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Mechanical or forced ventilation
“Mechanical” or “forced” ventilation is provided by an air handler and used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air.
Kitchens and bathrooms typically have mechanical exhausts to control odors and sometimes humidity. Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. Direct drive fans are available for many applications, and can reduce maintenance needs.
Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature by increasing evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor.
Natural ventilation is the ventilation of a building with outside air without the use of fans or other mechanical systems. It can be achieved with operable windows or trickle vents when the spaces to ventilate are small and the architecture permits. In more complex systems, warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus causing cool outside air to be drawn into the building naturally through openings in the lower areas. These systems use very little energy but care must be taken to ensure comfort. In warm or humid months in many climates maintaining thermal comfort solely via natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems in the forms of fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.
An important component of natural ventilation is air changes per hour: the rate of ventilation through a room with respect to its volume. For example, six air changes per hour means that the entire volume of the space is theoretically replaced with new air every ten minutes. For human comfort, a minimum of four air changes per hour is usually targeted, though warehouses might have only two. Too high of a replacement rate may be uncomfortable, akin to a wind tunnel which typically have thousands of changes per hour. The highest recommended replacement rates are for crowded spaces like bars, night clubs, and commercial kitchens at around 30 to 50 air changes per hour.
Room pressure can be either positive or negative with respect to outside the room. Positive pressure occurs when there is more air being supplied than exhausted, and is common to reduce the infiltration of outside contaminants.”
(Information Provided by Wikipedia)