Nuts & Bolts of Building Ventilation
The following is an article excerpt from the Center For Disease control that gives a nice overview of considerations regarding building ventilation. I thought it was a good starting point for the uninitiated and a comprehensive reminder for the rest of us – Neal.
The amount of air required to be delivered to a given space by an HVAC system is based primarily on the number of people occupying the space, the type and amount of equipment, and the overall size of the space. Proper distribution of ventilation air throughout all occupied spaces is essential. When areas in a building are used differently than their original purpose, the HVAC system may require modification to accommodate these changes. For example, if a storage area is converted into space occupied by people, the HVAC system may require alteration (balancing) to deliver enough conditioned air to the space.
Outdoor Air Supply
Adequate supply of outdoor air, typically delivered through the HVAC system, is necessary in any office environment to dilute pollutants that are released by equipment, building materials, furnishings, products, and people. CO2 levels are routinely collected in air quality studies because they can indicate whether a sufficient quantity of outdoor air is being introduced to an occupied space for acceptable odor control. ASHRAE notes that indoor CO2 concentrations no greater than 700 parts per million (ppm) above outdoor CO2 concentrations will satisfy a substantial majority (about 80%) of occupants [ANSI/ASHRAE 2013a].
However, CO2 is not an effective indicator of ventilation adequacy if the ventilated area is not occupied at its usual occupant density at the time the CO2 is measured. If CO2 concentrations are elevated, the amount of outdoor air introduced into the ventilated space may need to be increased.
In some cases, building owners/managers or occupants will open doors or windows to increase the amount of outdoor air coming into their building. However, relying on open doors may cause problems. For example, the air coming into the building through the doors may not reach all of the office areas in the building. The incoming air is unfiltered and may contain outdoor air pollutants such as pollen and dust. Additionally, open doors may affect the ability of the HVAC system to adequately control temperatures and humidity. ANSI/ASHRAE 62.1-2013 recommends outdoor air supply rates that take into account people-related sources as well as building-related sources. For office spaces, conference rooms, and reception areas, five cubic feet per minute of outdoor air per person (cfm/person) is recommended for people-related sources, and an additional 0.06 cfm for every square foot (cfm/ft2) of occupied space is recommended to account for building-related sources. In elementary and high school classrooms, 10 cfm/person plus 0.12 cfm/ft2 of outdoor air is suggested. To find rates for other indoor spaces, refer to Table 6-1 which is found in ANSI/ASHRAE 62.1-2013[ANSI/ASHRAE 2013a].
For spaces where airborne contaminants and odors are prevalent, ANSI/ASHRAE 62.1-2013 offers minimum exhaust rates from the space. For copy and printing rooms, the standard recommends an exhaust rate of at least 2.5 L/s•m2 (0.5 cfm/ft2) directly outdoors. The makeup air for this exhaust air can consist of any combination of outdoor air, recirculated air, or air transferred from adjacent spaces. When normal dilution ventilation does not reduce occupant exposures to emissions from office equipment to acceptable levels, some form of local exhaust ventilation must be considered to remove the contaminant from the source before it can be spread throughout the occupied space. However, little scientific research has been done to develop and/or test the performance of local exhaust systems for typical office equipment.
Outdoor Air Quality
When present, outdoor air pollutants such as carbon monoxide, pollen, and dust may affect indoor conditions when outside air is taken into the building's ventilation system. Properly installed and maintained filters can trap many of the particles in outdoor supply air. Controlling gaseous or chemical pollutants may require more specialized filtration equipment and sometimes relocation of the outdoor air intakes. Section 4 of ANSI/ASHRAE Standard 62.1 specifies that any outdoor air brought into occupied spaces must be in compliance with the U.S. Environmental Protection Agency’s (EPA) National Ambient Air Quality Standards (NAAQS). The standard further stipulates that a local outdoor air quality assessment should be conducted at a building and the immediate surroundings during periods the building is expected to be occupied to identify and locate contaminants of concern. If any outdoor contaminants exceed the NAAQS limits, the outdoor air must be appropriately treated prior to introduction of that air to the occupied spaces.
Diligent maintenance of HVAC system equipment is essential for the adequate delivery and quality of building air. All well-run buildings have preventive maintenance programs that help ensure the proper functioning of HVAC systems.
The materials contained in this article were taken from The Center For Disease Control And Prevention website and can be viewed in its entirety at http://www.cdc.gov/niosh/topics/indoorenv/hvac.html