For buildings which predominantly rely on
air infiltration to meet ventilation needs (most homes), the common
ventilation rate measure is the number of times the whole interior
volume of air is replaced per hour. It is called air changes per hour (I
or ACH; units of 1/h). ACHs of 0.5 to 1.5 are common in modern U.S.
homes under winter design weather conditions.
When an item such as a fireplace, gas
heater, or candle is burning, more oxygen is replaced by carbon dioxide
(and possibly other poisonous gases and smoke), thus more ventilation
air is needed. A chimney causes natural ventilation because rising heat
moves air from inside to outside, creating a negative pressure within
the home. This pressure change causes air to flow into the building--air
that is normally cooler than the air which left the building.
Ventilation in a structure is also needed
for removing water vapor, produced by such items as respiration and
cooking as well as for removing odors from an animal, toilet or kitchen.
If operating, an air conditioner usually removes excess moisture from
the air. A dehumidifier may also be appropriate for removing airborne
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