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How to Install a Tankless Water Heater

Ask This Old House plumbing and heating expert Richard Trethewey heads to Milwaukee to install an instantaneous water heater.


  1. An installation like this that involves gas piping is best left to a licensed, professional plumber.
  2. Shut off the water and the gas to the hot water tank. Drain the water out of the tank and remove the old tank.
  3. The new water heater may require a larger gas line, which will need to be run from the gas meter to the water heater.
  4. Using the hole saw, drill a hole on the outside of the house near the location of the water heater. Run PVC pipes for the fresh air intake and vent through that hole.
  5. Connect the cold water supply line to the water heater and the hot lines out of the heater using the ¾” copper pipe.
  6. An on-demand water heater requires an electrical outlet. An electrician may need to assist in adding one near the installation location.
  7. A crossover valve can be installed at the furthest plumbing fixture to allow the built-in recirculation pump to keep hot water at the ready.
  8. Turn on the cold water supply, gas supply, and power to the water heater and test.