15 years
Older furnaces that were made with heavy metal heat exchangers, like cast iron, have been known to last 30 to 40 years, but they're also known for being energy drainers. Forced air furnace heat exchangers have an average life expectancy of 15 years, with a range typically between 13 and 20 years. Most heat exchangers used in models today today will last between 15 and 25 years and are usually made from aluminized or stainless steel. On average, a furnace or air conditioner will last 10 - 20 years. Sometimes it pays to replace the old system sooner because of the higher efficiency (lower gas and electric bills) provided by newer equipment. Furnaces have become more energy-efficient over the years. A gas furnace made in the early 1970s typically has an AFUE of about 65 percent. The lowest efficiency allowed by law for new gas furnaces is 78 percent, and some new models achieve 97 percent--near-total efficiency. Furnaces. High-efficiency propane furnaces provide energy, cost, and carbon savings in both new and existing homes. Heated air leaves the propane furnace at temperatures typically near 120 degrees Fahrenheit, significantly warmer than the delivery temperatures offered by electric heat pumps. Knowing how to buy an energy efficient propane furnace can keep you from making a costly mistake. The new, highly efficient propane furnaces are about 92-94 percent efficient, and the prices are reasonable.
http://airconditioningsoutheast.com/tips/heating/how-long-will-my-furnace-last http://www.expertsatjthomas.com/faq.htm http://www.consumerreports.org/cro/gas-furnaces/buying-guide.htm http://www.propane.com/residential/products-that-use-propane/space-heating/ http://greenliving.lovetoknow.com/Energy_Efficient_Propane_Furnace
how long does a energy efficient propane furnace last