Under-Sink Water Filtration

Under-sink buyer's guide

Who an under-sink system fits

An under-sink system is for the water you drink and cook with. It's the right-sized fix when your complaints show up in the glass or the coffee pot, when you rent and can't touch the main water line, or when you're done hauling bottled water home. It can also take the place of an existing reverse osmosis unit: the RO-replacement option swaps in a new faucet and redirect valve without drilling a new hole.

The system and what goes under your sink

We carry one under-sink system, the Sentry H2O Wellness System, in two configurations. DualGuard uses two filters: ToxyGuard, a catalytic-carbon stage, and AlkaGuard, which is designed to add calcium and magnesium back. TriGuard adds a third filter, BioChemGuard, with submicron media; the evidence for that stage is at the component level only. Each configuration comes with either a city-water or a well-water cartridge set. The system mounts to the cabinet wall with a bracket and ships with its fittings, tubing and tube cutters.

Faucet, fridge line and shut-off: what connects to what

• Kitchen cold-water line. The standard under-sink connection uses a 3/8 inch fitting.
• Refrigerator water and ice. A 1/4 inch connection feeds the fridge line, on its own or along with a faucet.
• Dedicated faucet. A separate filtered-water faucet uses 1/4 inch fittings and a redirect valve. It needs a hole in the sink or countertop unless it replaces an existing reverse osmosis faucet.
• Shut-off and leaks. Installation starts at the cold-water shut-off valve under your sink, and the manufacturer recommends pressure regulation and leak protection where needed.

What it reduces, and the evidence behind it

Two kinds of evidence back the system, and they aren't the same thing. The design: ToxyGuard is a catalytic-carbon stage aimed at chlorine and chloramine, the disinfectants behind most tap-water taste and smell complaints. The lab data: in the 2025 Tempe, Arizona samples published on our Science page, lead, arsenic, uranium and fluoride were not detected above the lab's method detection limit (MDL) in the "Treated Sink" sample, and pH went from 6.83 untreated to 8.21. The Science page reports the 2024 PFAS results separately, with their own limits.

About these samples (provisional): Sentry H2O's draft, not-yet-final answers describe the 2025 "Treated Sink" sample as a TriGuard under-sink system at the 500-gallon mark and the "Kitchen Sink" sample as a whole-house system at about 100,000 gallons (roughly one year for a family of three). Sentry has not yet supplied the per-report sample crosswalk, so we do not attribute these results to any current SKU. Results are specific to the sampled water and configuration and are not a certification or a guarantee of identical performance in every water source.

Keeping minerals vs. removing them

This is where an under-sink catalytic-carbon system and reverse osmosis part ways. RO removes most dissolved minerals along with what you want gone, and it sends reject water down the drain: EPA WaterSense puts a typical point-of-use unit at five gallons or more of reject water per gallon made. The Wellness System sends no reject water to the drain, and its AlkaGuard stage is designed to add calcium and magnesium back. If you're weighing the two, start with our Sentry H2O vs. reverse osmosis comparison and our look at reverse osmosis with remineralization.

Space and installation

Most kitchens take about 30 minutes, and a standard installation doesn't need a licensed plumber. Turn off the cold-water shut-off under the sink, mount the bracket to the cabinet wall with a drill and two screws, seat the filters hand-tight, connect the inlet to the cold-water supply and the outlet to your faucet or fridge line, then restore water, check for leaks and flush for 5 to 10 minutes.

The manufacturer limits the system to cold potable water from a municipal supply or a properly treated well, at 40 to 80 PSI (50 to 70 PSI recommended), with feed water between pH 6.5 and 9.0 and a maximum flow of 1 gallon per minute. If your water is already salt-softened, check compatibility with us before you install.

Filter life and cost of ownership

Replace the installed filter set every 12 months or 3,000 gallons, whichever comes first, or sooner if performance declines. The 500-gallon mark you may see quoted is peak-performance context, not the replacement point. TriGuard owners replace all three cartridges together. Replacement sets, with an optional auto-ship subscription, are on the replacement filters page. Our guides on when to replace a water filter and how to read a filter spec sheet help you plan the yearly cost.