"A surprising number of homeowners rarely even go near the furnace," Kenyon says. But sometimes simply standing next to your furnace can tell you a lot about its condition. Check for cracks and corrosion, and in the process listen for any unfamiliar sounds. While squealing and screeching are a clue that the blower may need adjustment, other types of noises—grinding or banging—indicate the need for either repair or full replacement. Finally, Kenyon says, "pay attention to how the furnace room smells." When you start up the appliance for its first use of the season, you can expect a musty aroma, but if the smell lingers for more than a few days, schedule a service appointment right away.