Issue link: https://hi.iaq.net/i/630729
122 Appendix B Some buildings use only natural ventilation or exhaust fans to remove odors and contaminants. In these buildings, thermal discomfort and unacceptable indoor air quality are particularly likely when occupants keep the windows closed because of extreme hot or cold tempera- tures. Problems related to underventilation are also likely when infiltration forces are weakest (i.e., during the "swing seasons" and summer months). Modern public and commercial build- ings generally use mechanical ventilation systems to introduce outdoor air during the occupied mode. Thermal comfort is commonly maintained by mechanically distributing conditioned (heated or cooled) air throughout the building. In some designs, air systems are supplemented by piping systems that carry steam or water to the building perimeter zones. As this document is concerned with HVAC systems in relation to indoor air quality, the remainder of this discussion will focus on systems that distribute conditioned air to maintain occupant comfort. Roles of the HVAC System Operator and Facility Manager The system operator(s) and facility manager(s) (or IAQ manager) are among the most significant factors in determining whether IAQ problems will occur in a properly designed, constructed, and commissioned HVAC system. HVAC systems require preventive maintenance and prompt repairs if they are to operate correctly and provide comfortable condi- tions. The operator(s) must have an adequate understanding of the overall system design and its limitations. The HVAC system capacity and distribution characteristics should be evaluated before renovations to the building, changes in its occupancy, or changes in the use of an area. System operators must be able to respond appropriately to occupant com- plaints. For example, if an occupant complains that it is too cold or too hot and the observed (measured) conditions are outside of the ASHRAE comfort zone, then the HVAC system needs to be evaluated. Sometimes the problem can be relieved by fine tuning or repairing the HVAC system, but in some cases the system cannot perform as expected, and a long-term solution must be investigated. TYPES OF HVAC SYSTEMS Single Zone A single air handling unit can only serve more than one building area if the areas served have similar heating, cooling, and ventilation requirements, or if the control system compensates for differences in heating, cooling, and ventilation needs among the spaces served. Areas regulated by a common control (e.g., a single thermostat) are referred to as zones. Thermal comfort problems can result if the design does not adequately account for differences in heating and cooling loads between rooms that are in the same zone. This can easily occur if: ■ The cooling load in some area(s) with- in a zone changes due to an increased occupant population, increased lighting, or the introduction of new heat-produc- ing equipment (e.g., computers, copiers). ■ Areas within a zone have different solar exposures. This can produce radiant heat gains and losses that, in turn, create unevenly distributed heating or cooling needs (e.g., as the sun angle changes daily and seasonally). Multiple Zone Multiple zone systems can provide each zone with air at a different temperature by heating or cooling the airstream in each zone. Alternative design strategies involve delivering air at a constant temperature while varying the volume of airflow, or modulating room temperature with a