What is the minimum size of screen openings for exterior combustion air intakes covered by louvres or grills?

The minimum size of screen openings for exterior combustion air intakes that are covered by louvres or grills is indeed set at 1/4 inch. This size is specified to ensure adequate airflow while also preventing the entry of larger debris or pests that could obstruct the combustion air supply. The 1/4 inch openings strike a balance between maintaining sufficient air intake for efficient combustion and providing protection against elements that could hinder the system's performance. Proper ventilation is crucial for safe operation, as inadequate air intake can lead to poor combustion efficiency and increased emissions.