The vent system for a condensing furnace is classified as what by the National Field Gas Code?

The vent system for a condensing furnace is classified as Category IV by the National Fuel Gas Code. This classification is specific to high-efficiency appliances that produce condensate during the combustion process. Category IV venting systems are designed to safely exhaust flue gases at low temperatures, which is a characteristic of condensing furnaces.

These furnaces utilize a secondary heat exchanger to extract additional heat from the exhaust gases, resulting in water vapor that condenses and is drained away. Because of the low-temperature flue gases and the presence of condensate, venting materials used for Category IV systems are often made of PVC or other materials that can withstand the corrosive nature of the condensed water, making them distinct from higher-temperature venting categories. This classification helps ensure that the venting system is adequate for the safe and efficient operation of condensing furnaces while preventing potential issues that could arise from high temperatures associated with other categories.