Turn off the gas supply, water supply, and electrical power to the existing heater
Drain the old water heater completely
Disconnect the gas line, water lines, venting, and temperature-pressure relief valve discharge pipe
Remove the old water heater
Place the new gas water heater in the correct location on a level, code-compliant surface
Verify required clearances from walls, combustibles, and other equipment
Install a drain pan if required by code or location
Connect the cold-water inlet and hot-water outlet lines
Install a new temperature-pressure relief valve if needed
Connect the relief valve discharge pipe to an approved termination point
Connect the gas supply line using approved fittings and sealant
Install or reconnect the venting system according to manufacturer and code requirements
Check that the vent has proper slope, support, and termination
Open the cold-water supply and fill the tank completely
Purge air from the hot-water lines by opening faucets
Inspect all water connections for leaks
Turn on the gas supply and test all gas connections with leak-detection solution
Light the pilot or follow the manufacturer’s ignition procedure
Set the thermostat to the desired temperature
Verify proper burner operation and venting
Confirm the temperature-pressure relief valve is installed correctly
Secure all access panels and insulation
Arrange for required inspection if local code requires it