Before starting any work, it's crucial to gather all the necessary tools and supplies. This ensures you can complete the job efficiently without interruptions. You will need a new T&P valve that matches the specifications of your old one (check the PSI and temperature ratings). You'll also need a large pipe wrench to provide the leverage needed to remove the old valve and tighten the new one, Teflon tape for sealing the threads, a garden hose for draining, and a bucket to catch any residual water. Wearing work gloves is recommended for grip and protection against hot surfaces.

This is the most critical safety step. First, locate your home's main electrical panel (breaker box). Find the breaker that controls the water heater; it is usually a double-pole breaker labeled 'Water Heater' or 'WH'. Flip the breaker to the 'OFF' position. Next, find the cold water shutoff valve for the water heater. This is typically a lever or wheel-style valve on the cold water pipe entering the top of the tank. Turn the valve clockwise (for a wheel) or perpendicular to the pipe (for a lever) to shut off the water supply.

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.

The T&P valve has a discharge pipe, typically made of copper or CPVC, attached to its outlet. This pipe directs any discharged water safely towards the floor. Before you can remove the valve itself, you must first detach this pipe. It is usually connected with a threaded fitting. Place your bucket underneath to catch any trapped water. Using your pipe wrench or channel-lock pliers, grip the fitting on the pipe and turn it counter-clockwise to unscrew it from the T&P valve. Once it's loose, set the pipe aside.

Now you can remove the valve itself. Position your large pipe wrench onto the hexagonal body of the old T&P valve. Ensure you have a firm grip. Apply steady pressure in a counter-clockwise direction to break the seal and unscrew the valve from the tank. This may require significant force, especially if the valve is old and corroded. Once it's loose, you can typically finish unscrewing it by hand. Be prepared for a small amount of water to spill out; have your rag ready.

With the old valve removed, use a rag to clean the female threads on the water heater port, removing any old tape or sealant. Take your new T&P valve and wrap the male threads with 3 to 4 layers of Teflon tape. It is critical to wrap the tape in a clockwise direction (the same direction you will be turning the valve to tighten it). This ensures the tape tightens into the threads rather than unravelling. Carefully thread the new valve into the port by hand to avoid cross-threading. Hand-tighten it as much as you can.

Once the new valve is hand-tight, use the pipe wrench to give it another one to two full turns. The goal is to make it watertight and position the valve so its outlet is pointing straight down towards the floor. Do not overtighten, as this can crack the valve body or damage the threads on the tank. Once the valve is installed, re-attach the discharge pipe to the valve's outlet. Hand-tighten the fitting and then give it a final snug turn with the pipe wrench. The pipe must terminate no more than 6 inches from the floor and should not be threaded at its outlet.

The installation is now complete, and it's time to refill the tank. First, ensure the drain valve at the bottom of the tank is fully closed. Also, make sure the hot water tap you opened earlier is still open. Go to the cold water inlet valve and slowly open it to begin filling the tank. You will hear water flowing and air sputtering from the open hot water tap. Leave the tap open until you have a steady, air-free stream of water coming out. This signifies the tank is full and all air has been purged. Once the tank is full, close the hot water tap. Thoroughly inspect the new T&P valve and its connections for any signs of leaks.

Once you have confirmed that the tank is completely full and there are absolutely no leaks, you can safely restore power. Return to the electrical panel and flip the water heater's breaker back to the 'ON' position. The water heater will now begin to heat the new water in the tank. This can take an hour or more, depending on the tank size and the temperature of the incoming water.

As a final quality check, it's wise to test the new valve's mechanical function. Place a bucket under the end of the discharge pipe. Carefully and briefly lift the test lever on the top of the T&P valve. A burst of hot water should flow out of the pipe. Release the lever; it should snap back down into its original position, and the flow of water should stop completely. If the valve continues to drip or run after the test, it may be defective or have debris from the installation caught in the seat, which may require you to flush it again or replace it.