What size opening is required in the furnace room door for combustion air in the described setup?

In the context of combustion air requirements for a furnace room, the size of the opening for combustion air is critical for ensuring that the furnace receives enough air to operate efficiently and safely. Each installation has specific requirements based on the furnace's BTU rating, the room's volume, and the type of ventilation used.

Choosing 125 square inches as the required opening reflects knowledge of standard guidelines that often govern these requirements. For many residential setups, a general rule is that a certain amount of free air must be provided for every BTU of the furnace's capacity. This ensures that there is sufficient oxygen available for the furnace to complete the combustion process effectively.

If a specific calculation method leads to the requirement of 125 square inches, it indicates that this is the minimum needed to maintain proper airflow and prevent issues related to inadequate combustion air, such as the production of carbon monoxide or inefficient heating.

In contrast, the other sizes like 100 square inches, 150 square inches, or 200 square inches would either under-provide or over-provide airflow, which may not align with the furnace's operational needs. Thus, selecting the appropriate size is crucial for safety and efficiency, making 125 square inches the correct choice in this scenario.