I came across Berricks blog "Is outside air really needed for a furnace?" from 2008 and have the similar issues.
I have a cabin that is 47 years old with forced air gas heater in a closet with sliding louver doors and I have two openings in the closet to the attic which let in cold air. One opening is an approx 8 inch pipe that runs from the ceiling to 12 inches from the floor and the other is an opening in the ceiling. The cabin furnace is a Day and Night(R) model 800A-14 S/N 0263 57188, Input 80,000 BTU, Bonnet capacity 64,000 BTU, series 260, 12 amps max that I think is old. Also I have a fire place with vertical wire mesh in front of it and since I rent my cabin I need to keep the fireplace flue open. Also the cabin is in Big bear lake which has mild but cold (below 32 F) winters.
I understand the history and the theory CraigFL stated.
My questions are;
1) What is the approx thermal efficiency of my old furnace?( I'm trying to calculate the time to break even)
2) Can I modify my furnace to close the burner area with sheet metal and run a metal pipe to the attic and close off the other two openings?
BTW, my heater has a pilot flame as does my stove.