When venting Category I furnaces, where should the vent pipe size be adapted to a larger size?

In venting Category I furnaces, the vent pipe size should be adapted to a larger size two feet above the furnace connection. This approach is based on the principle of allowing proper draft and efficient ventilation of exhaust gases.

When the vent size is increased above the furnace connection, it helps to ensure that the flue gases rise effectively, facilitating proper airflow and minimizing the risk of backdraft or spillage. The larger pipe diameter reduces resistance to flow compared to a smaller diameter, allowing for better performance and safety.

Adjusting the vent pipe size at this height rather than directly at the furnace connection or at the top of the stack maintains a consistent drafting profile, which is critical for the safe operation of the furnace. Venting practices are designed to optimize the efficiency and safety of gas appliances, and adapting the size at the appropriate point contributes significantly to achieving those goals.