Installing a Water Filtration System Under Your Sink: A Step-by-Step Guide

Installing a Water Filtration System Under Your Sink: A Step-by-Step Guide is a project that pays for itself in reliability, efficiency, and peace of mind. Whether you are installing a sink for the first time or refining your approach, this comprehensive guide gives you everything you need to complete the job correctly and safely. We cover the exact tools and materials required, critical safety precautions specific to this type of work, detailed step-by-step procedures with professional tips at each stage, thorough testing protocols to verify your work, and a complete troubleshooting section for common problems you might encounter along the way.

Installing a new sink is one of the most rewarding home improvement projects you can tackle. A proper installation ensures years of trouble-free service and prevents the costly callbacks and water damage that result from shortcuts or improper techniques. Throughout this guide, we emphasize the details that separate a professional-quality installation from a problematic one: precise measurements before purchasing, thorough preparation of the work area, correct use of sealants and connection hardware, and a methodical testing process that catches problems before they become emergencies. We also explain the reasoning behind each step so you understand not just what to do, but why — which helps you adapt when your specific situation differs slightly from the standard procedure.

By following this guide, you will gain practical, hands-on experience with your sink that serves you well beyond this single project. The skills, techniques, and understanding of your plumbing system that you develop here apply directly to future maintenance and repairs, saving you money for years to come. Most homeowners who complete this type of work themselves save 50-80% compared to hiring a professional — and they gain the confidence and capability to handle similar projects independently in the future.

General Plumbing Safety: Before beginning any plumbing work, locate and test the relevant shut-off valve. For fixture-level work, use the dedicated shut-off valve directly below or behind the fixture. If no dedicated valve exists, or if the fixture valve is stuck or leaking, use the main house shut-off valve (typically located where the water line enters your home, often in the basement, crawl space, or near the water meter). After closing the valve, open a faucet downstream to verify water is fully off and to relieve residual pressure in the lines — there will always be some water remaining in the pipes between the valve and the fixture, so have towels and a bucket ready.

Protect Yourself and Your Home: Wear safety glasses whenever working with plumbing components, as pressurized water, debris, and small parts can become projectiles. Wear rubber or nitrile gloves when working on drain components, toilet internals, or any fixture that contacts waste water. Place drop cloths or old towels on floors below the work area to protect against water damage. If your work area has hardwood or laminate flooring, cover it thoroughly — even small amounts of water can cause irreversible warping if they seep into seams.

Shut Off Both Supply Valves: Close both the hot and cold shut-off valves located directly under the sink. Turn them clockwise until fully closed, then open the faucet to verify flow has stopped and to drain residual water from the lines above the valves. If the valves are old gate-style valves that do not fully close, plan to replace them with quarter-turn ball valves during this project.

Working in Tight Spaces: Under-sink work requires crouching or lying in a cramped cabinet, which increases the risk of back strain, bumped heads, and dropped tools. Use a headlamp for hands-free lighting so both hands are available for the work. Lay a folded towel on the cabinet floor for knee and back comfort. Remove everything stored under the sink before starting — chemicals, cleaning supplies, and personal items can get wet or contaminate the work area.

Preventing Contamination and Spills: Place a bucket or shallow container under every connection you plan to loosen. Even after shutting off valves and draining the faucet, there will be residual water in the supply lines, P-trap, and drain connections. Have several old towels at the ready. If you are working on a garbage disposal or drain connection, the water may contain food waste and bacteria — wear gloves.

Before purchasing your new sink, take detailed measurements of the existing space, connections, and mounting points. Use a tape measure and write down every dimension — do not rely on memory or estimates. Photograph the current setup from multiple angles, making sure to capture supply line sizes (measure the diameter with a ruler), connection types (threaded, compression, push-fit, or soldered), the distance between mounting holes, and the brand and model number of the current unit. Bring all of these measurements and photos to the store so a plumbing associate can help you select an exact match. For plumbing components, even a 1/8-inch difference in pipe diameter or a different thread pitch means the part will not work, so precision is critical. Purchase Teflon tape, plumber's putty, and fresh supply lines at the same time — installing a new sink with old, worn supply lines defeats the purpose of the upgrade.

Refer to the Tools & Materials list above for the complete inventory of everything you will need for this project. Before you start any work, lay out all tools and parts at the work site where you can see and reach them easily. Organize small parts like screws, nuts, washers, and O-rings in a small container or on a magnetic tray so nothing rolls away or falls down the drain. When purchasing replacement parts for your sink, always bring the old part to the hardware store for side-by-side matching — plumbing parts vary significantly across brands, model years, and even production batches, and visual similarity alone is frequently not sufficient to guarantee a proper fit. If you are purchasing online, measure the old part with calipers if possible and cross-reference with the manufacturer's specifications before ordering.

Before purchasing your sink, take precise measurements of the existing space and all connection points. Use a tape measure to record: the distance between supply line stubs or shut-off valves, the pipe diameter of existing supply lines (typically 3/8" or 1/2" for fixtures, 3/4" for main lines), the type of connections present (threaded, compression, push-fit, or soldered), and the available clearance around the mounting area. Photograph the current setup from multiple angles with your phone, including close-ups of every connection point. These photos serve as both a shopping reference and a reassembly guide. Verify that your new sink is compatible with the existing pipe sizes and connection types — you may need transition fittings or adapters if switching between different standards. Check your local building code requirements: some installations require a permit and inspection, particularly if you are modifying existing plumbing runs or adding new connections. It is far better to pull a permit before starting than to be required to tear out finished work for an inspection later.

Turn off the water supply at the appropriate shut-off valve — use the fixture-specific shut-off valves if they exist (typically located directly below sinks or behind toilets), or close the main house shut-off valve if no dedicated valves are available. After closing the valve, open the sink or a nearby faucet to verify the water is completely off and to drain any remaining water from the lines between the valve and the fixture. Place a bucket directly below the work area and spread old towels on the floor to protect against water that will inevitably drip during disconnection. Clear the work area of all stored items, cleaning supplies, and anything that could get wet or obstruct your access. Set up adequate lighting — a headlamp is ideal for under-sink and tight-space work since it keeps both hands free. If you are removing an existing unit, disconnect it now: close the supply valves, disconnect the supply lines (have the bucket ready), disconnect the drain, and remove the old unit. Take this opportunity to inspect the exposed connections — if you see corrosion on supply valves, deterioration on supply hoses, or mineral buildup on pipe threads, address these issues now while everything is accessible rather than reinstalling on top of worn components.

Unpack the new sink and read through the manufacturer's installation instructions completely before beginning — different brands have specific requirements that may differ from general guidance. Lay out all included hardware, gaskets, and mounting components and verify everything is present. Set the new unit in position and check alignment with existing supply and drain connections before making any permanent attachments. Use a level to verify the unit is plumb (vertically straight) and level (horizontally flat) — an unlevel installation causes drainage problems, uneven wear, and can look visibly crooked. Mark mounting hole locations with a pencil. If the unit requires wall or floor mounting, pre-drill holes to the appropriate size for the included anchors or screws. Secure the unit according to the manufacturer's instructions using the provided hardware. Apply plumber's putty or silicone sealant to any gaskets or contact points specified in the instructions — the manufacturer will indicate which sealant type to use at each joint. Do not overtighten any mounting fasteners: snug plus a quarter turn is typically sufficient, and overtightening can crack housings, strip threads, or compress gaskets so tightly that they fail to seal properly.

Connect the water supply lines to the sink using the fittings appropriate to your connection type. For threaded connections, wrap the male threads with Teflon tape: hold the tape taut, wrap clockwise (when looking at the end of the fitting) for 3-5 full wraps, then press the tape into the threads with your fingers so it conforms to the thread profile. Thread the connection hand-tight, then add a half turn with a wrench — Teflon-taped connections should not require more than this to seal. For compression fittings (common on supply lines and shut-off valves), slide the compression nut and ferrule onto the pipe before inserting the pipe into the fitting body. Hand-tighten the nut, then add one quarter turn with a wrench. For push-fit connections (such as SharkBite), ensure the pipe end is cut perfectly square with a pipe cutter (not a hacksaw, which leaves a rough edge), deburr the inside and outside edges, and mark the pipe at the insertion depth specified by the manufacturer before pushing it into the fitting. Connect the drain lines if applicable, ensuring a minimum slope of 1/4 inch per foot of horizontal run for proper gravity flow. Double-check every connection by visual inspection before turning on the water — verify that no fittings are cross-threaded, no gaskets are pinched or misaligned, and all connections are properly seated.

With all connections made, slowly open the water supply valve — turn it open gradually over 5-10 seconds rather than snapping it fully open, which can cause water hammer and stress new connections. As pressure builds, systematically inspect every connection point for leaks. The most reliable method is to wipe each joint with a dry paper towel, wait 30 seconds, then check the towel for any moisture — this detects leaks too small to see with the naked eye. Pay special attention to the points where supply lines connect to the shut-off valves and where they connect to the sink, as these are the most common leak locations on new installations. Once you have confirmed all connections are dry, run the sink through its complete range of operation: test every function, run both hot and cold water for at least 2 minutes each, verify smooth operation of all handles and controls, check that water temperature is correct, and confirm that drainage is flowing freely with no standing water or slow draining. After the initial test, leave the area for 30 minutes with a paper towel placed under each connection, then return and check — some leaks only develop after components settle under sustained pressure. If you find any leaks, tighten the affected connection by a quarter turn. If tightening does not stop the leak, disassemble the joint, inspect for cross-threading or a damaged gasket, clean the surfaces, apply fresh sealant, and reassemble.

Restore the water supply gradually. Inspect every connection:

1. Look for visible drips or spray at every joint 2. Wipe each connection with a dry paper towel — check for moisture after 5 minutes 3. Test the sink through its full range of operation 4. Check the surrounding area for water that should not be there 5. Listen for unusual sounds: hissing (leak) or banging (water hammer)

Monitor for 24 hours. Some leaks only appear after parts settle under sustained pressure.

Leak at a connection: Tighten slightly — most joint leaks need only a quarter turn more. If it persists, disassemble, clean surfaces, apply fresh sealant, and reassemble.

Parts don't match: Bring the old component to the store for exact matching. Photos of the brand and model number prevent return trips.

Fixture doesn't work after repair: Verify the supply valve is fully open. Check for debris in the line. Confirm all parts are correctly oriented.

Unusual noises: Air in lines causes sputtering — run faucets until smooth. Whistling indicates a partially closed valve or wrong part size.

How much would a plumber charge for this job? Professional rates for work on a sink typically range from $150-400 including parts and labor. Emergency calls add $50-150. The DIY approach in this guide saves 60-80% of that cost.

Do I need plumbing experience for this project? This guide is written for homeowners with basic tool skills. Follow the steps in order, take your time, and don't skip the safety section. If you encounter something unexpected or feel uncomfortable at any point, there is no shame in calling a licensed professional.

How do I prevent this problem from happening again? Regular maintenance is the best prevention. Inspect the sink quarterly, address small issues before they worsen, and follow the maintenance schedule in this guide. Most plumbing failures develop gradually and show warning signs before becoming emergencies.

When should I call a professional instead? Call a pro if: you are unsure about the diagnosis, the work involves gas lines or main sewer connections, permits are required, you find extensive corrosion or structural damage, or the problem persists after your DIY attempt.