References cited on this page:
|1.||Hitchcock PJ, Mair M, Inglesby TV, et al. Improving performance of HVAC systems to reduce exposure to aerosolized infectious agents in buildings; recommendations to reduce risks posed by biological attacks. Biosecur Bioterror. 2006; 4(1):41-54.|
|17.||Bearg, DW. Building Systems: HVAC Systems. In: Spengler JD, Samet JM, McCarthy JF, eds. Indoor Air Quality Handbook. New York: McGraw-Hill; 2001: 7.1–7.18.|
|18.||Orme, MS, Persily, AK, Rock, BA. et al. Ventilation and Infiltration. In: 2001 ASHRAE Handbook; Fundamentals (SI Edition). Atlanta, GA: ASHRAE, Inc.; 2001: 26.1-26.31.|
|19.||Emmerich, SJ, Dols, WS, Axley, JW. Natural Ventilation Review and Plan for Design and Analysis Tools (NISTIR 6781). Washington, DC: NIST. August 2001.|
Full list of references
HVAC System Basics
To fully appreciate building occupants’ risk of exposure to airborne biological threats and what can be done to reduce it, it is necessary to understand how air enters and moves through buildings.
Building ventilation is the process of bringing outdoor air into a building, circulating it, and later purging it to the environment.17 The main purpose of ventilation is to provide acceptable indoor air quality by diluting and removing contaminants from the indoor air.17,18 Building ventilation is achieved by natural or mechanical means, or by a combination of the two.18
Natural ventilation is the use of differences in air pressure that exist between the inside of a building relative to the outside of it, across the building envelope, to ventilate a building.18,19 These air pressure differences are created by natural forces such as wind and temperature. Air moves into and out of naturally ventilated buildings through windows, doors, vents and other openings incorporated into the building design and via infiltration/exfiltration.
Mechanical ventilation is the use of mechanical air handling systems—commonly referred to as heating, ventilation, and air conditioning (HVAC) systems—to ventilate buildings.17,18 Most commercial buildings use mechanical ventilation, which is more controllable and responsive than natural ventilation in providing adequate indoor air quality. However, mechanical ventilation also can exacerbate infiltration/exfiltration, which can compromise indoor air quality.
Purpose, Design Basics, and Function of HVAC Systems
An HVAC system provides adequate indoor air quality by: conditioning the air in the occupied space of a building in order to provide for the comfort of its occupants; diluting and removing contaminants from indoor air through ventilation; and providing proper building pressurization.17
While there are many different HVAC system designs and operational approaches to achieving proper system functionality, and every building is unique in its design and operation, HVAC systems generally share a few basic design elements (figure 1):
- Outside air intake
- Air handling unit—a system of fans, heating and cooling coils, air-flow control dampers, air filters, etc.
- Air distribution system
- Air exhaust system.17
In general, outside (“supply”) air is drawn into a building’s HVAC system through the air intake by the air handling unit (AHU). Once in the system, supply air is filtered to remove particulate matter (mold, allergens, dust), heated or cooled, and then circulated throughout the building via the air distribution system, which is typically a system of supply ducts and registers.17
In many buildings, the air distribution system also includes a return air system so that conditioned supply air is returned to the AHU (“return air”) where it is mixed with supply air, re-filtered, re-conditioned, and re-circulated throughout the building. This is usually accomplished by drawing air from the occupied space and returning it to the AHU by: (1) ducted returns, wherein air is collected from each room or zone using return air devices in the ceiling or walls that are directly connected by ductwork to the air-handling unit; or (2) plenum returns, wherein air is collected from several rooms or zones through return air devices that empty into the negatively pressurized ceiling plenum (the space between the drop ceiling and the real ceiling); the air is then returned to the air-handling unit by ductwork or structural conduits.1,17
Finally, some portion of the air within is exhausted from the building. The air exhaust system might be directly connected to the AHU and/or may stand-alone.