When venting Category 1 furnaces, where should the vent pipe size be increased?

Increasing the vent pipe size at the furnace connection is essential for ensuring optimal performance and efficiency in Category 1 furnaces. This type of furnace utilizes gravity for venting, and the size of the vent pipe directly affects the ability of the combustion gases to escape safely.

When the vent pipe is sized appropriately at the furnace connection, it allows for adequate airflow and minimizes the risk of back drafting, where combustion gases re-enter the living space instead of venting outside. A proper size at this connection supports effective draft, helping to carry exhaust gases away efficiently while maintaining safe operating conditions.

In contrast, changing the size at other locations does not address the primary need for proper venting right at the source of the gases produced. The pressures and flow dynamics created at the furnace's exhaust outlet require that the size be optimized at this initial point to prevent any obstruction or inefficiency during operation.