Before any work begins, you must isolate the water heater from its power and water sources. Go to your home's main electrical panel and locate the circuit breaker dedicated to the water heater. Switch this breaker to the 'OFF' position. To be certain, you can use a voltage tester at the heater's electrical connection. Next, locate the cold water inlet pipe on the water heater, usually marked with a blue ring or handle. Turn the shut-off valve on this pipe clockwise until it stops, cutting off the water supply to the tank.

To access the T&P valve without a major flood, you must lower the water level inside the tank. Connect a standard garden hose to the drain valve spigot located near the bottom of the water heater. Route the other end of the hose to a floor drain, utility sink, or outside. To prevent a vacuum from forming and to speed up the draining process, open a hot water faucet in a nearby sink or tub. Now, open the drain valve on the water heater. Since the T&P valve is on the side, you only need to drain a few gallons until the water level is below the valve's location.

The T&P valve has a discharge pipe that directs released water safely toward the floor. This pipe must be removed before you can access the valve itself. Using a large pipe wrench or adjustable wrench, grip the fitting where the copper discharge pipe connects to the T&P valve's outlet. Turn the fitting counter-clockwise to unscrew it. Have a rag or small bucket ready to catch any residual water trapped in the pipe.

With the discharge pipe removed, you can now remove the valve itself. Position a bucket or rags beneath the valve to catch any remaining water. Firmly attach the pipe wrench to the hexagonal body of the old T&P relief valve. Apply steady pressure in a counter-clockwise direction to break the seal and unscrew the valve from the threaded port on the water heater tank. Once loosened, you should be able to finish removing it by hand.

Prepare the new T&P valve for installation. Take the roll of Teflon tape and wrap it clockwise around the male threads of the new valve 3 to 4 times. Wrapping in this direction ensures the tape tightens as you screw the valve in, rather than unraveling. Carefully thread the new valve into the port on the water heater by hand to avoid cross-threading. Once it is hand-tight, use the pipe wrench on the valve body to tighten it another 1 to 2 full turns, until it is snug and the outlet is pointing directly downwards towards the floor.

With the new valve in place, you must re-install the discharge pipe. If desired, clean the threads on the pipe and apply fresh Teflon tape. Screw the discharge pipe into the outlet of the new T&P valve. Tighten it using your pipe wrench or adjustable wrench until it is secure. Per universal plumbing code, the pipe must extend down to within 6 inches of the floor and its end must not be capped, threaded, or otherwise obstructed to ensure a safe path for water discharge.

It is time to return the water heater to service. First, ensure the drain valve at the bottom of the tank is fully closed. Go to the hot water faucet you previously opened and leave it open. Slowly turn the cold water supply valve counter-clockwise to begin refilling the tank. Air will hiss and sputter from the open hot water faucet. Keep the water filling until you have a steady, air-free stream of water coming from the faucet, then turn the faucet off. Meticulously inspect the new T&P valve and discharge pipe connection for any signs of leaks. If everything is dry, return to the electrical panel and switch the water heater's circuit breaker back to the 'ON' position.

The final step is to verify the mechanical operation of the new valve. Allow the water heater some time to heat the water. Place a bucket under the end of the discharge pipe. Wearing gloves to protect from hot water, carefully lift the manual release lever on the top of the new T&P valve. A gush of hot water should exit the pipe. Let it run for 2-3 seconds, then release the lever. It should snap back into its original position, and the water flow must stop completely. If the valve continues to drip, lift the lever again briefly to flush any potential debris from the seal. If it still leaks, the new valve may be defective and should be replaced.