Patent Publication Number: US-2020292197-A1

Title: Hvac terminal unit systems and methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/817,957, entitled “HVAC TERMINAL UNIT SYSTEMS AND METHODS,” filed Mar. 13, 2019, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     A heating, ventilation, and/or air conditioning (HVAC) system is often utilized to regulate environmental conditions, such as temperature and/or humidity, within a building or other conditioned space. For example, an HVAC system may include equipment, such as one or more heat exchangers deployed in an HVAC unit, which operates to produce temperature-controlled air. To facilitate supply of the temperature-controlled air to a conditioned space, the HVAC system may include ductwork and a terminal unit. Generally, the terminal unit includes a primary air inlet for receiving the temperature-controlled air, a plenum air inlet for receiving plenum air, such as air within a plenum or space above a ceiling in the building, and an air outlet for providing the temperature-controlled air and the plenum air to the conditioned space. However, in some scenarios, the air flows may generate sound within the terminal unit that radiates from the plenum air inlet and causes undesired noise. 
     SUMMARY 
     This section provides a brief summary of certain embodiments described in the present disclosure to facilitate a better understanding of the present disclosure. Accordingly, it should be understood that this section should be read in this light and not to limit the scope of the present disclosure. Indeed, the present disclosure may encompass a variety of aspects not summarized in this section. 
     The present disclosure relates to a terminal unit for a heating, ventilation, and/or air conditioning (HVAC) system may include a housing defining a mixing chamber. The mixing chamber may have a primary air inlet, a plenum air inlet, and a mixed air outlet, and mix a primary air flow from the primary air inlet with a plenum air flow from the plenum air inlet to form a mixed air flow that is output through the mixed air outlet to a conditioned space. The terminal unit may also include a panel adjustably coupled to the housing to adjustably occlude the plenum air inlet. The terminal unit may also include an actuator coupled to the housing and the panel to move the panel and adjust a size of an air flow path corresponding to the plenum air inlet. 
     The present disclosure also relates to a heating, ventilation, and/or air conditioning (HVAC) system that includes a terminal unit. The terminal unit may include a first air inlet, a second air inlet, an air outlet, and movable panel. The first air inlet may receive a plenum air flow from a plenum space within a building, and the movable panel may regulate a size of an air flow path through the first air inlet. The HVAC system may also include a controller to determine a desired size of the air flow path and to adjust a position of the movable panel based on the desired size of the air flow path. 
     The present disclosure also relates to a terminal unit having a primary air inlet to receive a primary air flow from a heating, ventilation, and/or air conditioning (HVAC) unit via ductwork. The terminal unit may also include a plenum air inlet to receive a plenum air flow from a plenum space of a building and an air outlet to discharge an output air flow from the terminal unit to a conditioned space of the building. The terminal unit may also include a plenum door panel to adjust a size of an air flow path through the plenum air inlet to reduce a noise output of the terminal unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of the present disclosure may be better understood upon reading the detailed description and upon reference to the drawings, in which: 
         FIG. 1  is a partial cross-sectional view of an embodiment of a building that includes a heating, ventilating, and air conditioning (HVAC) system, in accordance with aspects of the present disclosure; 
         FIG. 2  is a schematic of a an embodiment of a portion of the building of  FIG. 1 , illustrating air flow to a conditioned space via a terminal unit, in accordance with aspects of the present disclosure; 
         FIG. 3  is a top cross-sectional view of an embodiment of the terminal unit with a sound attenuator and a controller for use with the terminal unit, in accordance with aspects of the present disclosure; 
         FIG. 4  is a flowchart of an embodiment of a process for operating the terminal unit, in accordance with aspects of the present disclosure; 
         FIG. 5  is a perspective view of an embodiment of the terminal unit with a plenum door panel open, in accordance with aspects of the present disclosure; 
         FIG. 6  is a perspective view of an embodiment of the terminal unit with the plenum door panel partially closed, in accordance with aspects of the present disclosure; and 
         FIG. 7  is a perspective view of an embodiment of the terminal unit with the plenum door panel closed, in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     As will be discussed in further detail below, a heating, ventilation, and/or air conditioning (HVAC) system, such as one including an air conditioner and/or heat pump, may include a terminal unit for delivering air to a conditioned space of a structure. In general, the terminal unit may be located near or within the conditioned space, and air flows may be facilitated to and/or from the terminal unit via ductwork. For example, the terminal unit may receive a primary air flow, via a primary air inlet, and a plenum air flow, via a plenum air inlet. The primary air flow and the plenum air flow may mix, for example, if both air flows are received simultaneously within the terminal unit, and the mixed air flow may be output to the conditioned space. 
     In some embodiments, the terminal unit may use the plenum air flow to supplement the primary air flow, for example, to increase the volume of air flow supplied to the conditioned space and/or to utilize heated or cooled air within the plenum space. In a further example, the air within the plenum space may be heated or cooled from various surrounding elements, such as walls, people in the conditioned space, equipment within the structure, and/or from an air flow, such as a previously-supplied primary air flow, that has traveled into the plenum space. Thus, the air within the plenum space may provide auxiliary heating or cooling to the conditioned space along with or instead of primary air flow. 
     The plenum air flow may be received through a plenum air inlet of the terminal unit. However, sounds generated from the primary air flow passing through the ductwork, sounds generated from the plenum and/or primary air flow passing into, out of, or through the terminal unit, and/or sounds corresponding to other equipment, such as fans, diffusers, valves, or auxiliary heat exchangers, may radiate out of the terminal unit via the plenum air inlet, thereby causing undesired noise. As such, in some embodiments, a plenum door panel may be adjusted to regulate the size of the plenum air inlet, for example, to a reduced size in order to cause a reduction in the radiated noise. For example, the size of the plenum air inlet may be reduced or closed entirely by the plenum door panel based on a desired amount of plenum air to be output to the conditioned space via the terminal unit. 
     Turning now to the drawings,  FIG. 1  illustrates an embodiment of a heating, ventilation, and/or air conditioning (HVAC) system for environmental management that may employ one or more HVAC units. As used herein, an HVAC system includes any number of components configured to enable regulation of parameters related to climate characteristics, such as temperature, humidity, air flow, pressure, air quality, and so forth. For example, an “HVAC system” as used herein is defined as conventionally understood and as further described herein. Components or parts of an “HVAC system” may include, but are not limited to, all, some of, or individual parts such as a heat exchanger, a heater, an air flow control device, such as a fan, a sensor configured to detect a climate characteristic or operating parameter, a filter, a control device configured to regulate operation of an HVAC system component, a component configured to enable regulation of climate characteristics, or a combination thereof. An “HVAC system” is a system configured to provide such functions as heating, cooling, ventilation, dehumidification, pressurization, refrigeration, filtration, or any combination thereof. The embodiments described herein may be utilized in a variety of applications to control climate characteristics, such as residential, commercial, industrial, transportation, or other applications where climate control is desired. 
     In the illustrated embodiment, a building  10  is air conditioned by a system that includes an HVAC unit  12 . The building  10  may be a commercial structure or a residential structure. As shown, the HVAC unit  12  is disposed on the roof of the building  10 . However, the HVAC unit  12  may be located in other equipment rooms or areas adjacent the building  10 . The HVAC unit  12  may be a single packaged unit containing other equipment, such as a blower, heat exchangers, integrated air handler, and/or auxiliary heating unit. In other embodiments, the HVAC unit  12  may be part of a split HVAC system, which includes an outdoor HVAC portion and an indoor HVAC portion. 
     The HVAC unit  12  is an air cooled device that implements a refrigeration cycle to provide conditioned air to the building  10 . Specifically, the HVAC unit  12  may include one or more heat exchangers across which an air flow is passed to condition the air flow before the primary air flow is supplied to the building. In the illustrated embodiment, the HVAC unit  12  is a rooftop unit (RTU) that conditions a supply air stream, such as environmental air and/or a return air flow drawn from the building  10 . After the HVAC unit  12  conditions the air flow, the air flow, also referred to herein as a primary air flow, is supplied to the building  10  via ductwork  14  extending throughout the building  10  from the HVAC unit  12 . For example, the ductwork  14  may extend to various individual floors or other sections of the building  10 . In certain embodiments, the HVAC unit  12  may be a heat pump that provides both heating and cooling to the building with one refrigeration circuit configured to operate in different modes. In other embodiments, the HVAC unit  12  may include one or more refrigeration circuits for cooling an air flow and a furnace for heating the air flow. The primary air flow supplied to the building  10  by the HVAC unit  12  may include environmental air, such as air from outside the building  10 , and/or recirculated air from within the building  10 , which may or may not be actively and/or passively heated or cooled by the HVAC unit  12 . For example, the HVAC unit  12  may operate in a recirculating or economizer mode, such that the supply air flow, and thus the primary air flow, is not actively heated or cooled. 
     A control device  16 , one type of which may be a thermostat, may be used to designate a desired temperature of a conditioned space  18  within the building  10 . The control device  16  also may be used to control the flow of air, such as volume, through the ductwork  14  to different areas within the conditioned space  18 . For example, the control device  16  may be used to regulate operation of one or more components of the HVAC unit  12  or other components, such as dampers, fans, and/or terminal units  20  within the building  10  that may control the flow of air through and/or from the ductwork  14 . In some embodiments, other devices may be included in the system, such as pressure and/or temperature transducers or switches that sense the temperatures and pressures of the conditioned air, return air, and so forth. Moreover, the control device  16  may include computer systems that are integrated with or separate from other building control or monitoring systems, including systems that are remote from the building  10 . 
     As discussed above, a plenum air flow may be drawn from a plenum space  22 . In some embodiments, the plenum space  22  may facilitate transfer of return air back to the HVAC unit  12 . For example, the plenum space  22  may be above a drop ceiling  24  that separates the plenum space  22  from the conditioned space  18 . Moreover, in some embodiments, the terminal unit  20  may be implemented in the building  10  without the drop ceiling  24 , and the plenum air flow may be drawn from a portion of air near the ceiling of the conditioned space  18 . 
       FIG. 2  is a schematic diagram of an embodiment of a portion  26  of the building  10 , illustrating an embodiment of the terminal unit  20  implemented within the building  10 . As discussed above, the terminal unit  20  may receive a primary air flow  28 , via ductwork  14 , and a plenum air flow  30  from the plenum space  22 . In the illustrated embodiment, the terminal unit  20  includes a primary air inlet  38  configured to receive the primary air flow  28  and a plenum air inlet  40  configured to receive the plenum air flow  30 . The terminal unit  20  is configured to mix the primary air flow  28  and the plenum air flow  30  to generate a mixed air flow  32  that is then supplied to the conditioned space  18  via an air outlet  33 . In some implementations, the plenum space  22  may receive a return air flow  34 , for example, drawn into the plenum space  22  via a vent  36 . The return air flow  34  may then be subsequently returned to the HVAC unit  12  for conditioning or may be drawn into the terminal unit  20  as the plenum air flow  30  via the plenum air inlet  40 . As discussed herein, the mixed air flow  32  may include the primary air flow  28  and/or the plenum air flow  30 . For example, if the primary air inlet  38  of the terminal unit  20  or the plenum air inlet  40  of the terminal unit  20  is closed, the mixed air flow  32  may include air from a single source. In other words, if the primary air inlet  38  is closed, the terminal unit  20  may receive and supply the plenum air flow  30  alone to the conditioned space  18 , and if the plenum air inlet  40  is closed, the terminal unit  20  may receive and supply the primary air flow  28  alone to the conditioned space  18 . 
     The size of the air flow path through the plenum air inlet  40  may be regulated by a plenum door panel  42  adjustably affixed to the terminal unit  20 . For example, the plenum door panel  42  may be motorized to enable actuation between a fully open state, a fully closed state, and a plurality of partially closed states therebetween. Depending on operating conditions, the plenum door panel  42  may be closed or partially closed to reduce noise radiated from the terminal unit  20 . 
     To help further illustrate, an embodiment of the terminal unit  20  having the adjustable plenum door panel  42  is shown in  FIG. 3 . As discussed further below, the plenum door panel  42  is configured to be actuated to adjust a size of an air flow path  41  through the plenum air inlet  40 . In general, the plenum air inlet  40  may be sized to intake up to 100 percent of the desired output flow of the mixed air flow  32  into a mixing chamber  44  of the terminal unit  20 . However, in some circumstances, the volume of air received through the plenum air inlet  40  may be less than 100 percent of the mixed air flow  32  output from the terminal unit  20 . For example, 25 percent, 50 percent, 75 percent, or 100 percent of the mixed air flow  32  may stem from the plenum air flow  30  entering into the mixing chamber  44  via the plenum air inlet  40 , and a remaining portion or percentage of the mixed air flow  32  may be composed of the primary air flow  28  entering the terminal unit  20  via the primary air inlet  38 . As will be appreciated, the particular amounts or portions of the primary air flow  28  and the plenum air flow  30  that are combined to generate the mixed air flow  32  may be based on various operating parameters, such as a temperature of the primary air flow  28 , a temperature of the plenum air flow  30 , a measured temperature of the conditioned space  18 , a temperature set point of the conditioned space  18 , and so forth. When a desired amount of plenum air flow  30  as a component of the mixed air flow  32  is less than 100 percent, the size of the air flow path  41  through the plenum air inlet  40  may be reduced in order to reduce noise associated with or generated by the air flows entering the terminal unit  20 . As such, in accordance with present techniques, the plenum door panel  42  may be moved between multiple different positions by an actuator  46  to adjust the size of the air flow path  41  through the plenum air inlet  40  to minimize the noise radiated from the plenum air inlet  40  while also enabling a desired amount of the plenum air flow  30  to flow into the mixing chamber  44 . 
     The plenum door panel  42  and the actuator  46  may include any suitable components and may be of any suitable arrangement for changing the size of the air flow path  41  through the plenum air inlet  40 . For example, the plenum door panel  42  and/or the actuator  46  may be coupled to a housing  48  of the terminal unit  20  defining the mixing chamber  44 , where the housing  48  includes the plenum air inlet  40  and the primary air inlet  38 . The plenum door panel  42  and the actuator  46  may be disposed within the housing  48  or may be external to the housing  48 . In one embodiment, the housing  48  and the plenum door panel  42  may be formed of the same material, such as sheet metal. The plenum door panel  42  may be disposed along a track  49  affixed to the housing  48 , and the actuator  46  may be configured to move the plenum door panel  42  along the track  49  to adjust a size of the air flow path  41  through the plenum air inlet  40 . Indeed, the plenum door panel  42  may be sized to enable complete occlusion or blockage of the plenum air inlet  40  when no plenum air flow  30  into the mixing chamber  44  is desired. Further, the plenum door panel  42  and the actuator  46  may be positioned, such as along the track  49  coupled to the housing  48 , to enable full retraction of the plenum door panel  42  from the plenum air inlet  40  when  100  percent of plenum air flow  30  into the mixing chamber  44  is desired. 
     The actuator  46  may be a linear actuator, rotary actuator, or other suitable actuator for facilitating movement of the plenum door panel  42  within, behind, and/or in front of the plenum air inlet  40 , relative to a flow direction of the plenum air flow  30  through the plenum air inlet  40 . For example, the actuator  46  may translate the plenum door panel  42  horizontally or vertically and/or rotate the plenum door panel  42 . Additionally, in some embodiments, the plenum door panel  42  may include multiple sections and/or expand and contract in an accordion-like manner. Furthermore, depending on implementation, multiple actuators  46  may also be used to actuate one or more plenum door panels  42 . The actuator  46  may include a motor, a chain, linkages, or any other suitable components to enable movement of the plenum door panel  42 . In some embodiments, the actuator  46  may include a stepper motor and/or a sensor to measure the position of the plenum door panel  42  relative to the plenum air inlet  40 . It should be noted that the plenum door panel  42  and the actuator  46  may have any other suitable configurations to enable adjustment of the size of the plenum air inlet  40 . For example, the plenum door panel  42  may be part of a valve such as a butterfly valve, ball valve, or other suitable fluid control valve. 
     In some embodiments, the terminal unit  20  may also include a sound attenuator  50  coupled to the housing  48  or integrated with the housing  48  and adjacent to the mixing chamber  44  as an additional sound reducing feature. The sound attenuator  50  may define an introductory or initial flow path to enable the plenum air flow  30  to reach the plenum air inlet  40  while attenuating sounds reverberated out of the plenum air inlet  40 . For example, the sound attenuator  50  may use geometric sound damping and/or a sound attenuating material  51  disposed within. Moreover, in some embodiments, the plenum door panel  42  and/or the interior of the housing  48 , such as within the mixing chamber  44 , may also have sound attenuating material  51  disposed thereon. 
     Depending on implementation, the terminal unit  20  may include the plenum door panel  42 , the sound attenuator  50 , or both. For example, in implementations where more sound reduction is desired, the terminal unit  20  may include both the plenum door panel  42  and the sound attenuator  50 . Additionally, in embodiments where the sound attenuator  50  may not be suitable due to space constraints or limitations, the plenum door panel  42  may be implemented on the terminal unit  20  without the sound attenuator  50 . In embodiments of the terminal unit  20  including the sound attenuator  50 , the plenum door panel  42  and/or the actuator  46  may be disposed at an attenuator opening  52  to enable adjustment of a size of the air flow path  41  for the plenum air flow  30  at the attenuator opening  52 . 
     In some embodiments, the plenum door panel  42  and actuator  46  may be retrofitted to an existing terminal unit. For example, the actuator  46  and plenum door panel  42  may be affixed to the exterior of an existing terminal unit via one or more fasteners, such as screw or rivets. Additionally or alternatively, the existing terminal unit may be replaced by the terminal unit  20  with the plenum door panel  42  and actuator  46  already installed. Because the terminal unit  20  with the plenum door panel  42  can be utilized with or without the sound attenuator  50 , the terminal unit  20  may have a similar form factor to that of the existing terminal unit it replaces. As such, existing HVAC systems may be retrofitted with, and new installations may be fitted with, terminal units  20  employing the movable plenum door panel  42  to decrease noise associated with the air flows into and through the terminal unit  20 . 
     The terminal unit  20  may also include additional components, such as a fan  54 , one or more sensors  56 , an air diffuser  58 , a primary inlet valve  60 , and/or an auxiliary conditioner  62 . The operation of these components is discussed in further detail below. Moreover, a controller  64  may regulate operation of the terminal unit  20  to provide desired conditioning to the conditioned spaces  18  of the building  10  via the mixed air flow  32  based on one or more operating parameters. The operating parameters may include a user setting, which may be received via the control device  16  discussed above, and/or operating conditions monitored by the controller  64 , such as temperatures and/or static pressures of the primary air flow  28 , plenum air flow  30 , and mixed air flow  32 , as may be measured by the one or more sensors  56 . The controller  64  may include one or more processors  66 , one or more memory devices  68 , such as a non-transitory machine readable media, and/or one or more input/output (I/O) interfaces  70 , which may be configured to communicate with the control device  16 , the one or more sensors  56 , the fan  54 , the primary inlet valve  60 , the auxiliary conditioner  62 , and/or the actuator  46 . Furthermore, the controller  64  may be implemented as part of the control circuitry of the HVAC unit  12  or may be implemented separately as stand-alone circuitry, such as local or remote to the terminal unit  20 . 
     The fan  54  of the terminal unit  20  may draw the primary air flow  28  and/or plenum air flow  30  into the mixing chamber  44 , promote mixing of the primary air flow  28  and plenum air flow  30  to generate the mixed air flow  32 , and/or force the mixed air flow  32  out of the terminal unit  20  toward the conditioned space  18 . As should be appreciated, the primary air flow  28  may be forced into the mixing chamber  44 , for example via blowers of the HVAC unit  12 , and/or may be drawn into the mixing chamber  44  via the fan  54 . Additionally, the primary inlet valve  60  may regulate the primary air flow  28  into the mixing chamber  44 . Operation of the primary inlet valve  60  may be regulated by the controller  64  based on an operating parameter, such as a thermostat or zoning setting. In some embodiments, the fan  54  may also motivate the mixed air flow  32  across the auxiliary conditioner  62 , which may be disposed adjacent to the outlet  33  of the terminal unit  20 . The auxiliary conditioner  62  may include heating and/or cooling capabilities. For example, the auxiliary conditioner  62  may include a heat exchanger having coils  72  configured to circulate a heated or chilled fluid therethrough. Additionally or alternatively, the auxiliary conditioner  62  may include an electric heating element for heating the mixed air flow  32  prior to discharge of the mixed air flow  32  from the terminal unit  20 . 
     In some embodiments, the speed of the fan  54  may correspond to a desired volumetric flow rate of the mixed air flow  32  discharged by the terminal unit  20 . To this end, the one or more sensors  56  may measure the amount of the primary air flow  28  received by the mixing chamber  44 , and the amount of plenum air flow  30  received via the plenum air inlet  40  may be calculated by the difference between the amount of mixed air flow  32  and the amount of primary air flow  28 . For example, the one or more sensors  56  may directly measure a flow rate of the primary air flow  28  and/or measure the static pressure of the primary air flow  28  relative to the pressure within the mixing chamber  44  or relative to the pressure downstream of the terminal unit  20 . From these measurements, a desired volumetric flow rate of the plenum air flow  30  received by the terminal unit  20  may be determined. Based on the desired amount of plenum air flow  30  to be received by the terminal unit  20 , the controller  64  may control the size of the air flow path through the plenum air inlet  40  to enable a desired volumetric flow rate of plenum air flow  30  into the terminal unit  20  while also reducing noise reverberating out of the plenum air inlet  40 . In other words, based on the desired amount of plenum air flow  30  to be received by the terminal unit  20 , a position of the plenum door panel  42  may be selected or determined that enables a flow of the desired amount of plenum air flow  30  into the terminal unit  20  and that also reduces reverberation of noise out of the plenum air inlet  40 . In some embodiments, an empirically-determined look-up table associating flow rates of the primary air flow  28 , static pressure measurements of the primary air flow  28 , flow rates of the plenum air flow  30 , and/or positions of the plenum door panel  42  may be stored in the memory  68  of the controller  64 . Based on measurement of one or more operating parameters, the controller  64  may reference the look-up table to determine a desired position of the plenum door panel  42 . 
       FIG. 4  is a flowchart  74  of an embodiment of a process for operating the terminal unit  20 . As discussed herein, the controller  64  may monitor operating parameters of the primary air flow  28  and/or the mixed air flow  32 , as indicated by process block  76 . For example, via feedback received from the sensors  56 , the controller  64  may monitor temperatures, pressures, flow rates, or other parameters of the primary air flow  28  and/or the mixed air flow  32 . Based on parameters of the primary air flow  28  and/or the mixed air flow  32 , the controller  64  may determine a desired flow rate or amount of the plenum air flow  30  to be received by the terminal unit  20 , as indicated by process block  78 . As an example, the controller  64  may determine a desired amount of the plenum air flow  30  to be included in the mixed air flow  32  based on a temperature of the plenum air flow  30 , a temperature of the primary air flow  28 , a flow rate of the primary air flow  28 , a static pressure of the primary air flow  28  at the primary inlet valve  60 , a temperature set point of the conditioned space  18 , and/or a desired temperature of the mixed air flow  32 . Based on the desired amount of plenum air flow  30  to be received into the mixing chamber  44  of the terminal unit  20 , the controller  64  may determine a desired or suitable position of the plenum door panel  42 , as indicated by process block  80 . For example, the controller  64  may reference a look-up table stored in the memory  68  to determine or select an empirically-determined position of the plenum door panel  42  based on the desired amount of plenum air flow  30  and/or based on other operating parameters. The plenum door panel  42  may then be actuated to the desired position to enable the desired flow rate of plenum air flow  30  into the terminal unit  20 , while reducing radiated sound from the terminal unit  20 , as dictated process block  82 . 
     As discussed above, the desired position of the plenum door panel  42  may be determined based on the desired volume of the plenum air flow  30  to be received into the terminal unit  20 . However, in order to maintain the desired volume of air through the reduced size of the air flow path  41 , the velocity of the plenum air flow  30  through the plenum air inlet  40  may increase. As such, depending on the volume of the plenum air flow  30  and the position of the plenum door panel  42 , the increased velocity of the plenum air flow  30  may produce as much or more noise than would be reduced by the partial closure of the plenum door panel  42 . As such, the position of the plenum door panel  42  may be determined such that the total noise, including noise associated with the primary air flow  28  and the noise associated with the plenum air flow  30 , reverberated from the terminal unit  20  is minimized. Furthermore, as discussed herein, the desired position of the plenum door panel  42  may be determined based on a thermostat setting and/or the monitored state of the terminal unit  20 , for example via periodic or continuous monitoring of the plenum air flow  30 , the primary air flow  28 , and/or the static pressure of the primary air flow  28 . 
     Additionally or alternatively, the speed of the fan  54  and/or the voltage supplied to the fan  54  may be monitored and used via the controller  64  to determine the desired position of the plenum door panel  42 . For example, a relatively low signal voltage, such as two volts, may correspond to a low fan speed, while a relatively high signal voltage, such as 10 volts may correspond to a high fan speed. As should be appreciated, the voltage levels and corresponding fan  54  speeds may be dependent on implementation and are given as non-limiting examples. Further, the fan  54  speed may correlate to the volume of the mixed air flow  32 . Further, even if the volume of the primary air flow  28  is unknown, an estimate of the plenum air flow  30  may be made based on the speed of the fan  54  or signal indicative thereof. As such, at low fan  54  speeds and, thus, relatively low volumes of plenum air flow  30 , the plenum door panel  42  may be positioned to reduce the air flow path  41  more than at high fan  54  speeds, which may correspond to relatively high volumes of plenum air flow  30 . 
       FIGS. 5, 6, and 7  are perspective views of an embodiment of the terminal unit  20  without the sound attenuator  50  and with the plenum door panel  42  in a fully open position, partially closed position, and fully closed position, respectively. In some embodiments, when the plenum door panel  42  is fully open, the air flow path  41  of the plenum air inlet  40  may be approximately equal in size to the opening of the plenum air inlet  40  formed in the housing  48 . As the plenum door panel  42  is closed, such that the plenum door panel  42  overlaps with the plenum air inlet  40 , the size of the air flow path  41  may decrease. Moreover, when the plenum door panel  42  is fully closed, such as when the plenum door panel  42  substantially completely overlaps with the plenum air inlet  40 , the air flow path  41  may be completely blocked by the plenum door panel  42 , and the plenum air flow  30  may not be permitted to flow through the plenum air inlet  40 . Further, in some embodiments, the actuator  46  may position the plenum door panel  42  to one of multiple set or predetermined positions. For example, the controller  64  may associate a range of plenum air flow volumes and/or fan speeds to a set number of predetermined positions. Moreover, in some embodiments, the actuator  46  may have a full range of motion to position the plenum door panel  42  at any position along the plenum air inlet  40 . 
     As discussed herein, noise associated with the flow of air into and/or out of the terminal unit  20  may be minimized by reducing the air flow path  41  though the plenum air inlet  40 . As such, the plenum door panel  42  of the terminal unit  20  may be actuated to a determined position to decrease the size of the air flow path  41  and reduce the noise reverberated from the terminal unit  20  through the plenum air inlet  40 . Additionally, in some embodiments, the terminal unit may also include sound attenuating material  51  and/or a sound attenuator to further reduce noise. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.