Feeling cold indoors even though the heat is on provides perhaps the most obvious motivation for taking a close look at your furnace. Kenyon advises, however, that "you can’t rule out the possibility of another cause.” It may feel chillier as a result of newly formed air leaks or a broken or malfunctioning thermostat. But in the absence of any alternative explanations, Kenyon advises homeowners not to push the heating system into overdrive. Instead, "bring in a professional"—especially if you notice significantly different temperatures in different rooms. “The hallmark of old, single-speed blower furnaces,” uneven heating may be the inevitable result of an increasingly outmoded technology.