A water heater replacement is a sequence. Each step depends on the one before it. Work through the job in order, and confirm each check before moving on, because the most common problems come from skipping a step or reversing two.
Before the job starts
Confirm the fuel type of the old heater and the new one. Check that the new unit matches it. Look at the existing water connections, the drain pan if there is one, and the path for the relief valve discharge line. Note how the flue or vent runs, if the unit burns fuel. Confirm with the local permit office whether a permit and inspection apply, and when the inspection happens relative to the work.
Shut off and drain
Turn off the gas or the power to the old unit at its own controls. Close the cold water supply. Attach a hose to the drain valve and run it to a safe drain or outdoors. Open a hot tap elsewhere in the house so the tank can empty. Let the tank drain fully before you disconnect anything, because the water is hot and the tank is heavy.
Remove and set
Disconnect the water lines, the gas or electrical connections, and the vent or flue. Move the old unit out, and set the new one on its base or pan. Keep it level and clear of combustibles. Leave room around the unit for service access, as the manufacturer manual describes.
Connect in order
Connect the water lines first, then the relief valve and its discharge pipe, routed to a safe spot that the manual describes. If the system has an expansion control, install it per the manual. Then connect the gas line or the wiring, and finish the vent or flue. Check each joint for tightness before the next step.
Fill and purge before power
Open a hot tap in the house, then open the cold supply to fill the tank. Keep the tap open until water runs steadily without spitting, which means the air has cleared. Only then turn the power on, or light the gas, following the manual. Running a heater dry can damage it.
Check the start-up
Watch the unit through its first heat-up. Look for leaks at every joint and listen for unusual sounds. Check the relief valve discharge path. Test a hot tap to confirm the water is warm, not just running. Keep the manual and the permit paperwork with the unit.
Plan the next check
Mark the install date on the unit or in your records. The water heater's typical service life is a planning input, so note when to start looking at the next replacement window. Local rules and the manufacturer manual govern gas and electrical work, and some areas require a licensed installer for those steps.