Ventilation is the changing of air in any space to remove moisture, odors,
smoke, heat, and airborne bacteria. Ventilation includes both the exchange of
air to the outside as well as circulation of air within the building. It is one of
the most important factors for maintaining acceptable indoor air quality in
buildings. Methods for ventilating a building may be divided into
mechanical/forced and natural types.

Mechanical or Forced ventilation
"Mechanical" or "Forced" ventilation is used to control indoor air quality.
Excess humidity, odors, and contaminants can often be controlled via dilution
or replacement with outside air. But in humid climates, much energy is
required to remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust 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. If the ducting for the fans traverse unheated space (e.g., an
attic), the ducting should be insulated as well to prevent condensation on the
ducting. Direct drive fans are available for many applications, and can reduce
maintenance needs.

Heat recovery ventilation systems employ heat exchangers to recover some
heat from exhausted air to preheat the incoming outside air.

Ceiling fans and table/floor fans are very effective in circulating air within a
room. Paradoxically, because hot air rises, ceiling fans may be used to keep a
room warmer.

Natural ventilation is the ventilation of a building with outside air without the
use of a fan or other mechanical system. It can be achieved with operable
windows 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 forcing cool
outside air to be drawn into the building naturally though openings in the
lower areas. These systems use very little energy but care must be taken to
ensure the occupants' comfort. In warm or humid months, in many climates,
maintaining thermal comfort via solely natural ventilation may not be possible
so conventional air conditioning systems are used as backups.
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