The first and most critical step is to make the work area safe. Go to your home's main electrical panel and locate the circuit breaker labeled for the 'Water Heater'. Flip this breaker to the 'OFF' position. If you have a gas water heater, turn the gas control dial on the unit to 'OFF' and close the manual gas shut-off valve on the supply pipe. Next, find the cold water pipe that enters the top of the water heater and turn the handle of the shut-off valve clockwise until it stops. This prevents new water from entering the tank while you work.

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.

With a bucket positioned underneath to catch water, firmly grip the old expansion tank with one hand to support its full weight. Use a large pipe wrench or a pair of channel-lock pliers on the metal fitting at the base of the tank. Apply steady pressure counter-clockwise to break the seal. Once it's loosened, you should be able to continue unscrewing it by hand. As you remove it, tilt it carefully to drain the remaining water into the bucket. Be aware that a failed tank can be completely full of water and surprisingly heavy.

Before installing the new tank, you must ensure its internal air pressure matches your home's water pressure. Remove the plastic cap from the air valve on top of the new expansion tank. Use a standard tire pressure gauge to check the factory pre-charge. For this guide, we are assuming a target of 60 PSI. If the pressure is lower than 60 PSI, use a bicycle pump or an air compressor to add air until it reaches the correct pressure. If it's too high, gently press the pin in the center of the valve with a small screwdriver or the back of the gauge to release air. Re-check the pressure until it is accurate. Replace the valve cap once set.

Prepare the threads on the new expansion tank for a watertight seal. Take the roll of Teflon tape and, holding the tank with the threads facing you, wrap the tape clockwise around the threads. Use 3 to 4 full wraps, pulling the tape taut so it sinks into the grooves of the threads. Wrapping clockwise ensures the tape won't unwrap as you screw the tank in. Carefully lift the new tank and align its threads with the female fitting on the water pipe. Screw it in by hand for the first few turns to prevent cross-threading. Once it's hand-tight, use the pipe wrench to tighten it an additional 1 to 1.5 turns. The tank should feel snug and secure; do not overtighten.

Before restoring power, you must refill the tank and check for leaks. First, make sure the drain valve at the bottom of the water heater is fully closed and the garden hose is removed. Go back to the sink where you opened the hot water tap and close it. Return to the water heater and slowly open the cold water supply valve by turning it counter-clockwise. You will hear water rushing into the tank. While it fills, keep your eyes on the new expansion tank connection. Once the tank is full (the rushing sound will stop), thoroughly dry the connection with a paper towel and watch closely for any sign of moisture or dripping. If a leak appears, turn off the water and tighten the connection another quarter-turn.

With the tank full and leak-free, you can complete the job. Return to the electrical panel and flip the water heater's circuit breaker back to the 'ON' position. For a gas heater, follow the manufacturer's instructions printed on the tank to relight the pilot light and turn the gas control knob to 'ON'. To purge any remaining air from the system, go back to the hot water faucet you used earlier and open it fully. It will likely sputter and spit air for a minute. Let it run until a smooth, steady stream of water flows out, then turn it off. The water heater will now begin heating the water. Your expansion tank replacement is complete.