Natural ventilation is produced through doors and windows. The air in a room is changed by this method about three times an hour. If there is a fireplace in addition, the total number of changes per hour will be about four. A furnace will produce five changes of air per hour. Every room should be large enough to allow proper ventilation without too much draught. Authorities agree that not less than 300 cu. ft. of air space should be allowed for each person.
Forced ventilation is produced by forcing the air into a building with a fan or blower. Such a fan operates by means of the centrifugal force of a paddle wheel which sends the air off the edge of the blades.
Exhaust ventilation is that in which fans are placed at the top of the house, or ventilating flue, thus lessening the pressure within the building by producing a slight vacuum.