Turn off power to the old water heater at the breaker
Shut off the water supply to the existing unit
Drain the old water heater if replacing one
Remove the old water heater if necessary
Check local plumbing and electrical or gas code requirements
Verify the new on-demand water heater is the correct type and size
Choose the installation location with proper clearance and ventilation if gas-fired
Mount the unit securely on the wall or prepare the installation base
Install required water shutoff valves and service valves
Connect the cold water inlet and hot water outlet
Install a pressure relief valve if required by the manufacturer
Install a condensate drain if required
Connect the gas line for a gas unit or the electrical supply for an electric unit
Install venting for a gas unit according to manufacturer specifications
Install a dedicated circuit and proper breaker for an electric unit
Install a temperature and pressure gauge if required
Flush the water lines before final connection
Open the water supply and check for leaks
Purge air from the system
Restore power or gas supply
Set the desired water temperature
Test hot water flow at a fixture
Inspect all connections again after startup
Schedule professional inspection if required by local code