Furnaces do not always announce when they are dying. In fact, you really have to be paying close attention sometimes in order to tell that they are about to die or need repairs. The following signs, and keen senses, will help you prevent total furnace failure and get your furnace fixed in a timely fashion.
What You Smell
Most furnaces produce that whiff of "ozone" burning when you first fire them up in late fall. Yet, other smells that are unique to a furnace's operation should not be ignored. If you are used to the smell of a furnace's initial fire-up, you will be able to distinguish that from other furnace odors that are a sign something is wrong. Rotten smells, fire smells that are not the smell of wire burning, and anything that smells a lot worse than usual could be a sign that you need to call an HVAC technician.
What You Do Not Smell
What you do not smell can be just as bad for your furnace as what you do smell. Have a CO2 detector installed right next to your furnace. When a furnace starts to fail, it may produce more CO2, which in turn can asphyxiate you in your sleep while your furnace seems to be keeping everyone cozy at night. The CO2 detector will alert you to what you do not smell coming from your furnace and let you know that there is something wrong that you cannot see, hear, or smell.
What You Feel
The house seems to be warm and cozy, but there just does not seem to be much circulation. For a furnace and heating system to work properly, the cold air return vents suck the cold air out of the room, which is laying closer to the floor, and send the air back in after it has been heated in the furnace. When the cold air exchange does not work, you still get heat, but the circulation is not working. Ergo, you still have this sort of chilly draft close to the floor all the time, and lots of hot air above. This part of the furnace needs to be fixed, or the furnace will not be able to heat any more air and just what exists in the furnace itself will rise up into the house. The heat you feel is the furnace firing, but no new cold air is heated in the process. Reach out for furnace repair if this seems to be a problem.