no lower than 75. Even with these stringent requirements, older and less efficient furnaces still exist and may be rated as low as 55. For the purpose of energy conservation and heating costs, knowing the AFUE of a furnace is important in determining which brand or type of heating system will best suit your needs. These ratings are useful in evaluating the cost of a furnace relative to how much money it will save in the long run. Reliance Home Comfort has provided HVAC services in Ontario since 1999.  If you have any questions about your furnace and its efficiency, please contact us .