Your water heater, whether gas or electric, builds up sediment at the bottom of the tank. This sediment decreases the efficiency of the heater, shortens the life span of the appliance and reduces the hot water supply in your home. Therefore, it is recommended that you drain about two gallons of water from the tank once or twice a year. You could, if you choose to, drain the entire tank for a more thorough cleansing. You may have another reason to drain your water heater and that is explained here as well.
Caution: Please read our safety information before attempting any maintenance, installation or repair. When draining water from a water heater, the water is hot, possibly hot enough to scald and burn. Do not touch the water as it comes out and take appropriate measures to protect yourself from burns. Wearing heavy gloves and eye protection is recommended. Turn off your water heater before draining to prevent damage to the appliance.
A typical water heater has two valves on it. One is a safety valve that relieves pressure and is located near the top. The other is for draining water and is located near the bottom. We will use the bottom valve to drain water and the sediment from the tank.
If you will be draining the tank completely, you must turn off the water supply and turn off the thermostat so that the heater will not be damaged by heating an empty tank. Either turn off the circuit breaker for an electric model or turn the gas control to "Pilot" for gas models. To turn off the water supply, turn off the valve on the pipe where it enters the water heater. If there is no valve then you will have to turn of the water at the water main. Open a hot water tap in a shower to prevent a vacuum when draining the tank completely.