Issue link: https://hi.iaq.net/i/630729
HVAC Systems and Indoor Air Quality 123 supplementary system (e.g., perimeter hot water piping). Constant Volume Constant volume systems, as their name suggests, generally deliver a constant airflow to each space. Changes in space temperatures are made by heating or cooling the air or switching the air han- dling unit on and off, not by modulating the volume of air supplied. These systems often operate with a fixed minimum percentage of outdoor air or with an "air economizer" feature (described in the Outdoor Air Control discussion that follows). Variable Air Volume Variable air volume systems maintain thermal comfort by varying the amount of heated or cooled air delivered to each space, rather than by changing the air temperature. (However, many VAV systems also have provisions for resetting the temperature of the delivery air on a seasonal basis, depending on the severity of the weather). Overcooling or overheating can occur within a given zone if the system is not adjusted to respond to the load. Underventilation frequently occurs if the system is not arranged to introduce at least a minimum quantity (as opposed to percentage) of outdoor air as the VAV system throttles back from full airflow, or if the system supply air temperature is set too low for the loads present in the zone. BASIC COMPONENTS OF AN HVAC SYSTEM The basic components of an HVAC system that delivers conditioned air to maintain thermal comfort and indoor air quality are: ■ outdoor air intake ■ mixed-air plenum and outdoor air control ■ air filter ■ heating and cooling coils ■ humidification and/or de-humidification equipment TESTING AND BALANCING Modern HVAC systems typically use sophisticated, automatic controls to supply the proper amounts of air for heating, cooling, and ventilation in commercial buildings. Problems during installation, operation, mainte- nance, and servicing the HVAC system could prevent it from operating as designed. Each system should be tested to ensure its initial and contin- ued performance. In addition to providing acceptable thermal conditions and ventilation air, a properly adjusted and balanced system can also reduce operating costs and increase equipment life. Testing and balancing involves the testing, adjusting, and balancing of HVAC system components so that the entire system provides airflows that are in accordance with the design specifications. Typical components and system parameters tested include: ■ all supply, return, exhaust, and outdoor airflow rates ■ control settings and operation ■ air temperatures ■ fan speeds and power consumption ■ filter or collector resistance The typical test and balance agency or contractor coordinates with the control contractor to accomplish three goals: verify and ensure the most effective system operation within the design specifications, identify and correct any problems, and ensure the safety of the system. A test and balance report should provide a complete record of the design, preliminary measurements, and final test data. The report should include any discrepancies between the test data and the design specifica- tions, along with reasons for those discrepancies. To facilitate future performance checks and adjustments, appropriate records should be kept on all damper positions, equipment capacities, control types and loca- tions, control settings and operating logic, airflow rates, static pressures, fan speeds, and horsepowers. Testing and balancing of existing building systems should be per- formed whenever there is reason to believe the system is not functioning as designed or when current records do not accurately reflect the actual operation of the system. The Associated Air Balance Council recom- mends the following guidelines in determining whether testing and balancing is required: ■ When space has been renovated or changed to provide for new occu- pancy. ■ When HVAC equipment has been replaced or modified. ■ When control settings have been readjusted by maintenance or other personnel. ■ After the air conveyance system has been cleaned. ■ When accurate records are required to conduct an IAQ investigation. ■ When the building owner is unable to obtain design documents or appropriate air exchange rates for compliance with IAQ standards or guidelines. Because of the diversity of system types and the interrelationship of system components, effective balancing requires a skilled technician with the proper experience and instruments. Due to the nature of the work, which involves the detection and remediation of problems, it is recom- mended that an independent test and balance contractor be used and that this contractor report directly to the building owner, facility manager, or IAQ manager.