For furnaces or boilers adjacent to outside walls, what is the minimum required permanent air supply inlet?

The correct answer is based on the specific requirements outlined for combustion air supply in heating appliances, particularly for those situated close to outside walls.

The requirement of 1 square inch of air supply per 4000 BTU, with a minimum inlet size of 35 square inches, ensures that there is adequate air for combustion. This is crucial for maintaining efficient operation of the furnace or boiler, allowing it to draw in sufficient air for combustion, which in turn helps to prevent issues such as incomplete burning, which can lead to higher emissions and potentially hazardous carbon monoxide production.

The established minimum size of 35 square inches guarantees that even in situations where the BTU rating may be lower, there is enough air supply for proper combustion, preventing operational inefficiencies. This requirement reflects the importance of balancing air and fuel to achieve safe and effective heating.

In contrast, the other options present different ratios and minimum sizes that do not align with current standards or regulations for appliances positioned adjacent to outside walls. Each of these alternatives either provides insufficient air based on BTU output or fails to meet the necessary minimum inlet size specified for safe operation.