The issue is that an older furnace may have a crack or corrosion in the heat exchanger. The heat exchanger separates the flame from the air flowing over the heat exchanger. If there is a crack or other opening in this part, carbon monoxide can get into the heated air that goes into where you are living.
Carbon monoxide is colorless, odorless, and can build up in your blood. It does not come out of your blood cells easily, and if it builds up enough will kill you, period. Not a good way to die.
The heater, if it is original, could have cracked heat exchanger metal in it, and is therefore dangerous to use. You are getting good advice.
Consider replacing the heater with a new one. Your life is not worth trying to use a possibly dangerous heater. You should also have a battery powered carbon monoxide detector permanently installed in your AS as well as a smoke detector for your safety.
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