How to Install an Under-Sink Water Filter

Before You Begin: Start by turning off the shutoff valve for the cold-water supply to the kitchen sink, which is typically located in the basement, crawl space, or utility closet. If your home doesn’t have a dedicated 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 pliers so any water trapped in the pipe between the sink and the valve can drain out.

In this video, This Old House plumbing and heating expert Richard Trethewey explains how to install an under-sink water filter.

Steps:
1. Close the cold-water shut-off valve leading to the kitchen sink.
2. Disconnect the copper cold-water pipe from the supply line that runs up to the faucet.
3. Wrap Teflon tape around threads of tee fitting and adapter.
4. Tighten outlet fitting to the tee fitting using an adjustable wrench.
5. Connect the tee fitting to the threaded adapter onto copper cold-water pipe.
6. Join ¼-inch tubing to inline shut-off valve.
7. Connect tubing with inline valve to tee on cold-water line; tighten with adjustable wrench.
8. Screw filter housing to inside of cabinet.
9. Remove soap dispenser or spray hose from sink deck; if necessary, drill new hole in sink deck.
10. Place bead of plumber’s putty under faucet.
11. Set faucet in hole in sink deck.
12. Use basin wrench to tighten faucet nut from below.
13. Connect tubing to faucet stem with compression fitting; tighten with basin wrench.
14. Connect cold-water tubing to inlet side of filter housing.
15. Connect cold-water tubing to outlet side of filter housing.
16. Turn on the cold-water shut-off valve, and check all fittings for leaks.
17. Test the water-filter faucet.

Why a Dedicated Faucet? As described in This Old House Magazine, Richard Trethewey chose a dual-cartridge water filter for this project — a model with both carbon-block and granular-carbon filters, a dedicated faucet, and an electronic monitor that signals when it’s time to replace the filters. An undersink water-filtration system is a much more convenient and less expensive alternative to bottled water for putting clean, fresh-smelling drinking water into your glass.

Pro Tip: When soldering a new shutoff valve assembly for the filter’s water supply, be sure the ball valve is open during soldering so that steam can escape. For maximum safety, solder the assembly together — copper tubing to ball valve, ball valve to the next piece of tubing, and tubing to the female adapter — before installing it under the sink, rather than soldering in the tight confines of the cabinet.