DIY Network

How to Install an Under-Sink Water Filtration System

These basic instructions show how to install an under-sink reverse osmosis water filtration system.

More in Plumbing

Step 1: Assess the Project

Determine where the filter system will go and where the spout will be mounted.

Remove all items from underneath the sink.

Locate the cold water line. This is the line that the filtration unit will be connected to.

Shut off the main water supply.

shut off main water supply

Step 2: Secure the Valves

The saddle valve (Image 1) has a rubber seal to stop leaks along with a hollow needle that taps into the water line.

Take one of the provided 2" bolts and feed it through the valve into the back-plate. Screw it in about 1", make sure that the notch in the back and the notch in the front are aligned (Image 2). This will insure that the valve will squeeze against the pipe.

Place the valve around the pipe with the nozzle pointing downward; thread the second bolt into the back-plate. Use a flathead screwdriver to tighten it into place. Caution: Don't over tighten, or the pipe will crimp which will cause a leak.

Twist the valve stem until the needle punctures the pipe and water begins to drip out. Once the pipe is punctured, back off the valve to purge the system.

Re-tighten to shut off the drip. Now the valve is ready to feed the filtration system.

Safety Tip: When working on drain lines, it's a good idea to wear gloves and have a bucket ready.

Step 3: Attach the Drain Line

Unscrew the bottom section of the P-trap as well as the disposal drain connection. Loosen the drain nut then remove the tailpipe. Cut off the tailpipe leaving about 2" below the drain, and reattach.

Take the drain line adaptor and mark the length to just at the disposal drain connection. Cut the drain adaptor to length with a PVC saw, then attach to the main drain line and reconnect all the joints.

Note: Be sure to tighten everything down to prevent leaks.

attach the drain line

Step 4: Install the Filter Faucet

Place the base on the threaded stem and attach to the sink top. Place the large gasket on the stem under the sink surface, making sure the two barbed studs fit through the opening.

Attach the mounting plate and the spacer followed by the metal washer and hex nut.

Finally, insert the washer into the tubing adapter before securing it tightly to the faucet stud.

loosen drain nut then remove tailpipe

Step 5: Measure the Mounting Holes

Mark the position of the mounting holes. Measure up three inches and make another mark. This extra space will keep the unit off the base and allow for easier filter replacement.

Attach 1" screws and mounting-washers into the side of the cabinet with a normal drill or screwdriver.

Measure the distance between mounting holes, in this case it will be 10".
Transfer the marks to the other end of the cabinet and repeat the screw installation.

mark position of mounting holes

Step 6: Hang the Filter System

Hang the filter system off the mounting washers and double check that there is 2" to 3" of height off the cabinet deck to allow for easy filter access.

hang filter system off mounting washers

Step 7: Attach the Tubing

Attach the 1/4" tubing marked 1/4" barb to the barb fitting on the end of the faucet. Then, attach the 1/4" tube marked supply line directly to the T-valve installed earlier.

Next, attach the tubing adapter to the end of the faucet stud and tighten.

Slide the tubing into the compression nut and attach it to the tubing adapter hand tight. Then, take the basin wrench and tighten it as tight as possible to make sure the seal between the nut and the tubing is secure.

Run the 3/8" tubing marked storage tank to the tank shutoff valve. Use a small bit of Teflon tape around the threads to prevent leaks.

attach tubing adapter

Step 8: Purge the System

Turn on the cold water supply to check for leaks.

Purge the system four times before using it. This allows all the systems to filter out any manufacturing sediments and breaks in the filtering process.

Now it's time to enjoy filtered water right out of the faucet.