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Ventilation (architecture)
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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 wood stoves, their exhausts are often called vents or flues — this should not be confused with ventilation. The vents or flues carry the products of combustion which have to be expelled from the building in a way which does not cause harm to the occupants of the building.
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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 wood stoves, their exhausts are often called vents or flues — this should not be confused with ventilation. The vents or flues carry the products of combustion which have to be expelled from the building in a way which does not cause harm to the occupants of the building. 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 odours and limit the concentration of carbon dioxide and airborne pollutants such as dust, smoke and volatile organic compounds (VOCs). Ventilation air is often delivered to spaces by mechanical systems which may also heat, cool, humidify and dehumidify the space. Air movement into buildings can occur due to uncontrolled infiltration of outside air through the building fabric (see stack effect) or the use of deliberate natural ventilation strategies. Advanced air filtration and treatment processes such as scrubbing, can provide ventilation air by cleaning and recirculating a proportion of the air inside a building.
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 normally have sufficient air leakage to prevent dangerous levels of carbon dioxide. Inadequate ventilation in a densely occupied room can cause the level of carbon dioxide to increase leading to sleepiness and reduced efficiency at work. This is a matter of concern in schools where attentiveness and learning ability may be adversely affected.
In commercial, industrial, and institutional (CII) buildings, and modern jet aircraft, return air is often recirculated to the air handling unit. A portion of the supply air is normally exfiltrated through the building envelope or exhausted from the building (e.g., toilet or kitchen exhaust) and is replaced by outside air introduced into the return air stream. The rate of ventilation air required, most often provided by this mechanically-induced outside air, is often 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 (CFM) or liters per second (L/s). The ventilation rate can also be expressed on a per person or per unit floor area basis, such as CFM/p or CFM/ft², or as air changes per hour.
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 smoking is allowed indoors, ventilation air is needed in sufficient quantities to dilute the airborne contaminants. Banning indoor tobacco smoking and the use of candles, air fresheners, incense, and other generators of air contaminants is much more effective for improving indoor air quality.
Combustion (e.g., fireplace, gas heater, candle, oil lamp, etc.) consumes oxygen while producing carbon dioxide and other unhealthy gases and smoke, requiring ventilation air. An open chimney promotes infiltration (i.e. natural ventilation) because of the negative pressure change induced by the buoyant, warmer air leaving through the chimney. The warm air is typically replaced by heavier, cold air.
Ventilation in a structure is also needed for removing water vapor, produced by respiration, burning, and cooking, and for removing odors. 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: 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 through operable windows but it can also be achieved through temperature and pressure differences between spaces. Open windows or vents are not a good choice for ventilating a basement or other below ground structure. Allowing outside air into a cooler below ground space will cause problems with humidity and condensation.
- Infiltration is separate from ventilation, but is often used to provide ventilation air
Ventilation equipment
Natural ventilation
Natural ventilation involves harnessing naturally available forces to supply and removing air through an enclosed space. There are two types of natural ventilation occurring in buildings: wind driven ventilation and stack ventilation. The pressures generated by 'the stack effect'relies upon the buoyancy of heated or rising air. wind driven ventilation relies upon the force of the prevailings wind to pull and push air through the enclosed space. Both are examples of passive engineering and have applications in renewable energy.
Demand Controlled Ventilation (DCV)
DCV makes it possible to maintain proper ventilation and improve air quality while saving energy. ASHRAE has determined that: "It is consistent with the Ventilation rate procedure that Demand Control be permitted for use to reduce the total outdoor air supply during periods of less occupancy." CO2 sensors will control the amount of ventilation for the actual number of occupants. During design occupancy, a unit with the DCV system will deliver the same amount of outdoor air as a unit using the ventilation-rate procedure. However, DCV can generate substantial energy savings whenever the space is occupied below the design level.
See also
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