A vent is defined as a pipe or conduit that contains a passageway for conveying combustion products and air to the atmosphere. Products of combustion contain carbon dioxide, water, and sometimes carbon monoxide. Therefore, it is important to safely vent a gas furnace.
Modern gas furnaces are available with efficiencies that range from 80% to 98% Annual Fuel Utilization Efficiency (AFUE) ratio. Furnaces that are 90 to 98% AFUE are considered very efficient and utilize a category 4 vent system. This means that the flue gases operate under a positive vent stack pressure with a low vent gas temperature, causing the gases to condense before discharging from the vent. Venting materials used for category 4 furnaces are those recognized by the furnace’s installation instructions such as stainless steel, PVC, and ABS. Furnaces one tier lower in efficiency use more traditional venting methods than the 90+% units. Even though these systems are associated with conventional chimneys and vents, special attention must be given to their installation.