In a Category IV two-pipe system that terminates through the roof, the elbow for the combustion air intake should:

In a Category IV two-pipe gas furnace system, the proper installation of the combustion air intake elbow is crucial for optimal performance and safety. When the elbow for the combustion air intake is directed downwards, it prevents potential issues such as water intrusion, debris accumulation, and ice formation at the termination point. Pointing the elbow down allows any precipitation or condensation to easily drain away from the pipe, ensuring that the combustion air remains clean and dry for the furnace to operate efficiently.

This orientation also enhances the overall function of the system by reducing the risk of blockages that could hinder airflow. By ensuring a clear path for combustion air, the furnace can maintain proper pressure and efficiency, contributing to a safe and effective heating operation.

Other orientations, such as horizontally or upwards, may create vulnerabilities where water could accumulate and potentially affect the combustion air quality, leading to inefficient operation or even safety hazards. Thus, directing the elbow downwards is the most effective choice for maintaining a reliable and efficient Category IV gas furnace system.