Hot water tanks are typically heated by gas. Electrical hot water heaters use internal controls to maintain proper pressure levels, but gas hot water tanks use a pressure relief valve that opens when the pressure becomes too high. Performing maintenance or repairs on any kind of water tank requires that you drain out the water. You should drain a pressurised hot water tank, whether it's a boiler or a water heater, while the water is hot. Water expands as it cools, increasing the internal pressure. The process doesn't take a long time.
Turn off the electricity or gas to the hot water heater.
Attach a garden hose to the drain valve, located near the water tank's base.
Position the end of the garden hose in a safe area. The water coming out will be hot enough to burn your skin.
Open the drain valve. Allow the tank to drain until the flow starts to taper off, then open the pressure relief valve on top of the water tank. Position a bucket beneath the relief valve opening, as some water will come out. Opening the pressure relief valve will allow air into the system, forcing the water out of the lower drain valve.
Wear heat-proof gloves. The elements of the water tank are likely to be very hot and could burn your skin if you contact the metal.