What is the required combustion air supply from outdoors per 1,000 BTU/h of total input rating?

The requirement for combustion air supply is an important consideration in ensuring safe and efficient operation of HVAC systems, particularly when dealing with combustion appliances. In many residential and commercial systems, the standard guideline dictates that a supply of 0.35 cubic feet per minute (CFM) of combustion air is necessary for each 1,000 BTU/h of total input rating.

This figure ensures that there is sufficient air available for complete combustion, which helps in preventing the buildup of dangerous gases like carbon monoxide. Adequate combustion air also aids in improving energy efficiency by facilitating optimal combustion processes.

In evaluating the other options, it's clear that the provided alternatives either overestimate or underestimate the necessary airflow requirements. The other values do not align with established codes and best practices for combustion air supply, which are primarily set forth in standards such as the National Fuel Gas Code and other local regulations that govern safe HVAC installation practices. Adhering to the guideline of 0.35 CFM ensures proper ventilation and contributes to overall system safety and efficacy.