Air Movement
From Swikipedia
Introduction
Air movement is an important influence on indoor environments because it commonly moves across the building envelope. We generally want to keep control of indoor air quality, and air movement across the building envelope has the potential to affect the quality of indoor air dramatically. Circular air movement happening within a building envelope is called circulation. Air movement between the building interior and the outside of the building’s conditioned-air envelope, such as the exterior, crawlspace, attic or unheated basement is called infiltration (if air is moving into the conditioned space) and exfiltration, if it’s moving out.
Air movement in a building can be a concern when it creates undesirable conditions. These conditions can be uncomfortable moisture or temperature levels, or the introduction into the building of dust, pollen, mold spores, radon or other pollutants or health hazards.
Brief Description
Indoor Air Movement
Air movement is affected by the following:
• Differences in air pressure as air moves from areas of high pressure to areas of low pressure.
o Circulation- heating and cooling equipment both use blowers to distribute conditioned air throughout buildings. Depending on how well the system is balanced, this can establish air pressure differences in various areas of a building which can cause air to move in or out through the building envelope.
o Ventilation fans for bathrooms, laundries and range hoods all vent conditioned air to the outside which must be replaced. In the past, this make-up air has come from air infiltration around doors and windows and through other gaps in the building envelope.
o Combustion processes- appliances such as boilers, furnaces, heating stoves and water heaters pull air from the building interior as they exhaust the products of combustion to the exterior.
• Differences in temperature
o Thermal buoyancy- describes the action of air as it is heated. Because heated air is less dense it rises, moving from a cool, high-density area to ward a warm, low-density area.
o Stack effect- describes the action of warm air rising through a building. As warm air rises, it pulls cold make-up air into the building through the lower building envelope and pushes warm air out through the upper building envelope. Stack effect can have a significant effect on buildings, pulling undesirable hot or cold air, moisture or environmental pollutants and hazards (radon) into the building.
o Convection currents- The movement of cooler air moving in to replace rising warm air will establish convection currents any place in the building in which temperature differentials exist, with main areas of concern being the living space and attics. Supply and return registers are key points of temperature differentials and also key points of pressure differences caused by heating and cooling system hair handlers.
Outdoor Air Movement
How does wind effect buildings?
As wind blows against a building, a high-pressure area builds up next to the upwind exterior wall and roof surfaces. Air pressure is lower on the other (interior) side of this wall, so on the upwind side of the building air will be sucked into the building. As air blows past the building, air flowing next to the walls and roof (laminar flow) can’t turn sharply enough to cling to the downwind side of the building and so a vacuum is created on the downwind side of the building. On the downwind side of the building this vacuum will suck conditioned air out of the building. This process can lead to unwanted heat loss or gain depending on whether it’s the heating or cooling season.
Make-up Air
As air is exhausted from the building by the methods mentioned above, it must be replaced by make-up air. Unless ventilation devices are deliberately installed to provide make-up air, it will be pulled into the building through the building envelope. Uncontrolled make-up air may carry with it excessive moisture or heat (or lack of heat). It may infiltrate from the exterior, the crawlspace or the attic. In extremely tightly-built buildings, make-up air has been supplied from sewers after water was sucked out of the plumbing traps. Installing a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV) offers more control over the supply of make-up air, allows for more efficient use of heating and cooling equipment and reduces heating and cooling costs.
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