Project Guide

How to Install a Gas Tankless Water Heater

Remove the Old Water Heater
A man removing a water heater.

We highly recommend having your water heater installed by a professional as it is a difficult and sometimes dangerous process. Please follow all plumbing and electrical codes when sourcing and installing parts for your new water heater. Local codes can vary.

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 sizes of all water heater tanks have increased, while the amount of water 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.

  • Ask your gas provider about the size of your gas main. Tankless water heaters usually need more gas than even the biggest home furnaces. You may have to run new, larger gas pipe from your gas meter.
  • Turn off the water, gas supply valves and power.
  • Drain and disconnect the old water heater. Check with your local sanitation or recycling company for information on discarding it.
Install a New Gas Line
A newly installed gas line.
  • If your current gas line is adequately sized, install a tee and/or 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
A person installing a water line for a 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. If your water system has a check valve or anti-siphon valve on the supply line, an expansion tank near the cold intake to the heater will likely be necessary. See the heater owner's manual for more information.
  • 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
A person using a hammer and lumber to mount a 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 four inches from the wall.
  • Mount the tankless water heater securely to the platform.
  • Be safe. Choose a location for the new water heater that will allow you to vent it to the outside easily.
Connect the Water Heater
A person connecting a new gas tankless water heater line.
  • 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.
  • Make sure you have the purge and drain valves on hand since they do not come with many water heaters.
  • Connect the water supply line to the water heater valves following the manufacturer’s instructions.
  • Turn on the water supply and check for leaks.
  • Turn on the gas, bleed the line if necessary and check for leaks. After checking for leaks, turn the gas line back off until the installation is complete
Install the Vent
A reciprocating saw cutting a hole to install a gas tankless water heater vent.
  • 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. 
  • 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.
  • Some models use PVC pipe for venting, but the process is similar to the instructions above.
Turn on the Water Heater
A man turning on and adjusting the settings for a gas tankless water heater.
  • Open a faucet to full "hot" to purge air from the water pipes and heater. Turn the faucet off.
  • 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.