The diameter of the flue pipe should be what in relation to the furnace outlet?

The diameter of the flue pipe should be the same as the outlet size of the furnace. This principle is based on the need for proper venting to ensure that combustion gases are efficiently and safely expelled from the furnace to the outside atmosphere. A flue pipe that matches the outlet size facilitates optimal airflow and prevents excessive back pressure or resistance that could impede the exhaust process.

If the diameter were greater than the outlet size, it could lead to temperature drops and reduced draft performance. Conversely, a size smaller than the outlet could create a bottleneck effect, increasing resistance and potentially causing dangerous backflows of gases into the living space. Having the flue pipe diameter equal to the outlet size ensures that the system operates within designed parameters for safety and efficiency.