Fuel-burning appliances require air for combustion. When the appliances are located in enclosed spaces, provision must be made for supplying the required amounts of air. Depending on the specifics of the appliances and the enclosure, additional air may be required for draft hood dilution and space conditioning. An enclosed space can be a mechanical room in a building, a furnace room in a residence or the entire floor of a building if a separate enclosure is not used to isolate the combustion appliance(s). An example of the latter is an unenclosed warm-air furnace in a basement of a residence.