What is the minimum free air requirement for vertical installation?

The minimum free air requirement for vertical installations is determined to ensure that there is adequate space for air circulation and ventilation around equipment and piping systems, thereby preventing overheating and allowing for efficient operation.

In this case, the correct answer indicates that a minimum of 1 cubic inch of free air is required for every 4000 BTUs of input. This ratio is designed to ensure that enough cooling air is available when these systems are in operation, which is particularly crucial in avoiding condensation, allowing proper combustion for heating systems, or ensuring that any gases produced can disperse safely.

This standard can vary based on specific local codes and manufacturer specifications, but in the context of the question, it aligns with recognized practices in plumbing and HVAC standards. Adequate free air helps maintain safety and efficiency in the installation and operation of plumbing and heating systems.