Gas appliances are divided into four venting categories based on vent operating pressure and whether they are condensing or non-condensing. Category I is negative pressure, non-condensing. Category II is negative pressure, condensing. Category III is positive pressure, non-condensing. Category IV is positive pressure, condensing. These categories are used to determine the type of vent that should be used for the appliance.
Appliances that have a combustion efficiency of greater than 83 percent under American National Standard Institute (ANSI) test conditions are rated as condensing and will fall under Category II or IV.