Today, we’ll be covering an important topic: water heater efficiency. For tank-type water heaters that use natural gas, the standard efficiency is about 60%. This means that only 60% of the gas that is burned will play a direct role in heating the water within a tank. The other 40% will escape out of the flue. As a water heater ages, though, the percentage of gas that escapes will begin to increase, causing the heater’s efficiency to decrease.