water coming out of a kitchen faucet
Steps // How to Install a Water Filter
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Water Filter Overview

Step One // How to Install a Water Filter

Water Filter Overview

water filter
Illustration by Gregory Nemec

Americans spend nearly $6 billion a year on bottled water, and the rest of the world chugs down another $29 billion worth of the stuff. Given this trend, we asked This Old House plumbing and heating expert Richard Trethewey to show us how to install an undersink water-filtration system.

Richard chose a dual-cartridge water filter for this kitchen. This model has both carbon-block and granular-carbon filters, as well as a dedicated faucet and an electronic monitor that signals when it's time to replace the filters (after 500 gallons have passed through them, or six months). Dual filters reduce most common chemicals and contaminants found in tap water—from a municipal source or ground well—including lead, mercury, asbestos, pesticides, and cysts. They'll also eliminate sediment, and bad taste and odors caused by the chlorine added to municipal water supplies.

The system shown here comes with all the necessary hardware and tubing to complete the installation. However, Richard upgraded the system by adding a new ball valve to the water-supply tubing that feeds the filters. This allows the homeowners to shut off the water to the unit to change the filter without cutting off the sink's main faucet.

2 ×

Turn off the water supply

Step Two // How to Install a Water Filter

Turn off the water supply

turning off the water
Photo by Kolin Smith

Start by turning off the shutoff valve for the cold-water supply to the kitchen sink area, which is typically located in the basement, crawl space, or utility.

If your home doesn't have a shutoff for the kitchen, you'll have to turn off the water at the main valve for the house.

Ask a helper in the kitchen to open the sink's cold-water faucet to release pressure in the pipe.

Check the side of the shutoff valve for a small nut, called a drain cap. If you see one, hold a bucket under the valve and loosen the nut with your fingers or a pair of pliers. Any water trapped in the pipe between the sink and the valve will drain out.

3 ×

Cut into the water-supply line

Step Three // How to Install a Water Filter

Cut into the water-supply line

water supply line
Photo by Kolin Smith

Empty the sink cabinet and spread a tarp or blanket on the floor.

Locate the cold-water shutoff valve under the sink.

Take a piece of emery cloth and buff clean the copper tubing just directly below the valve.

Next, use a close-quarters tubing cutter to remove a 1-inch-long section of tubing and discard.

Brush flux onto the freshly cut tubing ends and a ½-inch copper tee fitting.

Slip the tee onto the tubing below the shutoff valve and solder it in place. Be sure to protect the cabinet with a fire-resistant cloth or metal shield.

4 ×

Solder on a new shut-off valve

Step Four // How to Install a Water Filter

Solder on a new shut-off valve

new shutoff valve
Photo by Kolin Smith

Fit a ½-inch ball valve to two pieces of copper tubing and a female adapter, as illustrated at far left.

Solder the assembly together before installing it under the sink, for maximum safety. Work from the bottom up, first soldering the copper tubing to the ball valve, then the ball valve to the next bit of tubing, and finally the tubing to the female adapter.

Fit the assembly onto the tee fitting coming off the cold-water supply with a 90-degree elbow. Solder both ends of the elbow in place.

Wrap Teflon tape around the reducer fitting and thread it into the top of the female adapter. Hold the adapter with pliers while tightening the reducer with a wrench.

Tip: Be sure the ball valve is open when soldering so that steam can escape.

5 ×

Drill a hole for the new filter faucet

Step Five // How to Install a Water Filter

Drill a hole for the new filter faucet

Drilling a hole for the filter faucet
Photo by Kolin Smith

Measure and mark the hole location for the filter's faucet on the stainless steel sink deck.

For a cast-iron sink, locate the hole on the countertop or mount the filter's faucet in the sprayer hole.

Drill a ½-inch-diameter hole through the stainless steel deck with a step drill. Each graduated step on the bit is 1/8 inch wider in diameter than the previous step.

Tip: To keep the drill from "walking" on stainless steel at teh start, cover the location with a strip of masking tape or strike a starting point with a center punch.

6 ×

Install the filter faucet

Step Six // How to Install a Water Filter

Install the filter faucet

install filter faucet
Photo by Kolin Smith

After drilling the faucet hole, use a wet/dry vacuum to collect the metal chips from the sink top and from inside the cabinet.

Now place the washer, aluminum base (escutcheon), and rubber gasket onto the threaded stem of the filter faucet.

Slip the faucet into the hole and make sure it sits flat on top of the sink deck.

From inside the cabinet, reach up and place the mounting bracket, lock washer, and hex nut on the faucet's stem.

Tighten the hex nut carefully with a basin wrench to secure the faucet.

7 ×

Attach the plastic tubing to the filter

Step Seven // How to Install a Water Filter

Attach the plastic tubing to the filter

Attaching plastic tubing
Photo by Kolin Smith

Remove the plastic cover from the top of the filter and install the 9V battery, which powers the electronic filter monitor.

Next, cut a length of the plastic tubing that comes with the filter kit to reach from the reducer fitting on the cold-water supply line to the filter's mounting location on the back wall of the cabinet.
Push one end of the tubing onto the inlet fitting on the left side of the filter.

Cut another length of tubing long enough to extend from the outlet fitting on the right side of the filter up to the faucet stem.

8 ×

Mount the filter in the cabinet

Step Eight // How to Install a Water Filter

Mount the filter in the cabinet

water filter
Photo by Kolin Smith

Select a spot on the back wall of the sink base cabinet to mount the filter; mark two screw holes, spaced to match the keyhole slots on the back of the filter.

Drive a screw into each hole; stop when the screw heads are about ½ inch from the cabinet's surface.

Align the keyhole slots with the two mounting screws.

Push the filter flat against the cabinet wall, then pull down to lock the keyhole slots onto the screw heads.

9 ×

Connect the filter's water-supply tubing

Step Nine // How to Install a Water Filter

Connect the filter's water-supply tubing

Connecting the tubing
Photo by Kolin Smith

Attach a compression fitting onto the end of the plastic tubing that's connected to the inlet side of the filter.

Thread the compression fitting onto the reducer that's attached to the cold-water supply line; tighten the fitting with a wrench while holding the reducer with pliers.

Connect the other length of tubing to the outlet side of the filter and to the faucet stem, using a compression fitting.

Open all the shutoff valves and check your work for leaks.

Remove the aerator from the sink's faucet and turn on the cold water for a minute or two to flush out any flux.

Finally, open the filter faucet and let it run for five minutes to remove air and carbon particles from the cartridges.


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