Fire Hazards in the Home: Recently I went on a heating call where the homeowner had smelled burning inside the house in the middle of the night. Luckily she called the fire department and they identified the heating unit as a probable source of the smell but no flames, thank goodness!
On first investigation, the unit appeared to operate just fine electrically. But the fan stayed on and kept running. The smell did return in the bedrooms and was just a faint smoky odor. So I set my Carbon monoxide detector down while I began working in the attic. I ran through all the cycles and everything appeared fine.
Cracked Heat Exchanger: But one test indicated a possible cracked heat exchanger. So I tested it over several times and it appeared I was on the right track. I went downstairs to check the carbon monoxide and found levels twice the acceptable limit. This confirmed the cracked heat exchanger diagnosis. The furnace was 18 years old and was also a consideration when diagnosing the heat exchanger.
Burning Smell: The fan running continuously gave me another concern. The homeowner later told me that the fan was running all night and would not shut off even with the thermostat set to auto. So the fan circuitry is shorted somewhere maintaining a closed hot lead to the fan keeping it running. This was probably the source for the smoky smell. The smell did go away once the unit was shut down and the rooms aired out.
Good thing for the homeowner that she was able to smell the odor and wake up and evacuate the home so no one got hurt.