Evidence of a possible cracked heat exchanger is as follows:
- Turn the thermostat up, wait for the burners to ignite.
- After the burners ignite wait for the blower to start.
- 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.