About Ventilation


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Ventilation (architecture)
For other articles with similar names, see Ventilation.
Return inlet (left) Supply outlet (right).
Return inlet (left) Supply outlet (right).


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 may be used to control humidity or odors. Kitchens and bathrooms typically have mechanical ventilation to control both. 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 (these save the owner the costs of maintaining/replacing drive belts).

Heat recovery ventilation systems employ heat exchangers to bring the fresh air temperature to room temperature.

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

[edit] Natural ventilation

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 fresh cool 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 soley natural ventilation may not be possible at times.

Ventilation is the intentional movement of air from outside a building to the inside. It is the V in HVAC. With clothes dryers, and combustion equipment such as water heaters, boilers, fireplaces, and woodstoves, their exhausts are often called vents or flues -- this should not be confused with ventilation. Their airflows are exhaust air. Movement of air between indoor spaces, and not the outside, is called transfer air.

Ventilation air, as defined in ASHRAE Standard 62.1 and the ASHRAE Handbook, is that air used for providing acceptable indoor air quality. When people or animals are present in buildings, ventilation air is necessary to dilute and remove airborne pollutants such as respirable suspended particles (RSPs) and volatile organic compounds (VOCs). Ventilation air is often delivered to spaces as part of the supply air which heats, cools, humidifies, and dehumidifies the space. Ventilation air can also be provided via infiltration, natural ventilation, or advanced air filtration and treatment processes such as scrubbing, for example.

In certain applications, such as submarines, pressurized aircraft, and spacecraft, ventilation air is also needed to provide oxygen, and to dilute carbon dioxide for survival. Buildings on Earth have significant air leakage, and rarely have been found to have dangerous O2 or CO2 levels. But inadequate ventilation in a densely-occupied room can cause the level of carbon dioxide to increase leading to sleepiness.

In commercial, industrial, and institutional (CII) buildings, and modern jet aircraft, air is often recirculated. But a percentage of the return air is normally exhausted and replaced by outside air. The rate of ventilation air required, most often provided by this mechanically-induced outside air, is determined from ASHRAE Standard 62.1 for CII buildings, or 62.2 for low-rise residential buildings, or similar standards.

The ventilation rate, for CII buildings, is normally expressed by the volumetric flowrate of outside air being introduced to the building. The typical units used are cubic feet per minute (commonly abbreviated as CFM), or, in metric units, liters per second (L/s). Often, the ventilation rate is expessed on a per person or per unit floor area basis, such as CFM/p or CFM/ft².

For residential buildings, which mostly rely on infiltration for meeting their ventilation needs, the common ventilation rate measure is the number of times the whole interior volume of air is replaced per hour, and 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.

If there is something burning (a fireplace, gas heater, candle, oil lamp, etc.) more oxygen is replaced by carbon dioxide (and possibly other poisonous gases and smoke) and more ventilation air is needed. A chimney causes infiltration or natural ventilation because it moves air from inside to outside. The resulting negative pressure change induced in the building typically causes a flow of air to move into the building replacing the warmer air that leaves by buoyancy through the chimney.

Ventilation in a structure is also needed for removing water vapor, produced by respiration, burning, and cooking, and for removing odors, e.g., from a toilet or kitchen. If water vapor is permitted to accumulate, it may damage the structure, insulation, or finishes. When operating, an air conditioner usually removes excess moisture from the air. A dehumidifier may also be appropriate for removing airborne moisture.


Types of ventilation

  • Mechanical or forced ventilation: as in through an air handling unit or direct injection to a space by a fan. A local exhaust fan can enhance infiltration or natural ventilation, thus increasing the ventilation air flow rate.
  • Natural ventilation occurs when the air in a space is changed with outdoor air without the use mechanical systems, such as a fan. Most often natural ventilation is assured though operable windows but it can also be achieved through temperature and pressure differences between spaces.
  • Infiltration is separate from ventilation, but is often used to provide ventilation air

Ventilation equipment

Natural ventilation

Natural ventilation is the process of supplying and removing air through an indoor space by natural means. There are two types of natural ventilation occurring in buildings: wind driven ventilation and stack ventilation. The pressures generated by buoyancy, also known as 'the stack effect', are quite low while wind pressures are usually far greater.

See also

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