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

In a Category IV two-pipe vent system, the elbow for the combustion air intake should point down. This design is crucial because it helps prevent the possibility of any precipitation or debris entering the combustion air intake. If the elbow were to point up or straight out, there would be a higher chance of rain or snow entering the vent, which could interfere with the proper operation of the heating system. By positioning the elbow downwards, any moisture is directed away from the intake, ensuring that the combustion air remains clean and dry, which is essential for efficient combustion and system performance. This setup also effectively mitigates the risk of restricting airflow, maintaining the necessary supply of air for combustion.