How is a flue defined in relation to a tube?

In the context of the definitions related to a flue and a tube, the identification of a flue as measuring more than 4 inches externally while a tube measures more than 4 inches internally is accurate. This distinction is important because it characterizes the difference in sizing and purpose between the two elements.

Typically, tubes are identified based on their internal diameter, which corresponds to the internal space available for the passage of materials (such as gases or liquids). In contrast, a flue, used primarily for the venting of gases from appliances like furnaces or stoves, is often considered in terms of its external diameter, as it needs to safely contain and direct exhaust away from the living space.

This external measurement aspect of flues means they can be larger in terms of external dimensions compared to tubes, particularly in setups where insulation or clearance from combustible materials is a concern. Understanding these definitions helps in numerous applications, such as HVAC systems or plumbing, where correct installation and compliance with safety standards are critical.