Atleast once a year your furnace should be inspected for efficiency and safety. And one of the main things we check during a furnace checkout and tune up is the heat exchanger. if your heat exchanger gets a crack in it that will allow carbon monoxide to be released into your home which is a very dangerous gas that can't be seen, smelled, or tasted. And the carbon monoxide detectors that you can buy from any retail store only alert when levels reach 40 ppm. It takes a lot less than that to cause nausea and headaches. So that's why it's important to get a furnace checkout atleast once a year.