If you have a confirmed crack in your heat exchanger, you need the furnace or heat exchanger replaced ASAP. I am an HVAC tech and business owner and when I find a cracked heat exchanger, I shut the gas and electric off untill it is repaired or replaced. This is dangerous and can kill!! Since carbon monoxide is odorless, you wont be able to smell it. Condensation on windows is a good sign of a cracked heat exchanger. I hope your CO detector is a good one.