Vented gas appliances are classified for venting purposes into four categories as follows: (1) Category I — An appliance that operates with a non-positive vent static pressure and with a vent gas temperature that avoids excessive condensate production in the vent; (2) Category II — An appliance that operates with a non-positive vent static pressure and with a vent gas temperature that may cause excessive condensate production in the vent; (3) Category III — An appliance that operates with a positive vent static pressure and with a vent gas temperature that avoids excessive condensate production in the vent; (4) Category IV — An appliance that operates with a positive vent static pressure and with a vent gas temperature that may cause excessive condensate production in the vent.