How to Install a Tankless Water Heater

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

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Cost: 00

Skill Level: Expert

Tools List for Installing a Tankless Water Heater:
Pipe wrench
Hole saw

Shopping List:
1” gas line and fittings
4” PVC pipe and fittings
PVC glue
Teflon tape
¾” copper pipe and fittings
Pipe cutter

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 cross-over 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.

Richard installed an RUR Super High Efficiency Plus instantaneous hot water heater, manufactured by Rinnai (

Expert assistance for this project was provided by Budiac Plumbing & Heating, Inc of Milwaukee, WIsconsin.

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Here’s how to install an electric water heater. Find step by step project details here:

Call 1-877-GO-LOWES or visit to learn more about installation services through Lowe’s.

Read this if you’re not sure your water heater needs replacing.

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