“Makeup air required. Exhaust hoodsystems capable of exhausting in excess of 400 cubic feet per minute should be provided with makeup air at a rate approximately equal to exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with exhaust system.”
Backdating of combustion appliance------the makeup air is often pulled backwards through water-heater flues or down wood-burning chimneys— a phenomenon called backdrafting. Since the flue gases of some combustion appliances can include carbon monoxide, backdating is dangerous. In some cases, it can be life-threatening.