Annual Fuel Utilization Efficiency
The efficiency of a furnace is expressed as a percentage of its Annual Fuel Utilization Efficiency. This is the ratio of a furnace's annual heat output to input. An AFUE of 100 percent means a furnace converts all its natural gas to heat. Furnaces from the early 1970s have AFUEs of 65 percent, while new gas furnaces need a minimum AFUE of 65 percent. The latest models have an AFUE of almost 97 percent.
Costs
Calculations for the hourly cost of a gas furnace must include the price of the furnace amortized over its projected lifetime. A cheaper furnace might not be a good value if its AFUE costs more to run per hour. An expensive furnace might be the better buy if its AFUE lowers hourly gas cost. A common AFUE for a base model of a natural gas furnace is 80 percent. This uses 750 therms a year at an annual natural gas cost of $750. This breaks down to 36 cents an hour, according to the U.S. Department of Energy.
AFUE of 90 Percent
The U.S. Department of Energy requires an AFUE of at least 90 percent for residential gas furnaces to receive an ENERGY STAR rating. This means an annual gas use of 660 therms at a cost of $660, or almost 31.7 cents an hour.
AFUE of 95 Percent
Top-of-the-line units boast an AFUE of at least 95 percent. They use 635 therms per year at an annual cost of $635, or 30.5 cents per hour.