A hot water furnace, or a boiler, has different energy efficiencies depending on its model as well as its duct and piping system. The energy efficiency of a central hot water furnace and boiler is gauged by using a measurement called the "annual fuel utilization efficiency."
The AFUE is a ratio that measures the efficiency of any given appliance in converting its fuel into energy over the period of a year. In terms of a hot water furnace and boiler, the AFUE measures how much heat it emits when compared to the amount of fuel it consumes. As of 2015, the minimum AFUE rating of newly manufactured hot water furnaces and boilers are set at between 75 to 83 percent, depending on their product class.