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Understanding Single-Zone VAV Systems providing insights for today’s hvac system designer Engineers Newsletter volume 42 –2 Understanding Single-Zone VAV Systems Traditionally, single-zone VAV has been packaged rooftop units or direct expansion Single-zone variable-air-volume (VAV) used for larger, densely occupied zones is not a new concept, but due to new (DX) split systems, fan-coils, classroom unit energy code requirements and greater that have variable cooling loads. Common ventilators, and water-source heat pumps. attention to reducing energy use, it is examples include gymnasiums, cafeterias, being applied more frequently. This EN lecture halls, auditoriums, large meeting will review these new requirements, rooms, churches, and arenas. Therefore, it discuss the benefits and challenges of Requirements of ASHRAE single-zone VAV systems, and identify has been available primarily in larger air- common applications for this system. handling units and large, packaged rooftop Standard 90.1 equipment. This type of equipment has long been available with variable-speed Requirements for VAV control in single-zone A conventional, single-zone constant- fans and cooling/heating that can be systems were added to the 2010 edition of volume system uses a temperature staged or modulated to control discharge- ANSI/ASHRAE/IESNA Standard 90.1, sensor in the zone to vary cooling or air temperature. Energy Standard for Buildings Except Low- heating capacity, while the supply fan Rise Residential Buildings.1 This is delivers a constant quantity of air However, due to increased focus on especially significant because the U.S. whenever the system is operating. reducing energy use, single-zone VAV is Department of Energy has mandated that starting to be used more often in K-12 each state must update its commercial In a single-zone VAV system, however, classrooms, retail stores, dormitories, and building code to meet or exceed Standard the temperature sensor in the zone is even offices. As its popularity has 90.1-2010 by Oct. 18, 2013. used to vary the cooling or heating increased for these smaller zones, this capacity and the airflow delivered by functionality is beginning to be offered in These requirements are included in the the supply fan to maintain supply-air smaller equipment, such as small mandatory provisions of Section 6.4. This temperature at a desired setpoint means that projects must comply with the (Figure 1). requirements for single-zone VAV control, regardless of whether the Simplified Figure 1. Single-zone VAV system Approach (by reference from Section 6.3), prescriptive path (Section 6.5), or Energy Cost Budget (Section 11) is used for compliance. Section 6.4.3.10 (see inset, p.2) includes T zone two parts: the first addresses systems that use chilled water for cooling, while the variable-speed second part addresses systems that use supply fan direct expansion. T controller © 2013 Trane, a business of Ingersoll Rand. All rights reserved. 1 Section 6.4.3.10 (“Single Zone Figure 2. Example SZVAV control with variable-speed fan Variable-Air-Volume Controls”) of design ASHRAE Standard 90.1-2010. maximum SAT for heating airflow HVAC systems shall have variable airflow supply airflow SAT setpoint controls as follows: supply fan airflow (a) Air-handling and fan-coil units with supply airflow chilled-water cooling coils and supply fans with motors greater than or equal to 5 hp shall have their supply fans minimum controlled by two-speed motors or SAT setpoint airflow limit variable-speed drives. At cooling demands less than or equal to 50%, the supply fan controls shall be able to supply-air temperature setpoint reduce the airflow to no greater than the larger of the following: design SAT for cooling • One-half of the full fan speed, or design zone design zone • The volume of outdoor air required to heating load zone sensible load cooling load meet the ventilation requirements of Standard 62.1. System Operation Depending on the outdoor conditions, as the SAT setpoint increases, eventually (b) Effective January 1, 2012, all air- Figure 2 depicts an example control conditioning equipment and air-handling no mechanical cooling is needed to units with direct expansion cooling and a sequence for a single-zone VAV system make this temperature, so the cooling capacity at AHRI conditions that uses a variable-speed fan. (Note compressors are turned off. And even greater than or equal to 110,000 Btu/h that control can vary by manufacturer, the airside economizer modulates back that serve single zones shall have their and there are nuances that depend on supply fans controlled by two-speed to bring in only the minimum outdoor motors or variable-speed drives. At whether chilled water or DX is used for airflow required for ventilation. When cooling demands less than or equal to cooling and whether the type of heater that happens, and the load continues to 50%, the supply fan controls shall be used can support variable airflow.) decrease, the zone temperature begins able to reduce the airflow to no greater to drop below the zone cooling setpoint, than the larger of the following: When the zone is at design sensible into the deadband between cooling and • Two-thirds of the full fan speed, or cooling load (right-hand side of the heating setpoints. The fan continues to chart), this system delivers maximum • The volume of outdoor air required to operate at minimum airflow, with no supply airflow at the design supply-air meet the ventilation requirements of compressors or heaters operating, and Standard 62.1. temperature (SAT) for cooling (e.g., the zone temperature is allowed to float 55°F). As the zone cooling load within this deadband. Note from the author: In part (a) above, decreases, supply airflow is reduced as the phrase, “At cooling demands less than or equal to 50%…” may be needed to maintain the desired Now consider what happens when the confusing, so the Standard 90.1 User’s temperature in the zone. This is zone eventually requires heating—that 2 Manual clarifies by stating the following: accomplished by varying the speed of is, when the zone temperature drops to the fan motor. Cooling capacity is then “The term ‘cooling demand’ refers the heating setpoint. The fan continues to the zone sensible cooling load. staged or modulated to maintain SAT to operate at minimum airflow and the That is, when the zone sensible at the same design setpoint. If the SAT setpoint is reset even further cooling load decreases to 50% of system has an airside economizer, the upward. Heating capacity is staged or the design sensible cooling load for economizer may provide all or part of modulated to maintain this supply-air the zone, the supply fan controls the cooling needed to achieve the SAT shall have reduced airflow to the temperature. threshold described.” setpoint. Eventually, the zone heating load may In addition, the User’s Manual clarifies Eventually, the zone sensible cooling increase to the point where the SAT that the supply fan can be controlled by load decreases to the point where reaches a pre-set maximum limit (also either a two-speed motor, an supply airflow reaches a minimum limit electronically commutated motor (ECM), discussed on p.3). As the zone heating or a variable-frequency drive (VFD). (discussed on p.3). As the cooling load load continues to increase, supply continues to decrease, the fan remains airflow is again increased, while heating at minimum airflow, but now the SAT capacity is staged or modulated to setpoint is gradually reset upward to maintain the SAT at this maximum limit. avoid overcooling the zone. 2 Trane Engineers Newsletter volume 42-2 providing insights for today’s HVAC system designer Minimum airflow. The minimum airflow ventilation performance, requiring more Figure 4 depicts an example control limit (Figure 2) might be determined by how outdoor air to be brought in through the sequence when using a two-speed fan. far the variable-frequency drive (VFD) or system intake. When the thermostat calls for both stages electronically commutated motor (ECM) of cooling, the supply fan operates at high can be turned down. This will be equipment One way to limit or avoid this effect is to speed with both compressors on. When specific, and might be to ensure motor deliver the air at a “not-so-hot” only one stage of cooling is needed, the reliability, or in the case of DX equipment, temperature during heating. In this case, fan turns down to operate at low speed to ensure reliability of the refrigeration the maximum supply temperature for with only one compressor on. Then when system. Alternatively, for some applications heating might be set to 15°F above the the thermostat calls for no cooling, the fan this limit might need to be high enough to zone setpoint. continues to operate at low speed with meet some air distribution requirement. both compressors shut off. Constant airflow when heating. For Standard 90.1 (see inset, p.2) requires this some equipment, the type of heater Finally, if the thermostat calls for heat, the minimum airflow to be no higher than one- available may not be able to fan operates at high speed and the heater half of design airflow for chilled-water accommodate variable airflow. In that turns on. When the zone warms up, the equipment, or two-thirds of design airflow case, the control sequence will vary thermostat shuts off the heater and the for DX. But if the outdoor airflow required to airflow when cooling, but then increase to fan returns to operate at low speed. meet Standard 62.1 is higher than this some constant airflow whenever the minimum threshold, the higher ventilation- heater is activated (Figure 3).
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