How to Install an Under-Sink Water Filter: Faucet, Valve, Tubing and Flush

What Installing an Under-Sink Water Filter Involves

An under-sink cartridge filter goes in with three connections. A small valve taps your cold-water supply, tubing carries that water into the filter housings, and more tubing runs from the housings up to a dedicated drinking-water faucet on the sink or countertop. Add a mounting spot on the cabinet wall, a slow turn of the water back on, a leak check and a flush, and that's the whole job.

None of it is complicated. The parts that trip people up are the decisions before the first wrench turn: where the faucet will go, which valve fits your supply line, and whether your water pressure sits inside the system's limits. This guide walks through those choices, then the install itself, the first start and the date your first cartridge change is due. It covers cartridge systems only. Reverse osmosis adds a drain line and usually a storage tank, which makes it a different job.

That isn't fine print for its own sake. The U.S. EPA's guide to maintaining a water filter puts it plainly: filters "must be installed, maintained, and used properly to work," and it tells you to follow the maker's installation instructions exactly. Every model is a little different, so your manual wins wherever it differs from the general steps below.

Before You Start: Space, Pressure and Parts

Ten minutes of checking now saves a second trip to the hardware store halfway through.

Check the Cabinet Space

Measure the wall you plan to mount on, and the depth from the cabinet opening to the back wall. You also need room below the housings to unscrew them and drop a cartridge out at change time. Crystal Quest's SMART under-sink system, for example, is listed at 14 inches high and 5 inches wide, and 5, 10 or 15 inches long for one, two or three housings. Our guide to water filter dimensions and clearance covers the rest of the space check, including what a pull-out trash bin does to your options.

Check Your Water Pressure

Cartridge housings have a pressure ceiling. Crystal Quest's under-sink systems are rated up to 60 psi, and the manuals recommend a pressure regulator if your home runs higher. Too little pressure is a problem too: the product listings put the floor at 20 psi, and one manual says the system needs at least 30 psi to work. If you don't know your pressure, a simple gauge on an outdoor hose spigot will tell you.

Temperature matters as well. These filters are for cold water only, with a listed maximum of 90°F on the product pages and 85°F in the manuals. The EPA's filter guide gives the reason: never run hot water through a filter, "because it can damage your filter."

Tools and Parts

Crystal Quest's SMART, Lead and Classic under-sink kits ship with the faucet and the installation hardware, and you choose the supply valve when you order. The manual for the single, double and triple systems lists these tools:

• Safety glasses and a flashlight
• Adjustable wrench and screwdrivers
• Plastic tube cutter
• Thread seal tape
• Plastic anchors and screws for the cabinet wall
• A drill with 1/8-inch and 1/2-inch bits, plus a 1/4-inch bit if you'll be tapping brass or galvanized pipe
• A 1-1/4-inch porcelain hole cutter, only if you have to make a new faucet hole

Add a towel and a shallow pan for the water left in the line.

Two Choices to Make First: Faucet Spot and Supply Valve

Where the Faucet Goes

You'll see two kinds of setup. Some under-sink filters are plumbed into the cold line that feeds your main kitchen faucet, so there's no extra faucet at all. Others, including Crystal Quest's under-sink cartridge kits, send filtered water to a small faucet of its own.

The separate faucet is worth the extra hole. With a cold-line hookup, every drop of cold water passes through the cartridges, including the water you rinse dishes and fill the mop bucket with. The EPA notes that "flowing large amounts of water you won't be consuming through your filter will wear it out faster." A dedicated faucet keeps the cartridges working on drinking and cooking water only, which is the job they're sized for.

Look for a spot in this order:

• An unused hole in the sink deck. Many sinks have one for a sprayer or soap dispenser. The Crystal Quest manual says that if your sink has a hole that fits the faucet, no drilling is required. A sprayer hole works if you cap off the sprayer connection.
• The countertop beside the sink. The faucet needs about 2 inches of flat surface, and the mounting surface can be no more than 1-1/4 inches thick.
• A new hole in the sink itself. A stainless sink can take a new hole with a hole punch. Porcelain and enamel sinks chip easily: the manual calls for a sharp porcelain cutter at slow speed, kept cool with a dam of plumber's putty filled with water.

Stone and quartz countertops are where most people stop and call a pro. A cracked slab costs far more than the install.

Which Supply Valve Fits Your Line

Look under the sink at the line that runs from the cold shutoff valve up to your faucet. What it's made of decides the valve you need.

If you have a flexible hose, the T-valve is the easy, no-drill route. Note the size of the shutoff valve's outlet before you order, so the tee matches it. Your local plumbing code has the final word on which fittings are allowed, so check it if you're unsure about a saddle valve. And if the shutoff valve itself is corroded or won't close all the way, fix that before anything else. You can't safely install a filter on a line you can't shut off.

How to Install an Under-Sink Water Filter, Step by Step

These steps follow the order in Crystal Quest's under-sink manuals. Clear out the cabinet, lay down the towel and set the pan under the cold shutoff before you start.

1. Mount the faucet

   Set the faucet in its hole and tighten it from below, following the instructions packed with the faucet. You'll connect its tubing in step 5.

2. Shut off the cold water and relieve the pressure

   Close the cold shutoff valve under the sink, then open the kitchen faucet's cold side until it stops running. Keep the pan under the valve.

3. Install the supply valve

   For a T-valve, disconnect the supply hose from the shutoff, wrap the shutoff threads with thread seal tape, thread on the tee, and reconnect the hose to the tee. For a saddle valve, clamp it to the cold pipe according to the table above. Leave the new valve closed.

4. Mount the filter housings

   Fasten the unit to the cabinet wall with anchors and screws, high enough that you can still unscrew the housings and drop the cartridges out. If your kit ships with the cartridges separate, install them now with the arrow on each cartridge pointing toward the cap, as marked.

5. Run and connect the tubing

   Cut each tube clean and square. The line from the supply valve goes into the port marked INLET, and the line to the faucet goes into OUTLET. Push each tube fully into its fitting and reattach any locking clips. Route the tubing along the cabinet walls with no kinks or sharp bends.

6. Turn the water back on slowly

   Open the main cold shutoff, then open the new supply valve a little at a time. Watch every connection with the flashlight as the housings fill.

First Start: Leak Check and Flush

Before anyone drinks from it, check and flush.

Run your hand around every joint, including the tee or saddle valve, both ports on the filter and the faucet base. A paper towel shows a slow seep better than your eyes do. Check again an hour later and the next morning, because a drip inside a closed cabinet can run for days before anyone notices. Crystal Quest's product pages recommend a leak detector valve for the same reason, and we sell a leak detector smart valve that fits under-sink systems.

Then flush the new cartridges. The Crystal Quest manuals say to run the filter faucet 3 to 4 times for 5 to 10 minutes each. Black specks in the first water are loose carbon fines, and cloudy or milky water is trapped air. Both are harmless and clear as you flush. Our guide to flushing a new water filter covers what's normal in the first day or two and the signs that aren't.

Set Your First Cartridge Change Date

Write the install date on a piece of tape on the housing, or put a reminder in your phone, today. The EPA's advice for filters without an indicator light is to track the days or gallons from the day you install a new cartridge and set your own reminder.

For Crystal Quest's standard 10-inch under-sink cartridges, plan on a change every 12 to 24 months. How much water you draw and what's in it decide where you land in that range. A clear drop in flow at the filter faucet is a sign the cartridges are loading up. Use the cartridges made for your housings, like the SMART replacement cartridge, and follow the manual's steps for the change.

Don't let it slide. The CDC warns that germs from pipe slime "can also live and multiply in water filters" that aren't properly maintained and replaced, and it lists under-sink filters among the ones to replace on the maker's schedule.

When to Hand the Job to a Plumber

Even if your warranty allows a do-it-yourself install, some situations are worth a plumber's visit:

• Your manual or warranty requires a licensed plumber.
• The cold shutoff valve is stuck, corroded or won't close fully.
• There's no shutoff valve under the sink at all.
• The faucet hole has to go through stone, quartz or another countertop you can't afford to crack.
• Your home's pressure runs above 60 psi and needs a regulator.
• You find old galvanized pipe, active corrosion or a leak you can't stop by snugging a fitting.

Still deciding which system to put under the sink? Our guide to the best under-sink water filter covers how to match the cartridges to what's in your water. Renting? Our water filter guide for apartments covers what to ask your landlord first.