A furnace cycles on and off during the course of its normal operation. When the home dips below the temperature set on the thermostat, the furnace kicks on to bring the temperature back up. Once the thermostat reaches the target temperature, the furnace turns off, and the cycle continues from there. If the performance of your furnace gives you any reason to think it may be on its way out, track its pattern of operation. Or, as Kenyon puts it, ask yourself two questions: “Does the furnace cycle on and off very frequently? Or does it seem to run all the time?” If the answer is yes to either question, Kenyon continues, it’s a “clear sign the furnace isn’t working as designed.” Call in a qualified pro to look into the behavior.