Understanding Single-Zone VAV Systems

providing insights for today’s hvac system designer

Engineers Newsletter volume 42 –2

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.

controller

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). Like with variable-speed fan control, the related airflow can be used to avoid minimum airflow at low fan speed might underventilating the zone. When the zone temperature drops to be determined by equipment design, for heating setpoint, the fan ramps up to full safety or reliability reasons. But Standard Maximum supply temperature when speed, and the heater is cycled on or 90.1 requires this minimum airflow to be heating. The maximum supply-air staged to warm up the zone. Once the no higher than two-thirds of design airflow temperature (Figure 2) might be zone temperature rises back up to the for DX equipment or one-half of design determined by the equipment deadband between heating and cooling airflow for chilled water. manufacturer, for safety or reliability setpoints, the heater turns off and the fan reasons. Alternatively, for some again backs down to minimum speed. applications this temperature might be Ventilation Control limited to minimize stratification when Two-speed fan control. Standard 90.1 supplying hot air from overhead diffusers. states that the supply fan can use either a Another important aspect of system variable-speed or two-speed motor. The control involves ensuring proper zone When both supply-air diffusers and return most common implementation of a two- ventilation at all operating conditions. grilles are mounted in the ceiling, and the speed motor is probably a DX unit that With a constant-speed fan, proper supply air is hot, the buoyancy of this hot air has either two compressors or one two- ventilation is achieved by setting the causes some of it to stay up near the stage compressor. This allows the position of the outdoor-air damper during ceiling, and bypass from the supply equipment to be used with a conventional startup and balancing. But with a variable- diffusers to the return grilles. This bypass thermostat (with COOL1/COOL2 control). can cause comfort problems, and impacts speed fan, it is no longer that simple.

Figure 3. Example SZVAV control with variable-speed fan and constant airflow Figure 4. Example SZVAV control with two-speed fan when heating

design design supply airflow airflow high fan speed high fan speed airflow both compressors ON heating capacity heater ON

supply fan airflow

supply airflow low fan speed minimum airflow

minimum one compressor ON SAT setpoint airflow limit

supply-air temperature setpoint

stages of cooling or heating capacity both compressors design SAT for cooling OFF

design zone design zone heating load zone sensible load cooling load

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To demonstrate, Figure 5 depicts the the desired quantity of outdoor air enters necessary with a traditional, constant- static pressure as air moves through through the outdoor-air damper. volume system. the various components of simple, single-zone VAV system. In a single-zone VAV system, supply To address this issue and ensure airflow is reduced during part-load proper ventilation in a single-zone VAV The supply fan must create a high operation. Pushing less air through the system, consider one of the following enough pressure at the fan discharge supply ductwork and diffusers results in approaches. (A) to overcome the pressure losses less pressure loss, so the fan does not associated with pushing the air through have to create as high a pressure at the Two-position OA damper control. the supply ductwork and diffusers, and fan discharge (D). For a system with two-speed fan into the zone. control (Figure 4), two position In the same manner, moving less air setpoints can be used to control the In addition, the fan must create a low through the return-air path also results in outdoor-air damper. enough pressure at its inlet (B) to less pressure loss. This causes the static overcome pressure losses associated pressure inside the mixing box (E) to When the fan is operated at high with drawing the return air out of the increase. In other words, it is not as speed, the OA damper position is set zone and through the return-air path negative as at design airflow. to bring in the quantity of outdoor air (which might include a return grille, required by code. Then, when the The static pressure inside the mixing box ceiling plenum, and some ductwork), controller switches the fan to low (E) has a direct effect on how much and then to draw the air through the speed, the OA damper is opened outdoor air enters the system. If the return-air damper, filter, and coils inside more, in order to bring in the same outdoor-air damper remains set at a fixed the air-handling or rooftop unit. quantity (cfm) of outdoor air. position, the quantity of outdoor air For this example system, the mixing entering through this damper will Note that these are minimum position box is where outdoor air mixes with the decrease because the pressure inside the setpoints. If the system includes an recirculated air. Due to the pressure mixing box is not as negative. The result is airside economizer, the OA damper drop through the return-air path and that outdoor airflow will decrease as may be opened farther when damper, the static pressure inside this supply airflow is reduced. conditions are suitable for mixing box (C) is negative (lower than economizing. the pressure outside the building). This To ensure that the same quantity (cfm) of causes outdoor air to be drawn into this outdoor air enters the system, the Proportional control of OA damper. mixing box. During startup and outdoor-air damper needs to be opened For a system with variable-speed fan balancing, the return damper is farther when supply airflow is reduced. control, one solution could be to adjusted so that the pressure inside the This requires designing and programming modulate the position of the outdoor- mixing box is low enough to ensure that a control sequence that was not air damper in proportion to the changing supply fan speed (Figure 6). This is a relatively inexpensive solution, Figure 5. Need for ventilation control in SZVAV system so it is likely to be used in many of the smaller, single-zone VAV applications. OA mixing With the system operating at design box SA supply airflow (maximum fan speed), RA the OA damper position is set to bring in the quantity of outdoor air required return-air grille, filter, supply ductwork, by code. Then with the system ceiling plenum, coil diffusers operating at minimum airflow return ductwork (minimum fan speed), the OA damper is opened more, in order to bring in the A d same quantity of outdoor air. zone esig zone n s upp ly a D irflo + return-air supply w During system operation, the controller damper fan re duced suppl modulates the position of the OA y airfl E ow damper in proportion to the change in supply airflow, which is determined – C

static pressure (relative to outside) static (relative pressure from the signal sent to the variable- B speed drive on the fan. Again, this is the Static pressure inside the mixing box (E) increases (less negative) as supply minimum damper position, so it may be airflow is reduced, so the OA damper must open farther as supply airflow opened farther when conditions are is reduced. suitable for economizing.

4 Trane Engineers Newsletter volume 42-2 providing insights for today’s HVAC system designer This method is not perfectly accurate However, outdoor airflow over the entire range of airflows, since measurement does increase the cost Fixed-position OA damper and damper performance is nonlinear. And of the system, so it is more likely to be SZVAV it does not account for outside used in larger systems. ASHRAE Standard 62.1-2010, Section influences like wind or stack effect. 5.3, states: 3 Therefore, it does result in some over- Dedicated OA delivered directly to “The system shall be designed to ventilation in the middle of the fan each zone. The three previous maintain no less than the speed range, but much less over- approaches are suitable for a piece of minimum outdoor airflow as ventilation than if a fixed-position equipment that has a mixing box and required by Section 6 under any damper were used (see inset). load condition. Note: Variable Air outdoor-air damper—such as a Volume (VAV) systems with fixed packaged rooftop unit, air-handling unit, outdoor air damper positions must One way to minimize this inaccuracy is or classroom unit ventilator. comply with this requirement at to add a third damper position setpoint minimum system primary airflow.” for some intermediate fan speed But most models of fan-coils or water- This means that the OA damper position (Figure 6). This ensures proper source heat pumps just have a single must be set to bring in the minimum ventilation at this intermediate speed, inlet opening—no mixing box or required outdoor airflow when the fan is and minimizes any over-ventilation by outdoor-air damper. Ventilation is operating at minimum speed, which shrinking the ranges between results in significant over-ventilation and typically provided by a dedicated increased energy use when the fan setpoints. outdoor-air system. operates at higher speeds (Figure 6).

Flow-measuring OA damper. In some applications, the dedicated OA is reduced. Therefore, outdoor airflow Another method for controlling system delivers the conditioned will decrease as supply airflow is ventilation is to measure the outdoor outdoor air to the inlet side of each reduced. airflow and control it directly. This is local heat pump or fan-coil (Figure 8). typically accomplished by using a flow- For a horizontal-style unit, the air might An added challenge is that the outdoor measuring device in the outdoor air air is delivered by a separate fan, and stream, such as an airflow be ducted to discharge into the open ceiling plenum, near the intake of the the pressure inside the ventilation measurement station or a flow- ductwork is influenced by the changing measuring damper (Figure 7). unit. For a vertical-style unit, it might be ducted to a closet where the unit is operation of all the local fans served by the ventilation system. This method is more accurate over the installed. In either case, the outdoor air mixes with recirculated air in the range of airflow, and can respond to With this configuration, to ensure that plenum or closet before being drawn in pressure fluctuations caused by wind the required quantity of outdoor air is through the intake of the heat pump or or stack effect. This approach has the delivered, a pressure-independent VAV fan-coil. added benefit of providing a means to terminal should be installed in the document the outdoor airflow brought ventilation duct for each zone (Figure 8). into the system over time. As described earlier (Figure 5), if the local unit has a two-speed or variable- As the local fan speed changes, this speed fan, the static pressure at its damper modulates to maintain the

inlet is not as negative when fan speed same outdoor airflow, regardless of fan speed.

Figure 6. Proportional control of OA damper Figure 7. Flow-measuring OA damper

OA damper position at minimum fan speed fixed-position OA damper

OA damper position

OA damper position at intermediate fan speed intermediate speed OA damper position min speed max speed at maximum fan speed supply fan speed

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Alternatively, consider delivering the lower than the building component of Figure 8. Conditioned OA delivered to inlet of each local unit conditioned outdoor air directly to each the ventilation rate (Ra × Az) for that zone, rather than to the inlet of the fan- zone. OA dedicated OA coils or heat pumps (Figure 9). In this unit case, the local units condition only Section 8.3 clarifies that this applies whenever that zone is “expected” to CA recirculated air. CA be occupied. That is, the standard is RA Because the outdoor air is not not requiring this building component CA SA distributed through the local fan, that of the ventilation rate to be delivered to SA fan can operate with a two-speed or the zone 24 hours a day, 7 days a variable-speed motor, without week. Rather, it only requires this Pressure-independent VAV terminals impacting the quantity of outdoor air during the normally scheduled maintain constant outdoor airflow, regardless of local fan speed. delivered to the zone. occupied period. RA

This “direct-to-the-zone” approach also Because the mixing box pressure WSHP or affords the opportunity to deliver the varies as supply airflow changes fan-coil outdoor air at a cold temperature, (Figure 5), DCV must be combined rather than reheated to neutral. This with either proportional damper control Figure 9. Conditioned OA delivered to directly to each zone has several installed cost and energy- or a flow-measuring damper in order to 4,5 saving benefits. prevent outdoor airflow from dropping OA dedicated OA below this minimum threshold. unit CO -based DCV. Single-zone VAV CA 2 A previous EN and the Standard 62.1 systems are often used for densely User’s Manual both describe a CA CA occupied zones with varying RA proportional control sequence that can SA population. This typically makes them be used to implement DCV in a single- SA good candidates for using CO -based 2 zone system.6,7 demand-controlled ventilation (DCV). Local fans can operate with two-speed or variable-speed control, With this approach, a sensor measures without impacting outdoor airflow. RA the concentration of CO2 in the zone, Potential Benefits of and this concentration is then used by WSHP or the controller to reset outdoor airflow SZVAV fan-coil as population changes. This saves Compared to a constant-volume energy by not over-ventilating the zone system, typical benefits of single-zone For this example, two-speed fan during periods of partial occupancy. VAV include lower energy use, better control (constant fan speed when dehumidification at part load, and less heating) reduced HVAC energy use by While Standard 62.1 allows the use of fan-generated noise at reduced about 25 percent, while variable-speed DCV, it also includes a few speeds. requirements to make it work:3 fan control (capable of variable fan speed when heating) reduced HVAC “The breathing zone outdoor airflow Lower energy use. Compared to a energy use by about 30 percent. shall be reset in response to current constant-volume system, a single-zone occupancy and shall be no less than the VAV system can result in significant fan Of course, the impact of two-speed or building component (Ra × Az) of the energy savings at part-load conditions. variable-speed fan control on the zone.” (Section 6.2.7.1.2) energy use of a specific building Figure 10 shows the potential energy depends on climate, building layout, “Systems shall be operated such that savings of using two-speed or variable- and operating hours. For this example, spaces are ventilated in accordance speed fan control in an example single- high internal loads in the classrooms with Section 6 when they are expected story K-12 school. TRACE™ 700 was resulted in systems operating in to be occupied.” (Section 8.3) used to model the building with a cooling mode for most occupied hours packaged rooftop unit serving each (even in Minneapolis). Operation of the While the people component of the zone.8 The baseline building uses heater was mostly limited to morning ventilation rate (Rp × Pz) can be varied, conventional, constant-volume warm-up mode. Standard 62.1 states that the controller rooftops. cannot reduce outdoor airflow any

6 Trane Engineers Newsletter volume 42-2 providing insights for today’s HVAC system designer For applications with more occupied reducing airflow delivered to the zone, Another method for improving part- hours of operation in the heating while maintaining a constant supply-air load dehumidification could be to add mode, the difference in energy use temperature. At this same example hot gas reheat (HGRH). In order to limit between two-speed and variable- part-load condition, supply airflow is space relative humidity to 60 percent, a speed fan control will likely be larger, reduced from 1500 cfm to 900 cfm, constant-volume system will over-cool since the two-speed fan operates at while the system still delivers the air at the air to further dehumidify, and then high speed whenever the heater is on. 55°F. reheat it to prevent overcooling the zone. This requires more compressor Reducing the airflow allows the system Better dehumidification at part energy, along with more fan energy to continue supplying the air at a cool load. Compared to a constant-volume (Table 1). temperature, so the coil removes more system, a single-zone VAV system moisture. At this part-load condition, For many applications, the variable- improves dehumidification because the supply air is still cool and speed fan may do an adequate job of performance because it continues to dry, the relative humidity in the limiting indoor humidity levels, so that deliver cool, dry air at part-load classroom rises to only 57 percent with the system may not require any other conditions. the variable-speed fan, compared to 67 dehumidification enhancements—such To demonstrate this benefit, Figure 11 percent with the constant-speed fan as hot gas reheat. compares the performance of two (Figure 11). However, if a project requires lower systems serving an example In this comparison, for the variable- humidity levels than a single-zone VAV classroom in Jacksonville, Florida.9 At speed fan system, the load on the system can achieve, some types of cooling design conditions, this system cooling coil is 4 tons and the fan moves equipment can be equipped with hot mixes 450 cfm of outdoor air with 1050 900 cfm. So achieving this lower gas reheat also. In that case, the cfm of recirculated air, then delivers humidity level requires a bit more variable-speed fan may limit humidity the resulting 1500 cfm of supply air to compressor energy, while benefitting most of the time, but the reheat can be the classroom. from much lower fan energy (Table 1). used if needed. At the part-load, peak dew point As another point of comparison, on a In the case of fan-coils or water-source condition, it is not as hot outside, but mild rainy day, the variable-speed fan heat pumps, a more efficient approach more humid. The system with a results in much better dehumidification would typically be to use the dedicated constant-speed fan (CV) continues to in this same example classroom—60 outdoor-air system to dehumidify the supply a constant volume of air to the percent RH, compared to 73 percent outdoor air centrally, rather than equip zone (1500 cfm). Meanwhile, as the RH with a constant-speed fan (Table 1). each local unit with reheat.4,5 sensible cooling load in the zone decreases, the compressor cycles on and off, resulting in warmer air delivered to the zone—63°F for this example load condition. Figure 10. Example energy savings from two-speed or variable-speed fan control

Although delivering this warmer air K-12 school 100 prevents over-cooling the zone, the constant-speed fan cycling compressor results in a warmer average coil temperature and less 80 two-speed fan moisture removed. This 63°F air has not been dehumidified as much as if it variable-speed fan was cooled down to 55°F. This warmer, 60 wetter supply air causes zone relative humidity to rise to 67 percent at this 40 part-load condition.

In contrast, at part-load conditions, as 20 the sensible cooling load in the zone decreases, the system with a variable- % of base energy consumption, HVAC speed fan (SZVAV) responds by first 0 Atlanta Minneapolis St. Louis

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Less fan-generated noise at reduced Figure 11. Improved dehumidification at part load for an example classroom in fan speeds. Air distribution Jacksonville, FL equipment, including fans, ductwork, 180 CV SZVAV and diffusers, can have a significant OA 84°F DB, 84°F DB, 160 impact on background sound levels in 76°F DP 76°F DP 80 occupied spaces. (450 cfm) (450 cfm) peak DP 140 RA 74°F DB, 74°F DB, Because the fan operates at reduced 67% RH 57% RH 120 speed, a single-zone VAV system 70 MA 77°F DB 79°F DB MA benefits from less fan-generated noise MA 100 at part-load conditions. The actual SA 63°F DB 55°F DB (1500 cfm) (900 cfm) 60 80 RA sound level depends on the type of (3.7 tons) (4.0 tons) SA 60 fan, operating conditions, and 50 RA installation. But, in general, as fan SA speed decreases, the sound level will 40 40 30 also decrease. 20

30 40 50 60 70 8090 100 110 Application Considerations In order to realize these benefits, the specific application must be well-suited Design air distribution system to Ensure proper ventilation as supply to using single-zone VAV. Following are accommodate variable airflow. Use airflow changes. For smaller some of the challenges or diffusers that will provide proper air systems, two-position or proportional considerations when applying this type distribution at reduced airflows. damper control is more likely to be of system. Diffusers that are appropriate for used, while larger systems may use an conventional VAV systems are probably airflow-measurement device, such as a Loads throughout a large zone must best suited. flow-measuring damper. be fairly uniform. This is a single-zone Keeping duct runs as short and When a dedicated outdoor-air system system, responding to a single symmetric as possible is good general is used, consider delivering the temperature sensor, so it does not guidance. Systems with longer or conditioned outdoor air directly to each have the capability to satisfy asymmetric duct runs do not zone, rather than to the inlet of the simultaneous heating and cooling necessarily prohibit the use of single- local equipment. requirements. zone VAV, but they are more If implementing demand-controlled If the loads throughout a large zone are susceptible to unequal airflows ventilation, be sure to provide some not fairly uniform, it may result in between diffusers as the supply fan method to vary relief (exhaust) airflow undesirable temperature variations in modulates. to prevent negative building pressure some areas of the zone that are farther when DCV reduces the intake airflow. away from the temperature sensor. Likewise, if a zone has some type of minimum exhaust requirement—such Table 1. Part-load dehumidification comparison: CV versus SZVAV as a restroom, kitchen, or locker constant-speed variable-speed constant-speed fan room—DCV should not be allowed to fan fan with HGRH reduce intake airflow any lower than peak DPT the required exhaust, unless the zone (84°F DBT, 76°F DPT) receives makeup air from some other zone humidity, %RH 67% 57% source. cooling load, tons 3.7 4.0 fan airflow, cfm 1500 900 mild, rainy (70°F DBT, 69°F WBT) zone humidity, %RH 73% 60% 60% cooling load, tons 1.6 1. 9 2. 4 fan airflow, cfm 1500 750 1500

8 Trane Engineers Newsletter volume 42-2 providing insights for today’s HVAC system designer Consider system operation when heating. ASHRAE 90.1 addresses References single-zone VAV during cooling [1] American Society of Heating, Refrigerating, Engineers operation. If variable airflow is also and Air-Conditioning Engineers (ASHRAE). desired during heating operation, the ANSI/ASHRAE/IESNA Standard 90.1-2010: Newsletter equipment will likely require some type Energy Standard for Buildings Except Low- Rise Residential Buildings. LIVE! of modulating heat, such as a For event details and registration modulating gas heater or a hot-water [2] American Society of Heating, Refrigerating contact your local Trane office. heating coil. and Air Conditioning Engineers, Inc. (ASHRAE). Standard 90.1-2010 User’s Manual. Upcoming 2013 Otherwise, if the equipment has on/off gas or electric heat, the manufacturer [3] American Society of Heating, Refrigerating, may require it to operate at full airflow and Air-Conditioning Engineers (ASHRAE). Single-Zone VAV ANSI/ASHRAE Standard 62.1-2010: Ventilation whenever the heater is activated for Acceptable Indoor Air Quality. Systems (Figure 3). Therefore, check with the manufacturer when specifying or [4] Murphy, J. and B. Bakkum. Water-Source and purchasing single-zone VAV equipment. Ground-Source Heat Pump Systems, All-Variable-Speed application manual SYS-APM010-EN, 2011. Plant Control Variable-speed fan control requires a [5] Murphy, J., P. Solberg, M. Schwedler, and J. zone temperature sensor. A Harshaw, “Energy-Saving Strategies for Water- Source and Ground-Source Heat Pump conventional thermostat (with COOL1/ Systems,” Engineers Newsletter Live program COOL2 control) can typically be used (2012). for two-speed fan control, but variable- speed control requires a zone [6] Murphy. J. and B. Bradley. “CO2-Based temperature sensor connected to the Demand-Controlled Ventilation with ASHRAE Standard 62.1.” Engineers Newsletter 34-5 unit controller. (2005).

This is important to communicate [7] American Society of Heating, Refrigerating when specifying or purchasing single- and Air Conditioning Engineers, Inc. zone VAV equipment. If replacing an (ASHRAE). Standard 62.1-2010 User’s Manual. older piece of equipment (controlled by [8] Trane Air-Conditioning and Economics a conventional thermostat) with a new, (TRACE™ 700). Available at www.trane.com/ variable-speed fan unit, this will likely TRACE require replacing the thermostat with a zone sensor. Wireless zone sensors [9] Murphy, J. and B. Bradley. Dehumidification in HVAC Systems, application manual SYS- make this much easier, especially for APM004-EN, 2002. those projects where pulling wires may be difficult.

By John Murphy, applications engineer, and Beth Bakkum, information designer, Trane. You can find this and previous issues of the Engineers Newsletter at www.trane.com/ engineersnewsletter. To comment, e-mail us at [email protected].

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Air conditioning clinics. A series of educational presentations that teach HVAC fundamentals, equipment, and systems. The series includes full- color student workbooks, which can be purchased individually. Approved by the American Institute of Architects for 1.5 (Health, Safety and Welfare) learning units. Contact your local Trane office to sign up for training in your area.

Engineers Newsletter Live. A series of 90-minute programs that provide technical and educational information on specific aspects of HVAC design and control. Topics range from water- and airside system strategies to ASHRAE standards and industry codes. Contact your local Trane office for a schedule or view past programs by visiting www.trane.com/ENL.

On-demand continuing education credit for LEED® and AIA . These 90- minute on-demand programs are available at no charge. The list of HVAC topics includes many LEED-specific courses. All courses available at www.trane.com/continuingeducation. Application manuals. Comprehensive reference guides that can Engineers Newsletters. These quarterly articles cover timely topics increase your working knowledge of commercial HVAC systems. Topics related to the design, application and/or operation of commercial, range from component combinations and innovative design concepts to applied HVAC systems. Subscribe at www.trane.com/EN system control strategies, industry issues, and fundamentals. The following are just a few examples. Please visit www.trane.com/ bookstore for a complete list of manuals available to order. Central Geothermal Systems discusses proper design and control of central geothermal bidirectional cascade systems that use borefields. This manual covers central geothermal system piping, system design considerations, and airside considerations. (SYS-APM009-EN, February 2011) Chilled-Water VAV Systems focuses on chilled-water, variable-air- volume (VAV) systems. To encourage proper design and application of a chilled-water VAV system, this manual discusses the advantages and drawbacks of the system, reviews the various components that make up the system, proposes solutions to common design challenges, explores several system variations, and discusses system-level control. (SYS-APM008-EN, updated May 2012) Water-Source and Ground-Source Heat Pump Systems examines chilled-water-system components, configurations, options, and control strategies. The goal is to provide system designers with options they can use to satisfy the building owners’ desires. (SYS-APM010-EN, November 2011)

Trane, Trane believes the facts and suggestions presented here to be accurate. However, final design and A business of Ingersoll Rand application decisions are your responsibility. Trane disclaims any responsibility for actions taken on the material presented. For more information, contact your local Trane office or e-mail us at [email protected]

10 Trane Engineers Newsletter volume 42-2 ADM-APN047-EN (April 2013)