How to Install a Gas Tankless Water Heater

2-4 hours

Tankless water heaters quickly supply hot water to your home and can lower your heating bills

Install a Gas Tankless Water Heater

Tankless water heaters are a compact and efficient way to supply hot water to your home. Whether gas or electric, tankless water heaters heat water only as it is needed and can save you up to 70 percent on your yearly heating bill. This guide outlines the steps required for a typical gas tankless water heater installation.

The Home Depot carries a vast selection of water heaters with the latest technologies and offers same-day installation. 

We highly recommend having your water heater installed by a professional as it is a difficult and sometimes dangerous process.

Note: Nationwide energy-efficiency regulations set in 2015 impact people buying new or replacing old hot water heaters. Your old water heater model number may not be the same as your new model number, so make sure to purchase based on capacity size instead of model number. Additionally, the size of all water heater tanks have increased, while the amount of water that they hold has decreased. Measure the space you have available for your water heater carefully so you can accurately judge what size water heater will fit. Learn more through Rheem’s National Appliance Energy Conservation Act guide.


Remove the old water heater
Remove old water heater  - Gas Tankless Water Heater

• Turn off the water, gas supply valves and power.
• Drain and disconnect the old water heater.

Install a new gas line
Install a new gas line  - Gas Tankless Water Heater

• Install a tee and valve in the existing gas line if needed.
• Using flexible gas supply line or black pipe, run a new gas supply line from the tee to a point where it can be easily attached to the gas line of the new water heater.

Install new water lines
Install new water lines  - Gas Tankless Water Heater

• Tie in new hot and cold water lines to the existing water lines with copper pipe and run the lines to the new water heater.
• Use bell hangers to hold pipe securely in place against the wall – this will also make it easy to apply pipe insulation later.
• Clean the pipes and connectors before soldering to remove oxidation.
• Apply flux to the end of the pipe, slip pipe and connectors together, heat the copper with a torch, and solder pipes together.

Mount the water heater
Mount the water heater  - Gas Tankless Water Heater

• Build a platform with 2 x 4-inch pieces of lumber that will allow you to mount the new tankless water heater about 4 inches from the wall.
• Mount the tankless water heater securely to the platform.

Safety: Choose a location for the new water heater that will allow you to vent it to the outside easily.

Connect the water heater
Connect the water heater  - Gas Tankless Water Heater

• Connect the shutoff valve and the sediment trap to the water heater following the manufacturer’s instructions.
• Attach the gas supply line to the shutoff valve.
• Connect the water supply line to the water heater following the manufacturer’s instructions.
• Turn on the gas, bleed the line if necessary, and check for leaks. After checking for leaks, turn the gas line back off until installation is complete.

Install the vent
Install the vent  - Gas Tankless Water Heater

• Apply heat resistant silicone sealant to the vent shaft of the water heater, slip on a connector, and secure with a hose clamp.
• Slip on a section of stainless steel vent pipe, slide on a retaining ring and fold over the taps to secure it in place.
• Slide on an elbow and adjust it so that the opening is pointing in the direction where the water heater will vent to the outside.
• Drill a hole from the inside through the joist or wall for venting to the outside at a point that will be the center of the vent hole.
• Connect the water supply line to the water heater following the manufacturer’s instructions.
• Outline a circle on the outside wall using the vent thimble and bore a 1-inch hole near the top of the circle using a spade bit.
• Cut out the vent hole with a reciprocating saw.
• Remove shingles or siding from around the vent hole to allow the flange thimble to rest flush against the outside wall.
• Apply silicone caulk around the edge of the flange, then insert the flange into the vent hole.
• Secure tightly with screws to the outside wall, and seal the screws with silicone.
• Install interior flange from the inside and run a section of vent pipe through the flange opening, securing each section with the tabs.
• From the outside, connect a small section of vent pipe and connect the vent hood.
• Replace the shingles or siding that were removed from around the vent opening.

Turn on the water heater
Turn on the water heater - Gas Tankless Water Heater

• Plug in the water heater.
• Insulate the hot water pipe coming from the water heater.
• Turn on the gas.
• Follow any specific start up procedures provided by the manufacturer of your new water heater.