Recirculating direct-fired industrial air heaters shall incorporate a means to supply outside ventilation air to the space at a rate of not less than ______ cubic feet per minute per 1,000 btu/h of rated input of the heater.

The requirement for recirculating direct-fired industrial air heaters to provide outside ventilation air is crucial for ensuring safe operation and maintaining air quality. The standard specifies a minimum ventilation rate of 4 cubic feet per minute per 1,000 BTU/h of the heater's rated input capacity. This guideline is based on the need to dilute any combustion by-products and to ensure that there is adequate oxygen for the combustion process.

By supplying at least this amount of outside air, it helps to prevent the build-up of harmful gases within the space being heated. Furthermore, providing proper ventilation helps to control humidity levels and maintain a comfortable indoor environment.

The essence of this requirement supports not only the efficiency of the heating process but also the health and safety of the occupants in the space. Hence, 4 cubic feet per minute per 1,000 BTU/h is established as a balanced and effective measure.