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

The vent system for a condensing furnace is classified as Category IV by the National Field Gas Code. This classification indicates that the furnace can vent directly outside without requiring additional equipment to condition the flue gases. Category IV systems are designed to handle appliances that operate with positive pressure and can vent both the combustion products and intake combustion air through plastic piping, typically due to the lower temperatures of the flue gases produced by condensing furnaces.

Additionally, the use of Category IV venting is advantageous because the lower venting temperatures reduce the likelihood of condensation forming in the vent, making it efficient and safe for residential use. This classification aligns with the operational characteristics of condensing furnaces, which extract significant heat from the combustion process, leading to higher efficiency and lower emissions.

In contrast, Category I, II, and III venting systems cater to different types of appliances with varying venting requirements regarding pressure, temperature, and flue gas handling. Therefore, recognizing that condensing furnaces specifically fall under Category IV helps clarify their unique venting requirements per industry standards.