An annual test of the relief valve is recommended to make sure the relief valve will lift if need and not stay closed. A relief valve is designed to pop open when the water temperature in the tank goes above 212 degrees Fahrenheit. The reason for this is simple  - no one would want water any hotter than at the boiling point of water. If the water was hotter, when a sink faucet was opened, steam would come out. This would be very dangerous. So a relief valve is designed to pop open when the water temperature gets this hot. The valve works by allowing new cold water into the tank, cooling it so that a explosion would not occur, if the gas valve never turned off. So annually, the relief valve should be manually lifted to the open position, then closed again. This will insure that there will be an available path for water to escape in the event such a condition would occur. There should be a tail pipe on the end of the relief valve directing the water to the floor.