 Or even if you have, you're going to want to listen up. If you're air-conditioned or in a furnace over 10 years old, more prone to breakdown and worse. It always happens at the worst time. A lot less efficient. With the dog days of summer approaching, it feels like it today. You're going to want to be proactive. Don't get caught in this heat and not have an AC that works. Or in the wintertime in a furnace that doesn't work.