A method for examining water heater safety includes the steps of: setting a desired temperature and sensing a cold water temperature and a flow rate of water to calculate a theoretical gas consumption, and then supplying a combustor the theoretical gas consumption to heat water; sensing a hot water temperature, and calculating a temperature ratio by the desired temperature, the cold water temperature and the hot water temperature. Examining the temperature ratio may find that the water heater is abnormal or malfunctioning. An efficiency ratio may also be calculated by the theoretical gas consumption and the actual gas consumption. Examining the efficiency ratio may find that the water heater is abnormal or malfunctioning as well.