For gas-fired, central furnaces, the venting systems are classified and furnaces marked as follows: Category I - A central furnace that operates with a non-positive vent pressure and with a vent gas temperature at least 140 degrees F (60 degrees C) above its dew point. Category II - A central furnace that operates with a non-positive vent pressure and with a vent gas temperature less than 140 degrees F (60 degrees C) above its dew point. Category III - A central furnace that operates with a positive vent pressure and with a vent gas temperature at least 140 degrees F (60 degrees C) above its dew point. Category IV - A central furnace that operates with a positive vent pressure and with a vent gas temperature less than 140 degrees F (60 degrees C) above its dew point.