The efficiency of a boiler is evaluated using its rating in annual fuel utilization efficiency (AFUE). This compares the quantity of heat a boiler produces to the energy it consumes. The efficiency of the unit is described by its AFUE rating over the course of a year. The AFUE rating of your boiler is listed on the EnerGuide tag which is attached to the side of your appliance (if your boiler does not have an EnerGuide logo, it is probably outdated and needs to be replaced). Depending on the boiler type and fuel source, boiler efficiencies typically range from 80 to 88 percent for mid-efficiency boilers and from 90 to 98 percent for high-efficiency boilers. Although there is no efficiency requirement for Canada, we typically recommend homeowners use boilers with a minimum efficiency of 92%.