AFUE and furnace efficiency
The Annual Fuel Utilization Efficiency or AFUE (pronounced ‘A’-‘Foo’) rating for a furnace is the most widely used measure of a furnace’s efficiency.  It denotes the actual amount of heat that gets delivered to your home versus the amount of heating fuel that you must supply the furnace.
If you were to look at this in dollars and cents, an example would be for a 95% efficient gas furnace, $0.95 of every dollar you spend on heating gas actually goes into heating your home. Compare this to a 60% efficient furnace where $0.40 of every dollar you spend on fuel is heading straight out of your chimney.
The minimum efficiency standard in Canada for natural gas units being installed in new homes is currently 95%. Most furnaces manufactured before 1990 where 60% efficient. Furnaces manufactured today have AFUE ratings that range from 95% to as high as 98%.