan to use.

Reverse osmosis details

An RO system usually has a sediment prefilter, a carbon prefilter, the membrane, and a carbon polishing filter. Water fills a small pressurized tank and flows to the dedicated faucet. Some water goes to the drain while the system is making water.

Tankless RO systems use a pump and need an outlet under the sink, which must already exist. RO water is low in minerals; some systems add a remineralization stage for taste.

RO systems need decent incoming pressure. If house pressure is low, the system makes water slowly. A booster pump can help but needs power.

More questions

Can I move a filter when I move out? Yes. We disconnect, cap the tee and patch the faucet hole with a cover.

Will a filter fit with a disposal? Usually, with planning. Compact housings help.

What certifications should I look for? NSF/ANSI standards for the contaminants you care about, as listed in the product's certification.

Water quality information

Public water systems publish an annual water quality report, often called a Consumer Confidence Report, listing detected substances and how they compare with standards, and the EPA has a page that explains these reports and how to find yours. Reading your provider's report is the best starting point for choosing a filter, because it tells you what is actually in your water rather than relying on marketing claims. If your home is on a private well, a lab water test is the way to learn what is in it.

Filter makers list the contaminants their products are certified to reduce. Matching that list to your water report helps you choose a filter that does what you need without paying for stages you do not. Keep the certification sheet with your cartridges so the next person who services the system knows what is installed.