Evidence of a possible cracked heat exchanger is as follows:

  1. Turn the thermostat up, wait for the burners to ignite.
  2. After the burners ignite wait for the blower to start.
  3. If when the blower starts, the burner flames change color, the heat exchanger may be cracked, and you should recommend service by a qualified HVAC contractor. Do not state that the exchanger is cracked, only that it may be, due to the change in burner fame color at blower activation.