What category does BPI disallow drilling of the flue?

The correct answer is based on the guidelines established by BPI (Building Performance Institute) regarding the safety and integrity of venting systems for appliances. Category IV flue systems are specifically designed to vent high-efficiency appliances that use condensing technologies. These appliances typically operate at low temperatures and produce water vapor as a byproduct of combustion, which can accumulate in the venting system.

Drilling into or modifying Category IV flue systems is disallowed primarily because such actions can compromise the integrity of the venting system. Since these flues are designed to handle the condensation produced by the appliance, any alteration could lead to leaks, which may result in dangerous situations, including carbon monoxide buildup or damage to the appliance itself. The guidelines emphasize maintaining the original design and function of these systems to ensure safe and efficient operation.

In contrast, Categories I, II, and III have different applications and safety considerations in regard to their venting requirements, which may allow for different installation methods. Therefore, understanding the distinctions between these categories and adhering to the stipulations for Category IV is crucial for maintaining safety standards in building performance and appliance efficiency.