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HVAC management for better IEQ

HVAC systems alone can act as sources of pollutants. If these systems are not appropriately maintained, ventilation air filters can become saturated leading to potential microbial growth and odor concerns. Microbial growth can also result from stagnant water in drain pans or from uncontrolled moisture inside of air ducts and cooling coils.

Ventilation System Design

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. Carbon dioxide (CO2) is a normal constituent of exhaled breath; thus, CO2 will also increase during building occupancy. 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. The American National Standards Institute (ANSI) and ASHRAE have developed consensus standards and guidelines for HVAC systems. ASHRAE notes in Informative Appendix D to ANSI/ASHRAE Standard 62.1-2016: Ventilation for Acceptable Indoor Air Quality  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 2016]. This would typically correspond to indoor concentrations below 1200 ppm since outdoor CO2 concentrations usually range between 375 to 500 ppm. 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. Elevated CO2 concentrations suggest that other indoor contaminants may also be increased. 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 and windows may cause problems. For example, the air coming into the building through the doors may not reach all of the occupied 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. ASHRAE guidelines provide specific details on ventilation for acceptable indoor environmental quality. A ventilation system expert can help meet ASHRAE ventilation guidelines in the building. ANSI/ASHRAE 62.1-2016 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 which is found in ANSI/ASHRAE 62.1-2016 [ANSI/ASHRAE 2016].

Exhaust Rates

For spaces where airborne contaminants and odors are prevalent, ANSI/ASHRAE 62.1-2016 offers minimum exhaust rates from the space. For copy and printing rooms, the standard recommends an exhaust rate of at least 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.

Temperatures and Occupancy Settings

Temperature and relative humidity measurements are often collected as part of an indoor environmental quality investigation because these parameters affect the perception of comfort in an indoor environment. The perception of thermal comfort is related to one's metabolic heat production, the transfer of heat to the environment, physiological adjustments, and body temperature. Heat transfer from the body to the environment is influenced by factors such as temperature, humidity, air movement, personal activities, and clothing. The ANSI/ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy specifies the combinations of indoor environmental and personal factors that produce acceptable thermal conditions to a majority of occupants within a space [ANSI/ASHRAE 2013b]. Assuming slow air movement (less than 40 feet per minute) and 50% indoor relative humidity, the operative temperatures recommended by ASHRAE range from 68.5oF to 75oF in the winter, and from 75oF to 80.5oF in the summer. The difference in temperature ranges between the seasons is largely due to clothing selection. ASHRAE also recommends that indoor relative humidity be maintained at or below 65% [ANSI/ASHRAE 2013] with no prescribed lower humidity limit.

Occupied and Non-Occupied Settings

Buildings with simple HVAC systems often operate the ventilation system during occupied hours and then turn them off completely at night or other periods when the building is unoccupied.  While turning the system off may save energy, depending on outdoor conditions, it often increases demand on the HVAC system when it is turned back on. Essentially, the equipment has to operate longer and harder to reach desired indoor temperature and humidity set-points.  It can also create issues with condensation if humidity levels indoors become elevated during the “off” periods. This is particularly true in areas with warmer climates. More sophisticated HVAC systems with programmable thermostats or building automation systems allow for the ventilation equipment to be “set back” during unoccupied periods. This method still allows the indoor temperature and humidity to drift further from the occupied set-points, but eventually the HVAC system will come on to prevent fluctuations as extreme as they might otherwise be with the equipment powered off.  This “set back” method still provides significant energy savings, but it does not require the system to work as long or as hard to bring the indoor conditions back to set-points in preparation for building occupancy.  Whether the system runs continuously, is powered off during unoccupied periods, or is “set back” when spaces are unoccupied, the indoor temperature and humidity conditions should always meet recommendations found in ANSI/ASHRAE 55-2013 and ANSI/ASHRAE 62.1-2016 any time the building is occupied.