How to Install a Tankless Water Heater (DIY Tips)

How to Install a Tankless Water Heater (DIY Tips) is a project that pays for itself in reliability, efficiency, and peace of mind. Whether you are installing a tankless water heater 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 tankless water heater 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 tankless water heater 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.

Energy Source Safety — Critical: Gas water heaters: turn the gas control valve to the OFF position before beginning any work. If you smell gas at any point during the project — even faintly — stop work immediately, do not flip any electrical switches or create any sparks, leave the house, and call your gas utility's emergency line from outside. Natural gas and propane are explosive, and even a small spark can cause ignition. Electric water heaters: shut off the dedicated 30-amp or 40-amp circuit breaker and verify with a voltage tester that power is off at the unit before touching any wiring. Electric water heater elements operate at 240V, which is lethal.

Scalding Prevention: The water inside a water heater tank is maintained at 120-140°F and can cause severe burns within seconds of contact. Before draining or disconnecting any water lines, either let the tank cool for 2-4 hours after turning off the heat source, or run hot water at a faucet for several minutes to introduce cold water into the tank. When opening the drain valve, keep hands and body away from the discharge flow. Use a garden hose to route the hot water safely to a drain.

T&P (Temperature and Pressure) Relief Valve: The T&P valve is a critical safety device that prevents the tank from exploding due to excessive pressure or temperature. Never cap, plug, or remove this valve. The discharge pipe must route downward to within 6 inches of the floor. If the T&P valve is leaking, it may indicate a serious problem with the water heater that needs professional diagnosis — do not simply replace the valve without investigating the cause.

Before purchasing your new tankless water heater, 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 tankless water heater 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 tankless water heater, 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 tankless water heater, conduct a thorough assessment of the installation space and all connection requirements. Measure the existing space including the footprint dimensions, height clearances, and distances to all connection points (water supply, drain, electrical outlet or junction box, and gas line if applicable). Note the pipe diameter of existing supply lines (typically 3/8" or 1/2" for fixture supply, 3/4" for main connections), the type of connections present (threaded, compression, push-fit, or soldered), and the electrical circuit specifications (voltage, amperage, and wire gauge). Photograph the current setup from multiple angles, capturing every plumbing connection, electrical junction, mounting point, and any labels showing brand and model information. Verify that your new tankless water heater is compatible with all existing infrastructure — water pressure range, pipe sizes, electrical requirements, and gas type (natural gas vs propane) if applicable. Check local building codes: many jurisdictions require permits for installations that involve both plumbing and electrical connections, and the work may need to pass inspection before use. It is far better to obtain a permit before starting than to be required to tear out completed work for an after-the-fact inspection.

This type of installation involves multiple utility connections, so you must systematically shut off each one before beginning work. First, turn off the water supply at the appropriate shut-off valve — use the fixture-specific valve if available, otherwise close the main house valve. Open a downstream faucet to verify water is off and to drain residual pressure. Second, shut off the electrical circuit at the breaker panel and use a non-contact voltage tester to verify power is completely off at the wiring location — do not rely on turning off a switch, as switches can be miswired. Place tape over the breaker to prevent someone from turning it on while you work. Third, for gas-connected units, turn the gas shut-off valve to the OFF position (handle perpendicular to the pipe means off). Place buckets and old towels beneath all connection points to catch residual water that will drain during disconnection. Clear the work area completely and set up adequate lighting. If removing an existing unit, disconnect all utility connections in reverse order of risk: gas first, then electrical, then water, then drain. Remove the old unit and inspect all exposed connections for corrosion, damage, or code violations that should be addressed before installing the new unit.

Carefully move the new tankless water heater into position and verify alignment with all existing connection points — water supply, drain, electrical, and gas (if applicable) — before making any permanent attachments. Use a level to check that the unit is plumb and level in both directions; an unlevel installation can cause drainage problems, accelerate wear on moving parts, create noise, and in some cases void the manufacturer's warranty. If the unit requires wall or floor mounting, mark the mounting hole locations, pre-drill for anchors if needed, and secure the unit using the hardware provided by the manufacturer. For heavy units (water heaters, softeners, large pumps), have a helper assist with positioning to avoid injury and prevent damage to the unit or surrounding surfaces. Ensure the unit is completely stable and properly supported before making any utility connections — an unsecured unit that shifts after connections are made can crack pipes, pull apart joints, or damage electrical wiring. Verify that all connection points on the new unit line up with the existing infrastructure; if any connections do not align, now is the time to plan and obtain the necessary adapters, extensions, or transition fittings.

Make connections in the safest order: water and drain first, then electrical (breaker still off), then gas last. For water supply connections, apply Teflon tape to all threaded male fittings — wrap clockwise (when looking at the end) for 3-5 complete wraps, pressing the tape firmly into the threads. For compression fittings, slide the nut and ferrule onto the pipe before inserting into the fitting body, then hand-tighten plus a quarter turn with a wrench. Connect drain lines using the appropriate fittings and ensuring a minimum slope of 1/4 inch per foot for gravity drainage. For electrical connections, follow the manufacturer's wiring diagram exactly: match wire colors (black to black for hot, white to white for neutral, green or bare copper to the grounding screw), strip wire ends to the specified length, and secure each connection with a properly sized wire nut. Give each wire nut connection a firm tug to verify it is secure, then wrap the base with electrical tape. For gas connections, use an approved flexible gas connector of the correct size and length — never reuse old gas connectors. Apply gas-rated pipe thread sealant (yellow Teflon tape or gas-rated pipe dope) to all threaded gas joints. After connecting, test every gas joint individually with a soapy water solution — apply soapy water with a brush and watch for bubbles, which indicate a leak. Even a tiny bubble means the joint must be tightened or redone.

Restore utilities in a controlled sequence, testing at each stage before proceeding to the next. First, slowly open the water supply valve — turn it gradually over 5-10 seconds and immediately inspect every water connection for leaks. Wipe each joint with a dry paper towel and check for moisture. Do not proceed until all water connections are confirmed leak-free. Second, restore electrical power at the circuit breaker. Verify that the unit powers on, displays are active (if applicable), and there are no burning smells, sparking, or tripped breakers. Third, for gas units, open the gas supply valve and immediately re-test all gas joints with soapy water under pressure — some leaks only appear when the full gas supply pressure is present. Follow the manufacturer's instructions to light the pilot (standing pilot models) or initiate the electronic ignition (electronic models). Run the tankless water heater through a complete operating cycle and verify every function: water heats to the correct temperature, the pump activates at the proper trigger point, the motor runs smoothly without unusual noise, water flows in the correct direction, drainage is proper, and all controls and safety devices respond correctly. Check the area around the unit for any water, gas odor, or unusual heat. Monitor the installation closely for the first 24 hours — check for leaks, listen for unusual sounds, verify continued proper operation, and confirm that no circuit breakers trip. Do not consider the installation complete until it has operated normally for at least a full day.

After restoring power and water:

1. Wait 30-60 minutes for water to heat 2. Run hot water at the nearest faucet — confirm it arrives on time 3. Check temperature with a thermometer (target: 120°F) 4. Inspect every connection with a dry paper towel 5. For gas: check connections with soapy water — bubbles mean a leak 6. Monitor for 24 hours

No hot water after relighting pilot: Wait 30-60 minutes for the full tank to heat. If still cold, the burner may not be firing.

Pilot keeps going out: Replace the thermocouple first — it's the most common cause.

Drain valve drips: Replace with a full-port brass ball valve.

Rumbling sounds: Sediment buildup. Perform a full flush.

How much does water heater service cost? Diagnostics cost $75-150. Specific repairs range from $100-300+. Full replacement including installation: $800-2,500 depending on type and size.

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 maintain my water heater? Flush the tank annually (every 6 months with hard water). Check the anode rod every 2-3 years. Test the T&P valve yearly. Keep the area around the heater clear.

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.