With energy efficiency a leading factor in the optimization of furnaces for the home, the best modern models are condensing furnaces. By capturing and condensing water vapor in the air, condensing furnaces take advantage of the tighter, more closed system that characterize modern homes and yielding ever better energy conservation. But a condensing furnace may require fine-tuning the intake and exhaust of air to balance levels of humidity in the home.
In the simplest terms, the higher the SEER value of a heating and cooling system, the more efficient and more cost-effective the system is. Many older systems still in use have SEERs of 6 or below, whereas by 1994, the average SEER for all units shipped by manufacturers in the U.S. improved to 10.61 for central air conditioners and 10.94 for central heat pumps.