If your HVAC technician just told you that your heat exchanger is cracked, I need you to understand exactly what that means before you sign anything. The heat exchanger is the heart of your furnace, and when it fails, you're looking at the most expensive repair bill you'll ever see on a heating system. Your heat exchanger separates combustion gases from the air that gets blown into your home. Burner flames heat up these metal chambers, and your blower pushes air across them. That's how you get w