Step 3 · Partially Drain the Water Heater
To replace the side-mounted T&P valve, you only need to drain the tank until the water level is below the valve's opening. Attach one end of a garden hose to the drain valve located at the bottom of the water heater. Place the other end of the hose in a floor drain, utility sink, or run it outside to an area where hot water won't cause damage. Open the drain valve (usually requires a flathead screwdriver to turn). To allow air into the tank so it can drain properly, go to a sink inside the house and open a hot water tap. You will hear gurgling as the tank drains. Keep an eye on the T&P valve location and close the drain valve once you are confident the water level is a few inches below it.