When obtaining outdoor combustion air through horizontal ducts, what is the minimum free area required for each opening?

The minimum free area required for outdoor combustion air through horizontal ducts is determined based on the input rating of the appliance being served. For combustion appliances, a common standard is that each opening must provide 1 square inch of free area for every 2,000 Btu/h of input. This requirement ensures that there is adequate airflow to support combustion and prevent backdrafting or other safety issues associated with insufficient air for combustion processes.

Understanding this guideline is essential for proper installation and operation, as insufficient air can lead to incomplete combustion, increased emissions, and potential hazards such as carbon monoxide poisoning. The specified ratio helps ensure a safe and efficient burn by providing enough air necessary for the combustion process according to the appliance's thermal output.