Step 2 · Relieve Pressure and Partially Drain Tank
To prevent a large volume of water from spilling when you remove the tank, you must relieve the pressure in the system. Go to a nearby sink and open the hot water tap all the way. This will allow air to enter the system as it drains. Next, attach a standard garden hose to the drain valve located at the bottom of the water heater. Place the other end of the hose into a floor drain or a large bucket. Open the drain valve by turning its handle or using a flathead screwdriver. Let a few gallons of water drain out until the water level is below the expansion tank's connection point.