80+ or less installs (non self-contained combustion chamber)
National Fuel Gas Code section on Air for Combustion and Ventilation or applicable provision of the local building codes should apply to both 80 & 90.
Circa 93-96, we were required to bring in outside air and terminate down wall within 12 inches of floor adjacent to any furnace. Did not matter if space was considered confined or unconfined or if furnace was 80 or 90.
Definition back then from manufacturer requirement on an 80+ or less install.
UNCONFINED SPACE - An unconfined space must have at least 50 cu ft for each 1000 Btuh of input for all the appliances (such as furnaces, clothes dryer, water heaters, etc.) in the space.
If unconfined space is constructed usually tight, aire for combustion and ventilation MUST come from either the outdoors or spaces freely communicating with the outdoors. Combustion and ventilation openings must be sized the same as for a confined space. A minimum opening with a total of at least 1 sq in / 5000 Btuh of total input rating for all equipment must be provided. Return air must not be taken from the room unless an equal or greater amount of air is supplied to the room.
A confined space is one whose volume is less than 50 sq ft per 1000 Btuh of the total input rating for all appliances installed in that space. A confined space MUST have 2 permanent openings. 1 within 12 in of the ceiling and the other within 12 in of the floor. Different requirements then came into play depending on where one was drawing the required air from.