Sometimes known as fleet efficiency, furnace ratings are less about the overall quality of particular brands or types of furnaces than about the annual fuel utilization efficiency of available furnaces from brand to brand and model to model. Furnaces ratings are, in essence, a way to determine the comparative cost effectiveness, in terms of fuel usage, of each of the numerous models of home heaters. Furnace ratings are useful in comparing types, oil or gas, as well as brands and models of home heating systems.
Furnace ratings are based, in total, on a comparison of two or more furnaces of different manufacturers based on the measured fuel usage of each furnace. This usage is computed by using an Annual Fuel Utilization Efficiency (AFUE).
As an example of AFUE ratings, if furnace A receives an AFUE factor, or rating, of 85%, this means that 85% of the fuel burned by the home heater is producing heat and 15% of the fuel is unburned, and thus, is wasted. Furnace B, on the other hand, receives an AFUE factor of 90%. This obviously means that furnace B utilizes 90% of its fuel for heating the home and wastes 10%. Thus, furnace B is a more efficient furnace than is furnace A, as far as fuel utilization is concerned.
Though particular home heaters may have better furnace ratings, fuel efficiency-wise than others, this does not mean that they are the most durable, or the least costly to install. In other words, though fuel-efficient, those furnaces with higher AFUE ratings may not represent the best overall value on the market.
However, on 1 January 1992, the U.S. Department of Energy (DOE) mandated that furnace ratings for all home heating systems manufactured after that date must have a minimum AFUE of 78%. Mobile home heaters must have AFUE furnace ratings of 75% or better. This is, of course, in response to Environmental Protection Agency (EPA) guidelines, as well as the rising costs and dwindling supplies of home heating fuel.
Gas boilers have been found to have marginally better furnace ratings, on average, than oil-fired boilers. Furnace ratings for oil-fired boilers are, in turn, better than for non-condensing gas boilers. Forced hot air furnaces are usually rated somewhat lower than either forced hot water or steam heating systems.
Determining a particular type, not to mention a specific brand of home heating system, is sometimes a difficult choice. Furnace ratings help homeowners in selecting the best possible heater for the money.