Patent Publication Number: US-2021172779-A1

Title: Systems and methods for control of an air duct

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation in part of U.S. application Ser. No. 16/993,812, filed Aug. 14, 2020, which is a continuation of U.S. application Ser. No. 16/251,011, filed Jan. 17, 2019, which claims benefit of and priority to U.S. Provisional Application No. 62/618,142, filed Jan. 17, 2018, the entire disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     The present disclosure relates, in exemplary embodiments, to air duct airflow sensors. Air dampers are mechanical valves used to permit, block, and control the flow of air in air ducts. Typically, a pressure sensor is incorporated to detect and measure the air pressure in the air duct. Pressure measurement can be used to determine the desire amount of airflow and to actuate a damper to open or close, thus affecting airflow. 
     It would be desirable to have an airflow sensor that would not be dependent on airflow orientation so as to permit location of sensor closer to a bend in the air duct than conventional sensors can be positioned. It would be desirable to have an airflow sensor less susceptible to clogging. 
     SUMMARY 
     One implementation of the present disclosure is a heating, ventilation, or air conditioning (HVAC) system for a space. In some embodiments, the HVAC system includes a valve configured to control an exhaust flowrate of exhaust air that exits the space or a supply flowrate of inlet air that enters the space. In some embodiments, the valve is configured to provide sensor data indicating a flowrate therethrough. In some embodiments, the pressure differential sensor is configured to measure an actual pressure differential of the space relative to a reference pressure. In some embodiments, the controller is configured to determine an actual offset flowrate between the supply flowrate and the exhaust flowrate. In some embodiments, the controller is configured to operate the valve to adjust the exhaust flowrate or the supply flowrate to drive the actual offset flowrate toward an offset flowrate setpoint. In some embodiments, the controller is configured to obtain the actual pressure differential from the pressure differential sensor. In some embodiments, the controller is configured to update the offset flowrate setpoint in response to the actual pressure differential being greater than a maximum allowable pressure differential or less than a minimum allowable pressure differential. 
     In some embodiments, the valve is a first valve configured to measure an inlet pressure differential between two stream locations along the first valve and control the supply flowrate of air that enters the space, the inlet pressure differential indicating the supply flowrate through the first valve. In some embodiments, the HVAC system further includes a second valve configured to measure an outlet pressure differential between two stream locations along the second valve and control the exhaust flowrate of air that exits the space, the outlet pressure differential indicating the exhaust flowrate through the second valve. 
     In some embodiments, the controller is configured to obtain a value of the supply flowrate, a value of the exhaust flowrate, and a value of the actual pressure differential of the space. In some embodiments, the controller is configured to obtain the offset flowrate setpoint and a pressure deadband. In some embodiments, the controller is configured to operate the valve control the supply flowrate of inlet air that enters the space or the exhaust flowrate of exhaust air that exits the space based on the value of the supply flowrate, the value of the exhaust flowrate, the value of the actual pressure differential of the space, the offset flowrate setpoint, and the pressure deadband. 
     In some embodiments, updating the offset flowrate setpoint includes decreasing the offset flowrate setpoint by a decrease amount in response to the actual pressure differential being greater than the maximum allowable pressure differential. In some embodiments, updating the offset flowrate setpoint includes increasing the offset flowrate setpoint by an increase amount in response to the actual pressure differential being less than the minimum allowable pressure differential. 
     In some embodiments, the controller is configured to operate the valve to adjust the exhaust flowrate of the exhaust air or the supply flowrate of inlet air to drive the actual offset flowrate towards the updated offset flowrate setpoint. 
     In some embodiments, the controller is configured to detect a pressure override condition based on the actual pressure differential of the space, and detect whether a door of the space is open. In response to detecting that the door of the space is not open, the controller may update the offset flowrate setpoint in response to the pressure override condition, according to some embodiments. In response to detecting that the door of the space is open, the controller may delay update of the offset flowrate setpoint until the door of the space is not open. 
     In some embodiments, the controller is configured to operate the valve to adjust the exhausr flowrate of the exhaust air or the supply flowrate of the inlet air to drive an actual offset flowrate towards the updated offset flowrate setpoint. 
     In some embodiments, the valve includes a sidewall defining an inner surface, and an air damper assembly. In some embodiments, the air damper assembly includes multiple fingers positioned along a circumference of a structural member of the air damper assembly and having different lengths along the circumference of the structural member. In some embodiments, the first valve and the second valve each include an actuator configured to drive the air damper assembly to rotate. In some embodiments, the multiple fingers are configured to contact the inner surface of the sidewall to adjust a cross-sectional flow area and thereby adjust flowrate of air through the valve. 
     Another implementation of the present disclosure is a controller for an HVAC system, according to some embodiments. In some embodiments, the controller includes processing circuitry configured to perform a volumetric offset control (VOC) scheme based on a flowrate offset setpoint to operate a valve to drive an actual flowrate offset between a supply rate of air entering a space and an exhaust rate of air leaving the space toward the flowrate offset setpoint. In some embodiments, the valve is configured to control the supply rate of air entering the space or the exhaust rate of air leaving the space and to provide sensor data indicating a flowrate therethrough. In some embodiments, the processing circuitry is configured to monitor a pressure of the space to detect a pressure condition and update the flowrate offset setpoint in response to detecting the pressure condition. In some embodiments, the processing circuitry is configured to perform the VOC scheme based on the updated offset flowrate setpoint. 
     In some embodiments, the valve is an exhaust valve configured to control the exhaust rate of air leaving the space. In some embodiments, performing the VOC scheme includes operating the exhaust valve of the space to adjust the exhaust rate of air leaving the space to drive the actual flowrate offset toward the flowrate offset setpoint. In some embodiments, performing the VOC scheme also includes operating a supply valve of the space to adjust the supply rate of air entering the space to achieve a desired environmental condition of the space. 
     In some embodiments, monitoring the pressure of the space to detect the pressure condition includes obtaining the pressure of the space, comparing the pressure of the space to a high pressure limit and a low pressure limit, and detecting the pressure condition in response to the pressure of the space being greater than the high pressure limit or less than the low pressure limit. 
     In some embodiments, updating the flowrate offset setpoint includes increasing the flowrate offset setpoint by a predetermined increase amount in response to the pressure of the space being greater than the high pressure limit, and decreasing the flowrate offset setpoint by a predetermined decrease amount in response to the pressure of the space being less than the low pressure limit. 
     In some embodiments, the processing circuitry is configured to limit subsequent increases of the flowrate offset setpoint in response to the flowrate offset setpoint being equal to a maximum limit, and limit subsequent decreases of the flowrate offset setpoint in response to the flowrate offset setpoint being equal to a minimum limit. 
     In some embodiments, the processing circuitry is further configured to obtain a status from a door sensor, the status indicating whether a door of the space is opened or closed. In some embodiments, the processing circuitry is configured to prevent updates to the flowrate offset setpoint until the door of the space is closed in response to the status indicating that the door of the space is opened. In some embodiments, the processing circuitry is configured to prevent updates to the flowrate offset setpoint until a delay time period has passed in response to the status of the door transitioning from opened to closed. 
     Another implementation of the present disclosure is a method for controlling an HVAC system of a space, according to some embodiments. In some embodiments, the method includes operating a valve of the space according to a volumetric offset control (VOC) scheme to drive an actual offset flowrate between the supply valve and the exhaust valve to drive an actual offset flowrate between a supply rate of air entering the space and an exhaust rate of air leaving the space toward a VOC setpoint. In some embodiments, the valve is configured to control the supply rate of air entering the space or the exhaust rate of air leaving the space and to provide sensor data indicating a flowrate therethrough. In some embodiments, the method includes detecting a pressure condition of the space, and in response to detecting the pressure condition of the space, updating the VOC setpoint to determine an adjusted VOC setpoint, and operating the valve of the space according to the VOC scheme based on the adjusted VOC setpoint. 
     In some embodiments, the pressure condition includes at least one of a pressure of the space exceeding a high pressure limit, or the pressure of the space being less than a low pressure limit. 
     In some embodiments, updating the VOC setpoint includes increasing the VOC setpoint by an increase amount in response to the pressure of the space being less than the low pressure limit or decreasing the VOC setpoint by a decrease amount in response to the pressure of the space being greater than the high pressure limit. 
     In some embodiments, the VOC setpoint is a setpoint offset amount between the supply rate of air entering the space and the exhaust rate of air leaving the space. 
     In some embodiments, the method includes limiting updates to the VOC setpoint in response to a door of the space being opened until the door is closed for a predetermined amount of time, and delaying updates to the VOC setpoint for the predetermined amount of time in response to the door of the space transitioning from opened to closed. 
     In some embodiments, the valve includes a sidewall defining an inner surface, an air damper assembly, and an actuator. In some embodiments, the air damper assembly includes multiple fingers positioned along a circumference of a structural member of the air damper assembly and having different lengths along the circumference of the structural member. In some embodiments, the actuator is configured to drive the air damper assembly to rotate. In some embodiments, the multiple fingers are configured to contact the inner surface of the sidewall to adjust a cross-sectional flow area and thereby adjust a flowrate of air through the valve. 
     Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings disclose exemplary embodiments in which like reference characters designate the same or similar parts throughout the figures of which: 
         FIG. 1  is an isometric view of an air duct assembly, according to some embodiments. 
         FIG. 2  is a side cross-sectional view of an air duct airflow sensor assembly, according to some embodiments. 
         FIG. 3  is a side cutaway view of the air duct assembly of  FIG. 1 , according to some embodiments. 
         FIG. 4  is a top elevation view of the air duct assembly of  FIG. 1 , according to some embodiments. 
         FIG. 5  is an exploded perspective view of an air duct, ring and gasket components that can be utilized in the air duct assembly of  FIG. 1 , according to some embodiments. 
         FIG. 6  is another top view of the air duct assembly of  FIG. 1 , according to some embodiments. 
         FIG. 7  is a side cross-sectional view of the air duct assembly taken along the line B-B of  FIG. 6 , according to some embodiments. 
         FIG. 8  is a detail view C-C of the nipple, gasket and tube, according to some embodiments. 
         FIG. 9  is a detail view D-D of the gasket, according to some embodiments. 
         FIG. 10  is a side cross-sectional view of another air duct airflow sensor assembly, according to some embodiments. 
         FIG. 11  is a side cross-sectional view of another air duct airflow sensor assembly, according to some embodiments. 
         FIG. 12  is a diagram of a control system for a space that uses inlet and outlet valves, according to some embodiments. 
         FIG. 13  is a diagram of a controller of the control system of  FIG. 12 , according to some embodiments. 
         FIG. 14  is a flow diagram of a process for performing volumetric offset control (VOC), according to some embodiments. 
         FIG. 15  is a flow diagram of a process for performing pressure based control, according to some embodiments. 
         FIG. 16  is a flow diagram of a process for performing a hybrid VOC and pressure based control, according to some embodiments. 
         FIG. 17  is a block diagram of functionality of a hybrid VOC and pressure based control of the controller of  FIG. 13 , according to some embodiments. 
         FIG. 18  is a side view of a portion of the air duct assembly of  FIGS. 1-11 , according to some embodiments. 
         FIG. 19  is a diagram of a pressure differential sensor, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Unless otherwise indicated, the drawings are intended to be read (for example, cross-hatching, arrangements of parts, proportion, degree, or the like) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, “upper” and “lower” as well as adjectival and adverbial derivatives thereof (for example, horizontally”, “upwardly”, or the like), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
       FIG. 1  depicts an isometric view of a cylindrical air duct assembly  1 . As shown, the air duct assembly  1  includes a first end  2 , a second end  3 , and interior wall  4 , an exterior wall  5 , and a control assembly  100 . Air duct assembly  1  is further shown to include an air damper assembly  50  situated within the interior wall  4  to control the volume of air flowing through the cylindrical air duct assembly  1 . In some embodiments, the diameter of the interior wall  4  is approximately 10 inches or any other value. 
     Referring now to  FIGS. 2-9 , various views depicting the air duct airflow sensor assembly  10  are shown, according to some embodiments. Air may flow through the air duct airflow sensor assembly  10  in the direction indicated by arrow “A” as shown in  FIG. 2 . The air duct airflow sensor assembly  10  includes a low pressure detection device and a high pressure detection device. The low pressure detection device comprises a hollow ring  20  which is mounted to or otherwise associated with the interior wall  4 . In some embodiments, the outer diameter of the hollow ring  20  can range from 0.5 inches to 0.75 inches. In an exemplary embodiment, the outer diameter of the hollow ring  20  is 0.625 inches. The ring  20  has a plurality of apertures  22  defined in the inner periphery  23  of the ring (versus the outer periphery  24  which is proximate to the interior wall  4 ). In exemplary embodiments, the apertures  22  are disposed in the inner periphery of the ring  20  such that they are generally orthogonal to the orientation of airflow, so that air flows across the apertures  22 , rather than flowing into the apertures  22 . 
     A hollow connector nipple  28  is connected to an aperture defined in the ring  20  and an aperture defined in the duct  1 . A tube  32  is connected to the nipple  28 . Air flowing into the apertures  22  can flow through the ring  20 , into the nipple  28 , and through the tube  32 . The tube  32  is connected to a pressure sensor  34  such that the air flowing through the tube  32  is received and detected by the flow pressure sensor  34 . The ring  20  serves two purposes: as an air collection device, and as an airflow restriction obstacle, so as to create a measurable pressure differential. 
     The air duct  1  further includes multiple apertures  40  defined therein, the apertures  40  being arranged generally in a ring-shape around the interior wall  4 . A gasket  42  is associated with the exterior wall  5  and is located generally over the apertures  40 . The gasket  42  has a recessed area  43  such that when associated with the exterior wall  5  a chamber  43  is formed. Detail views of the apertures  40  and chamber  43  are specifically depicted in  FIGS. 8 and 9 . 
     A hollow connector nipple  44  is connected to the gasket  42 . In exemplary embodiments, a gasket guarding ring  45  may be used and is fitted over the gasket  42 . A tube  46  is connected to the nipple  44 . The tube  46  is connected to the pressure sensor  34 . In an alternative exemplary embodiment, a separate pressure sensor (not shown) can be connected to the tube  46 . The apertures  40 , gasket  42 , nipple  44 , tube  46  and pressure sensor  34  form a high pressure sensor detection device. 
     In exemplary embodiments, the pressure sensor  34  is part of a control assembly  6  that controls the opening and closing of a damper  50 . In one exemplary embodiment of a control assembly, specifically depicted in  FIG. 7 , a housing  100  is mounted to or otherwise associated with the air duct. A sensor  34 , processor  102 , actuator  104  and power supply  106  may be disposed within the housing  100 . A damper  50  is in operational communication with the actuator  104 . 
     In operation, air flowing through the duct  1  in the direction of arrow A first encounters the high pressure detection apertures  40 . A portion of the air enters the apertures  40  and flows into the chamber  43 . The air then moves into the tube  46  via the nipple  44 , and then into the pressure sensor  34 . The pressure detected is the “high” pressure in the duct  1 , i.e., the pressure upstream from the airflow restrictor which is the ring  20 . 
     Air flowing through the duct  1  next flows over the ring  20  and can enter the apertures  22  and travel through the nipple  28  and the tube  32 , and into the pressure sensor  34 . The pressure detected is the “low” pressure in the duct, i.e., the pressure at the point where airflow is restricted by the ring  20 . The differential between the high pressure measurement and the low pressure measurement is an indication of the air velocity through the duct, specifically a scaled square root of the measured pressure (i.e., an application of Bernoulli&#39;s principle). The sensor  34  can send a signal to the control assembly  6  that in turn can cause the damper  50  to rotate so as to open or close the air duct  1 . 
     In exemplary embodiments, the pressure sensor  34  is a “dead-end” pressure sensor (versus a flow-through sensor); i.e., after the initial pressure is established no further airflow goes through the sensor. This can reduce the chance of the apertures  22  and  40  becoming clogged. 
     In one exemplary embodiment, for an air duct having a 10 inch diameter, a 0.5 inch diameter ring  20  was used. With such a construction measurements of 850 CFM (cubic feet per minute) down to 35 CFM were obtainable with a 0.1 in Wg duct static. In other embodiments, a 0.625 inch diameter ring  20  may be utilized. 
     A benefit of the presently described sensor assembly is that because of the ring  20  design having the apertures  22  orthogonal to the airflow orientation, air to be diverted into the ring  20  flows over the apertures  22 , rather than directly into the apertures  22 . This can reduce the likelihood of the apertures  22  becoming clogged by dust, dirt and debris that accompanies the airstream. 
     Another benefit is that the presently disclosed apparatus is not dependent on airflow orientation. Typically, conventional pressure sensor apparatus, such as variable air volume (“VAV”) boxes, are dependent on airflow orientation, and having a bend or other transition in the duct in the general area where the sensor can result in inaccurate measurement due to the airflow disruption that naturally occurs proximate to the bend. With the air detection means of the presently disclosed apparatus, which is not airflow orientation dependent, the sensor assembly can be located closer to a bend or other transition in the air duct without affecting pressure measurement. This provides the duct system designer with greater flexibility in designing the placement of the valve assembly. 
     Another benefit of the presently described sensor assembly is that it presents minimal obstruction to the airflow and thus allows for greater CFM velocity at lower duct statics. Additionally, in the event any of the apertures  22  become blocked, it is easy to carry out periodic maintenance by disconnecting the sensor  34  and introducing a blast of compressed air into the tube  32  or tube  46 . Any clogging debris will be blown out of the apertures  22  or  40 , respectively. 
     Another benefit of the presently described sensor assembly as part of an overall sensor/controller/damper design is that it can operate off of a 0-10V control signal to provide the desired airflow. This allows a designer or operator to set a required CFM with a linear control signal from a control system. 
     Referring now to  FIGS. 10 and 11 , alternate embodiments for airflow restriction used in the low pressure detection device are depicted. Specifically,  FIG. 10  depicts an airflow sensor assembly including a shroud component  60 . In some embodiments, the shroud component  60  can be ring-shaped, with an interior wall attachment portion  62 , an inclined portion  64 , and an aperture shielding portion  66 , although any suitable shroud configuration or geometry may be utilized. In some embodiments, the aperture shielding portion  66  extends from the interior wall  4  a distance ranging from 0.5 inches to 0.75 inches. 
     The aperture shielding portion  66  is situated proximate apertures  22  disposed within the air duct  1 . A gasket  48  is associated with the exterior wall  5  and is located generally over the apertures  22 . In some embodiments, one or more gasket guarding rings (not shown) may be used and fitted over the gaskets  42 ,  48 . The gasket  48  has a recessed area  49  such that when associated with the exterior wall  5  a chamber  49  is formed. Air flowing through the duct  1  flows over the interior wall attachment portion  62 , the inclined portion  64 , and the aperture shielding portion  66  of the shroud component  60  and can enter the apertures  22 . The air can then travel through the chamber  49  into the nipple  28 . Similar to the pressure measurement process described above with reference to  FIGS. 1-9 , after passing through the nipple  28 , the air can travel through a tube and into a pressure sensor for the purpose of controlling an air damper assembly. 
     Turning now to  FIG. 11 , an airflow sensor assembly including a channel feature  70  is depicted. Similar to the shroud component  60  described above with reference to  FIG. 10 , the channel feature  70  may be utilized as an air restriction feature in place of the hollow ring  20  described above with reference to  FIGS. 1-9 . The channel feature  70  can include multiple apertures  22  distributed about a periphery of the channel feature  70 . In some embodiments, the depth of the channel feature  70  can range from 0.5 inches to 0.75 inches. In an exemplary embodiment, the depth of the channel feature  70  is 0.625 inches. In other words, if the air duct  1  is nominally 10 inches in diameter, the diameter may reduce to 8.75 inches in the region of the channel feature  70 . 
     A gasket  48  is associated with the exterior wall  5  and is located generally over the apertures  22 . In some embodiments, one or more gasket guarding rings (not shown) may be used and fitted over the gaskets  42 ,  48 . The gasket  48  has a recessed area  49  such that when associated with the exterior wall  5  a chamber  49  is formed. Air flowing through the duct  1  flows over the channel feature  70  and can enter the apertures  22 . The air can then travel through the chamber  49  into the nipple  28 . Similar to the pressure measurement process described above with reference to  FIGS. 1-9 , after passing through the nipple  28 , the air can travel through a tube and into a pressure sensor for the purpose of controlling an air damper assembly. 
     Referring now to  FIG. 12 , a heating, ventilation, and air conditioning (HVAC) system  200  for a space  216  is shown, according to some embodiments. The HVAC system  200  uses a first valve  206   a  for controlling inlet air that is provided to the space  216 , a second valve  206   b  for controlling outlet air that exits the space  216 , and an air handling unit (AHU)  202 . The HVAC system  200  also includes a control system  300  including a controller  208  that is configured to generate and provide control signals or airflow setpoints to any of the first valve  206   a , and/or the second valve  206   b , or a first controller  232   a  and/or a second controller  232   b  of the first valve  206   a  and the second valve  206   b . The first controller  232   a  and the second controller  232   b  can be components of the first valve  206   a  and the second valve  206   b , respectively, and are configured to obtain airflow setpoints from the controller  208  and generate control signals for a control valve  209  portion of the first valve  206   a  and the second valve  206   b . For example, the first controller  232   a  can be configured to obtain airflow setpoints for the first valve  206   a  and determine control signals for the control valve  209   a  (e.g., an actuator and damper of the first valve  206   a  thereof). Similarly, the second controller  232   b  can be configured to obtain airflow setpoints for the second valve  206   b  and determine control signals for the control valve  209   b  (e.g., an actuator and damper of the second valve  206   b  thereof). The first valve  206   a  and the second valve  206   b  can also include a pressure differential sensor  207   a  and a pressure differential sensor  207   b , respectively, configured to measure a pressure differential of the air (e.g., the supply or intake air or the exhaust air) and provide the pressure differential to the first controller  232   a  or the second controller  232   b , respectively. The first controller  232   a  and the second controller  232   b  are configured to use the pressure differential to determine a supply measured airflow and an exhaust measured airflow of air being provided to the space  216  and leaving the space  216  through the first valve  206   a  and the second valve  206   b , respectively. 
     The first valve  206   a  and the second valve  206   b  can be the same as or similar to the air duct  1 . In other embodiments, the first valve  206   a  and the second valve  206   b  are venturi valves. The air duct  1  can provide a higher range of duct pressure (e.g., a higher pressure drop across the air duct  1  while still being operational) than the venturi. In some embodiments, the venturi valve implemented as one or both of the first valve  206   a  and the second valve  206   b  is provided as a low pressure venturi valve or a medium pressure venturi valve. For example, the low pressure venturi valve can be configured to have a pressure drop thereacross of at least 0.3 inches water gauge, but less than 3 inches water gauge. In some embodiments, the medium pressure venturi valve can be configured to have a pressure drop thereacross of at least 0.6 inches water gauge, and at most 3 inches water gauge. In some embodiments, the air duct  1  is configured to have a 0.1 inch water gauge pressure drop thereacross (e.g., having a max cfm of 850 for a 10 inch valve). In some embodiments, the air duct  1  can still operate even if the pressure drop across the air duct  1  is less than 0.1 inch water gauge. 
     The space  216  can be an interior space of a room, building zone, etc., shown as zone  204 . The zone  204  may be substantially sealed so that air that exits the space  216  exits through the second valve  206   b . The HVAC system  200  includes an inlet vent  218  and an outlet vent  220 . The AHU  202  can draw intake air (e.g., fresh air, outdoor air, etc.) and recirculated air. The AHU  202  can output air that is provided through a duct  210  and the first valve  206   a . The duct  210  and the first valve  206   a  provide the air as inlet air to the space  216  through the inlet duct  218 . 
     Air within the space  216  may exit through the outlet vent  220  (and an exhaust fan  230 ) which is fluidly coupled with a duct  212 . The second valve  206   b  is fluidly coupled with the duct  212 . Air that exits the space  216  or the second valve  206   b  (e.g., return air) can be recirculated to the AHU  202  through a duct  214 , may egress to outside (e.g., outside a building), and/or may be filtered or otherwise processed. 
     The HVAC system  200  also includes the control system  300  including the controller  208 . The controller  208  is configured to generate control signals for any of the first valve  206   a , or the second valve  206   b . The first valve  206   a  can operate to affect a flowrate of air that is provided to the space  216 . The second valve  206   b  can operate to affect a flowrate of air that is removed from the space  216 . The valves  206  may be the same as or similar to the cylindrical air duct assembly  1  as described in greater detail above with reference to  FIGS. 1-11 . 
     The HVAC system  200  or the control system  300  also includes a pressure sensor  226 , and a pressure sensor  224 . The pressure sensor  226  and the pressure sensor  224  can be provided as a single pressure differential sensor (shown as pressure differential sensor  234  in  FIGS. 13 and 19 ) that includes both the pressure sensor  226  and the pressure sensor  224 . The pressure sensor  226  is configured to monitor pressure within the space  216  p space  and can provide readings or measurements of the pressure within the space  216  to the controller  208 . The pressure sensor  224  is configured to measure or monitor pressure within a reference space  222  p ref  and provide the readings or measurements of the pressure within the reference space  222  to the controller  208 . In some embodiments, the HVAC system  200  includes only a single pressure differential sensor. The zone  204  may include a door and a door sensor  228  that is configured to monitor a door status. The door sensor  228  can provide the door status to the controller  208 . 
     Each of the pressure differential sensors  207  of the valves  206  are configured to obtain or measure a pressure differential of the air that flows therethrough between two different positions. For example, the pressure differential sensor  207   a  of the first valve  206   a  can measure an inlet air pressure differential, and the pressure differential sensor  207   b  of the second valve  206   b  can measure an exhaust air or an outlet air pressure differential. The valves  206  can provide the pressure differentials to the corresponding first controller  232   a  and/or second controller  232   b  for determination of the supply measured airflow or the exhaust measured airflow thereof. The first controller  232   a  and the second controller  232   b  of the valves  206  are configured to provide the supply measured airflow and the exhaust measured airflow to the controller  208  for use in determining airflow setpoints for the first valve  206   a  and the second valve  206   b . The airflow setpoints are provided by the controller  208  to the first controller  232   a  of the first valve  206   a  and the second controller  232   b  of the second valve  206   b . The first controller  232   a  and the second controller  232   b  are configured to use the airflow setpoints to generate control signals for the control valve  209   a  and the control valve  209   b  of the first and second valves  206   a - 206   b  to control or adjust the supply measured airflow and the exhaust measured airflow to achieve or be driven towards the airflow setpoints. In this way, the controller  208  may determine airflow setpoints for the first valve  206   a  and the second valve  206   b  based on supply measured airflow and exhaust measured airflow, and the first controller  232   a  and the second controller  232   b  can be configured to generate control signals for the first valve  206   a  and the second valve  206   b  based on the airflow setpoints, and to determine the supply measured airflow and the exhaust measured airflow based on the pressure differentials obtained at the first valve  206   a  and the second valve  206   b.    
     The controller  208  is configured to obtain any of the supply measured airflow from the first controller  232   a  of first valve  206   a , the exhaust measured airflow from the second controller  232   b  of the second valve  206   b , the door status from the door sensor  228 , the space pressure p space  from the pressure sensor  226 , and the reference pressure p ref  from the pressure sensor  224  (or a pressure differential Δp from the pressure differential sensor  234 ), according to some embodiments. The controller  208  can also obtain feedback or operational data from the AHU  202 . The controller  208  uses the inputs described herein to generate airflow setpoints for the first valve  206   a , and/or the second valve  206   b  according to a control scheme. In some embodiments, the airflow setpoint for the first valve  206   a  and the airflow setpoint for the second valve  206   b  are used by the controllers  232  of the valves  206  to generate control signals for the control valves  209  thereof to adjust a flowrate of air that is provided to the space  216  or to adjust a flowrate of air that leaves or exits the space  216  (e.g., to adjust the supply measured airflow and/or the exhaust measured airflow). The controller  208  can operate according to a variety of different control schemes, including a volumetric offset control (VOC) scheme, a pressure based control scheme, or a hybrid control scheme that uses techniques from both the VOC scheme and the pressure based control scheme. The controller  208  can use the supply measured airflow and the exhaust measured airflow obtained from the first valve  206   a  and the second valve  206   b  to determine a volumetric flowrate (e.g., a cfm) of the inlet air and a volumetric flowrate of the outlet air (e.g., the airflow setpoints). 
     Referring now to  FIG. 13 , the control system  300  is shown in greater detail, according to some embodiments. The control system  300  includes the controller  208  that is in communication with the first valve  206   a  (or sensors thereof), the second valve  206   b  (or sensors thereof), the pressure sensor  224 , the pressure sensor  226  (and/or the pressure differential sensor  234 ), the AHU  202 , and/or the door sensor  228 . The controller  208  is shown to include processing circuitry  302  including a processor  304  and memory  306 . The processor  304  may be a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Processor  304  may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. Processor  304  also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some embodiments, particular processes and methods may be performed by circuitry that is specific to a given function. 
     Memory  306  (e.g., memory, memory unit, storage device) may include one or more devices (e.g., RAM, ROM, Flash memory, hard disk storage) for storing data and/or computer code for completing or facilitating the various processes, layers and modules described in the present disclosure. Memory  306  may be or include volatile memory or non-volatile memory, and may include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described herein. According to an exemplary embodiment, the memory  306  is communicably connected to the processor  304  via the processing circuitry  302  and include computer code for executing (e.g., by the processing circuitry  302  or the processor  304 ) the one or more processes described herein. 
     Referring still to  FIG. 13 , the memory  306  is shown to include a VOC controller  310 , a hybrid controller  312 , a pressure controller  314 , and a flowrate estimator  308 . The flowrate estimator  308  is configured to obtain the supply measured airflow from the first valve  206   a  (or more specifically, from the first controller  232   a ) and the exhaust measured airflow from the second valve  206   b  (or more specifically from the second controller  232   b ). The flowrate estimator  308  is configured to use the supply measured airflow and the exhaust measured airflow to determine a volumetric flowrate of air that enters and leaves the space  216  (e.g., in appropriate units for the VOC controller  310  and/or the hybrid controller  312 ). For example, the volumetric flowrate of air that enters the space  216  may be expressed in units of cubic feet per minute cfm inlet  and the volumetric flowrate of air that leaves the space  216  may be expressed in units of cubic feet per minute cfm exhaust . The flowrate estimator  308  can estimate, calculate, determine, etc., the flowrates, and can provide the flowrates (e.g., cfm inlet  and cfm exhaust ) to any of the VOC controller  310 , the hybrid controller  312 , and/or the pressure controller  314 . 
     The VOC controller  310  is configured to receive the flowrates cfm inlet  and cfm exhaust  and generate the airflow setpoints for the first valve  206   a  and/or the second valve  206   b  according to a VOC control scheme. The VOC controller  310  can determine a difference between the flowrates: 
       Δcfm actual =cfm inlet −cfm exhaust  
 
     according to some embodiments. In some embodiments, the difference Δcfm actual  is an actual difference between the volumetric flowrate of the inlet air and the volumetric flowrate of the exhaust or outlet air. 
     The VOC controller  310  also uses a volumetric offset setpoint Δcfm set , according to some embodiments. In some embodiments, the volumetric offset setpoint Δcfm set  is a desired difference between inlet and outlet air of the space  216 . The volumetric offset setpoint Δcfm set  may be a positive value, indicating that the inlet air should be provided at a rate greater than the outlet air (e.g., a positive isolation mode) or may be a negative value, indicating that the inlet air should be provided at a rate less than the outlet air (e.g., a negative isolation mode). 
     The VOC controller  310  compares the difference Δcfm actual  to the volumetric offset setpoint Δcfm set  to determine if volumetric offset setpoint Δcfm set  has been achieved. The VOC controller  310  can examine differences between the difference Δcfm actual  and the volumetric offset setpoint Δcfm set  to determine if operation of any of the first valve  206   a  and/or the second valve  206   b  should be adjusted. The VOC controller  310  can operate the first valve  206   a  and/or the second valve  206   b  to drive the difference Δcfm actual  toward the volumetric offset setpoint Δcfm set  (e.g., using PID control, closed loop control, etc.). In some embodiments, the VOC controller  310  is configured to operate the first valve  206   a  and/or the second valve  206   b  to adjust the flowrate of air that is provided to the space  216  or that exits the space  216 , thereby adjusting the difference Δcfm actual  (e.g., by adjusting the cfm inlet  and/or the cfm exhaust ). For example, if the difference Δcfm actual  is less than the volumetric offset setpoint Δcfm set , the control signals generated by the VOC controller  310  can operate the second valve  206   b  to adjust a damper to decrease the outlet flowrate cfm exhaust , increase the inlet flowrate cfm inlet  by adjusting a damper of the first valve  206   a , or both adjust the dampers of the first valve  206   a  and the second valve  206   b  to decrease the outlet flowrate cfm exhaust  and increase the inlet flowrate cfm inlet . 
     Advantageously, the VOC controller  310  implements a VOC control scheme to operate the first valve  206   a  and the second valve  206   b  to achieve a desired airflow offset within the space  216 . The pressure can be achieved by operating the first valve  206   a  and the second valve  206   b  to provide a desired relative difference between incoming air to the space  216  and outgoing air from the space  216 . In some embodiments, the first valve  206   a  is operated by the VOC controller  310  to achieve a desired temperature and/or indoor air quality within the space  216  and the second valve  206   b  is operated to adjust the outlet flowrate cfm outlet  to achieve the volumetric offset setpoint Δcfm set . In some embodiments, since the VOC controller  310  does not require pressure measurements of the space  216 , the zone  204  may include stand-alone pressure sensors and displays throughout the space  216 . The stand-alone pressure sensors and displays can be units that are configured to measure the pressure within the space  216  and display the pressure of the space  216  relative to a reference space or a reference pressure. This can be done to provide positive feedback to occupants of the space  216 . 
     In some embodiments, the VOC controller  310  is used when the space  216  is a large space with air disturbance occurrences. For example, the VOC controller  310  can be used when the zone  204  includes multiple doors, multiple fume hoods, large air change rate requirements, and/or constant movement of occupants. The VOC controller  310  can also be implemented for spaces where there are multiple independent zones that are open to each other. 
     Referring still to  FIG. 13 , the memory  306  includes the pressure controller  314 , according to some embodiments. In some embodiments, the pressure controller  314  is configured to receive the reference pressure p ref  from the pressure sensor  224 , and the space pressure p space  from the pressure sensor  226 . The pressure controller  314  can also receive a door status from one or more door sensors  228  indicating which of one or more doors are currently opened or closed. 
     The pressure controller  314  is configured to determine a difference between the reference pressure p ref  and the space pressure p space , according to some embodiments. For example, the pressure controller  314  can be configured to determine an actual pressure differential: 
       Δ p   actual   =p   ref   −p   space  
 
       or: 
       Δ p   actual   =p   space   −p   ref  
 
     according to some embodiments. In some embodiments, the pressure differential Δp actual  is obtained directly from the pressure differential sensor  234 . 
     In some embodiments, the pressure controller  314  is configured to perform closed-loop feedback using the actual pressure differential Δp actual  to drive the actual pressure differential Δp actual  toward a pressure differential setpoint Δp setpoint . The pressure controller  314  can generate control signals or and airflow setpoint for the second valve  206   b  to adjust the damper of the second valve  206   b  to change a flowrate of air that exits the space  216 , thereby adjusting the actual pressure differential Δp actual . The pressure controller  314  can generate control signals or an airflow setpoint for the first valve  206   a  for temperature control and/or indoor air quality control of the space  216 . 
     The pressure controller  314  is also configured to receive the door statuses from the door sensor(s)  228  to determine if the space  216  is no longer sealed. If the pressure controller  314  detect that a door has been opened, the pressure controller  314  may delay updating or adjusting the control signals to prevent the system from going into overdrive. The pressure controller  314  may delay updating the control signals until the door status indicates that the doors are closed and that the space  216  is sealed. Once the door status indicates that the doors are closed that the space  216  is sealed, the pressure controller  314  can delay a predetermined amount of time before performing its functionality to determine adjustments to the control signals to control the second valve  206   b  to drive the actual pressure differential Δp actual  towards the pressure differential setpoint Δp setpoint . The door that the door sensor  228  measures the status of may fluidly couple the space  216  with the reference space  222  when opened. 
     Advantageously, the pressure controller  314  can be implemented when the space  216  is a substantially sealed space with minimal leakage. For example, the efficiency of the pressure controller  314  may increase when the space  216  is sealed with minimal leakage. The space  216  can be sealed to improve the efficiency of the pressure controller  314  through the use of good gap seals/skirts, ceiling tile seals, light fixture sealing, and/or window crack sealing, to facilitate minimal air use. In some embodiments, the pressure controller  314  is implemented when the space  216  is a smaller space with minimal disturbances that require a rapid and real-time response to changing conditions to maintain a desired pressure in the space  216  and a desired level of safety. 
     Referring still to  FIG. 13 , the memory  306  includes the hybrid controller  312 , according to some embodiments. The hybrid controller  312  can be configured to implement techniques or functionality of both the VOC controller  310  and the pressure controller  314 . For example, the hybrid controller  312  can obtain any of the supply measured airflow from the first valve  206   a  (or from the first controller  232   a  of the first valve  206   a ), the exhaust measured airflow from the second valve  206   b  (or from the second controller  232   b  of the second valve  206   b ), the reference pressure p ref  from the pressure sensor  224 , the space pressure p space  from the pressure sensor  226  (or the pressure differential Δp actual  from the pressure differential sensor  234 ), and/or the door status from the door sensor  228 . In some embodiments, the hybrid controller  312  is configured to perform the functionality of the VOC controller  310  as described above, while monitoring the actual pressure differential Δp actual  between the space  216  and the reference space  222 . The hybrid controller  312  can use a pressure deadband D; that defines a range of acceptable pressure differential values. If the hybrid controller  312  detects that the actual pressure differential Δp actual  is outside of or approaching a boundary of the pressure deadband DB p , the hybrid controller  312  is configured to update the volumetric offset setpoint Δcfm set  and operate according to the VOC controller  310  functionality described above using the updated volumetric offset setpoint Δcfm set  to drive the actual pressure differential Δp actual  back into the pressure deadband DB p . In this way, the hybrid controller  312  can operate the first valve  206   a  and the second valve  206   b  according to the functionality of the VOC controller  310  as described in greater detail above, while using the actual pressure differential Δp actual  as an indicator of whether or not the volumetric offset setpoint Δcfm set  should be updated. 
     Referring now to  FIG. 17 , a diagram  1700  of the functionality of the hybrid controller  312  is shown, according to some embodiments. The diagram  1700  illustrates the functionality of the controller  208  when the hybrid controller  312  is implemented, as well as the inputs and outputs of the controller  208  for the implementation of the hybrid controller  312 . 
     Room pressure  1738  can be obtained from the pressure sensor  224  and/or the pressure sensor  226 . The room pressure  1738  is compared to a room pressure high limit  1740  and a room pressure low limit  1742 . The room pressure high limit  1740  and the room pressure low limit  1742  can defined the pressure deadband DB p . If the room pressure  1738  is above the room pressure high limit  1740  or below the room pressure low limit  1742  (e.g., by comparing the room pressure  1738  to the room pressure high limit  1740  or the room pressure low limit  1742  at the comparison blocks  1744  and  1746 , respectively), a setpoint adjustment process can be enabled at enable block  1732 . In some embodiments, the setpoint adjustment process or functionality can be delayed to a later time based on an updated period  1734  and/or an updated timer  1736 . For example, the setpoint adjustment process can be delayed to a scheduled interval as determined or provided by the updated period  1734  and/or the updated timer  1736 . 
     In some embodiments, the setpoint adjustment process or functionality can be disabled or prevented from occurring (even if the room pressure  1738  is outside of the pressure deadband DB p ) based on a status of a door switch  1724 . The door switch  1724  can be the door sensor  228  as described in greater detail above. If the status indicates that a door that the door switch  1724  monitors is open, a disable block  1730  can disable either a setpoint increase or a setpoint decrease at disable blocks  1728  and  1726 , respectively. 
     In some embodiments, a VOC setpoint  1704  (e.g., a setpoint that is determined or adjusted for volumetric offset control) is determined based on a VOC base setpoint  1702  (e.g., a starting value, an initial value, a predetermined value, etc.), and one or more increases  1708  or decreases  1710 . If the room pressure  1738  is greater than the room pressure high limit  1740 , the VOC setpoint  1704  can be decreased by reducing the VOC setpoint  1704  by a decrease amount  1706  (this action is shown at the VOC decrease block  1708 ), according to some embodiments. Similarly, if the room pressure  1738  is less than the room pressure low limit  1742 , the VOC setpoint  1704  can be increased by increasing the VOC setpoint  1704  by an increase amount  1706  (this action is shown at the VOC increase block  1710 ), according to some embodiments. 
     The VOC setpoint  1704  can then be compared to a maximum limit  1712  and a minimum limit  1716  (shown as CFM limits). The VOC setpoint  1704  can be capped at the maximum limit  1712  and the minimum limit  1718 , respectively. For example, if the VOC setpoint  1704  is greater than or equal to the maximum limit  1712  (this action is shown at a comparison block  1714 ), setpoint increases can be disabled (shown at the disable block  1728 ). Similarly, if the VOC setpoint  1704  is less than or equal to the minimum limit  1718 , setpoint decreases can be disabled (shown at the disable block  1726 ). 
     The VOC setpoint  1704 , including any increases or decreases can be provided to a room balance controller  1722  as an adjusted VOC setpoint. The room balance controller  1722  can be the same as or similar to the VOC controller  310  as described in greater detail above with reference to  FIG. 13 . The room balance controller  1722  uses the adjusted VOC setpoint, an exhaust CFM measurement, and a supply CFM measurement. The room balance controller  1722  determines control signal outputs for an exhaust valve and a supply valve (e.g., the valve  206   a  and the valve  206   b ) and provides the control signals to the exhaust valve and the supply valve, respectively, so that the exhaust valve and the supply valve operate to achieve the adjusted VOC setpoint. It should be understood that the VOC setpoint  1704  and the adjusted VOC setpoint are setpoints for a flowrate differential between air entering and leaving (e.g., exhaust and supply) a space or a room that the controller which implements the functionality of the diagram  1700  serves. 
     Referring now to  FIG. 19 , a diagram  1900  of a configuration of the pressure differential sensor  234  is shown, according to some embodiments. The pressure differential sensor  234  can be provided between the space  216  and the reference space  222 . In this way, the pressure differential sensor  234  can be configured to measure both the space pressure p space  of the space  216  and the reference pressure p ref  of the reference space  222  to measure the actual pressure differential Δp actual  therebetween. The pressure differential sensor  234  can include the pressure sensor  224  and the pressure sensor  226 , or may include probes for sampling and measuring air in both the space  216  and the reference space  222 . In some embodiments, the space  216  is selectably fluidly coupled with the reference space  222  (e.g., through a door). The space  216  may be pressurized to a pressure that is greater than the reference space  222  and can be substantially sealed relative to the reference space  222  when the door is closed. 
     Referring now to  FIG. 14 , a flow diagram of a process  400  for operating an HVAC system according to a VOC control scheme is shown, according to some embodiments. The process  400  includes steps  402 - 412 , and can be performed, at least in part, using the control system  300  or the controller  208 . 
     Process  400  includes providing a space that is provided with input air through a first valve, where air exits the space through a second valve (step  402 ), according to some embodiments. The first valve may be the first valve  206   a  and the second valve may be the second valve  206   b . The first valve operate in conjunction with an AHU to provide air into the space, while the second valve may operate to control a rate at which air leaves the space. 
     Process  400  includes determining an offset flowrate setpoint (step  404 ), according to some embodiments. In some embodiments, the offset flowrate setpoint is a desired difference between a flowrate of air that enters the space and a flowrate of air that leaves the space. For example, the offset flowrate setpoint can be the volumetric offset setpoint Δcfm set  as described in greater detail above with reference to  FIG. 13 . The offset flowrate setpoint can be provided as a user input or can be stored in memory of the controller  208 . Step  404  can be performed by the VOC controller  310  of the controller  208 . 
     Process  400  includes obtaining a pressure differential from both the first valve and the second valve (step  406 ), according to some embodiments. The pressure differentials can be pressure differentials of different positions along the first valve and the second valve, respectively. In some embodiments, the pressure differentials are provided to the controller  208  from the first valve  206   a  and the second valve  206   b . The pressure differential of each of the first valve  206   a  and the second valve  206   b  can be obtained using any of the techniques described in greater detail above with reference to the air duct assembly  1 . Step  406  can be performed by the flowrate estimator  308  of the controller  208 . In some embodiments, step  406  is performed by controllers of the first valve and the second valve. For example, step  406  can be performed by the first controller  232   a  of the first valve  206   a  and the second controller  232   b  of the second valve  206   b.    
     Process  400  includes determining a flowrate of air entering the space and a flowrate of air exiting the space based on the pressure differentials obtained from the first valve and the second valve (step  408 ), according to some embodiments. In some embodiments, step  408  is performed by the first controller  232   a  and the second controller  232   b . The first controller  232   a  and the second controller  232   b  can use the pressure differential obtained from the first valve and the pressure differential obtained from the second valve, and Bernoulli&#39;s equation or technique to determine the flowrates (e.g., in cfm) of air entering and leaving the space. Steps  406 - 408  can optionally be performed by the controller  208  (e.g., by the flow rate estimator  308 ) or may be performed by the first controller  232   a  and the second controller  232   b . In some embodiments, steps  406 - 408  are performed by the first controller  232   a  and the second controller  232   b  and step  409  is performed by the controller  208  (or more specifically, by the flow rate estimator  308 ). 
     Process  400  includes obtaining a flowrate of air entering the space and a flowrate of air exiting the space (step  409 ), according to some embodiments. The flowrate of air entering the space may be the supply measured airflow, while the flowrate of air exiting the space may be the exhaust measured airflow. Step  409  can be performed by the controller  208 , or more specifically, by the flow rate estimator  308 . The flowrate of air entering the space and the flowrate of air exiting the space may be the flowrates determined in step  408  by the first controller  232   a  and the second controller  232   b.    
     Process  400  includes determining an actual offset flowrate between the flowrate of air entering the space and the flowrate of air exiting the space (step  410 ), according to some embodiments. Step  410  can be performed by the VOC controller  310  of the controller  208 . In some embodiments, the actual offset flowrate is a difference between the flowrate of air entering the space and the flowrate of air exiting the space (e.g., as obtained in step  409 ). 
     Process  400  includes adjusting operation of at least one of the first valve or the second valve while monitoring the actual offset flowrate to drive the actual offset flowrate to the offset flowrate setpoint (step  412 ), according to some embodiments. In some embodiments, step  412  is performed by the VOC controller  310 . Step  412  can include performing a closed-loop control scheme to drive the actual offset flowrate towards or to be substantially equal to the offset flowrate setpoint by generating control signals for the second valve to adjust a damper of the second valve, thereby adjusting flowrate of the air exiting the space. Step  412  can be performed by the VOC controller  310 , the first controller  232   a , and the second controller  232   b.    
     Referring now to  FIG. 15 , a process  500  for performing pressure based control of a space is shown, according to some embodiments. Process  500  can include steps  502 - 512  and can be performed by the controller  208 , or more specifically, by the pressure controller  314  of the controller  208 . 
     Process  500  includes providing a system for a space that includes a first valve through which air enters the space, a second valve through which air leaves the space, a reference pressure sensor, and a space pressure sensor (step  502 ), according to some embodiments. In some embodiments, the first valve is the first valve  206   a , the second valve is the second valve  206   b , the reference pressure sensor is the pressure sensor  224 , and the space pressure sensor is the pressure sensor  226 . The reference pressure sensor can be configured to read or monitor a pressure of a reference space, while the space pressure sensor is configured to read or monitor a pressure of the space that the system operates to affect. In some embodiments, the reference pressure sensor and the space pressure sensor are provided as a single pressure differential sensor that is configured to measure a pressure differential between a reference pressure and a space pressure. 
     Process  500  includes determining a pressure differential setpoint (step  504 ), according to some embodiments. In some embodiments, step  504  is performed by the pressure controller  314 . In some embodiments, the pressure differential setpoint is a desired pressure differential between the space and the reference space. The pressure differential setpoint can be set by a user, an occupant (e.g., through a wall-mounted thermostat or unit), or may be a predetermined value. The pressure differential setpoint may be used by the pressure controller  314  in a closed loop pressure based control scheme. 
     Process  500  includes determining if a door is opened (step  506 ), according to some embodiments. In some embodiments, step  506  is determined based on a door status obtained by the controller  208  from a door sensor (e.g., the door sensor  228 ). The door may fluidly couple the space with the reference space when opened. In response to the door being detected as open (step  506 , “YES”), process  500  returns to step  506  and waits for the door to close. In response to the door being detected as closed (step  506 , “NO”), process  500  proceeds to step  508 . Pressure differentials obtained from the pressure differential sensor can still be read and displayed, but do not need to be acted upon or used for control. 
     Process  500  includes delaying a predetermined amount of time (step  508 ) once the door is closed (step  506 , “NO”), according to some embodiments. In some embodiments, delaying the predetermined amount of time prevents the system from going into overdrive. Step  508  can be performed by the pressure controller  314 . 
     Process  500  includes determining an actual pressure differential between the reference pressure sensor and the space pressure sensor (step  510 ), according to some embodiments. In some embodiments, step  510  includes determining a difference between the space pressure and the reference pressure (e.g., real-time sensor data). Step  510  can be performed by the pressure controller  314  based on the sensor data obtained from the pressure sensor  224  and the pressure sensor  226 . 
     Process  500  includes adjusting operation of at least one of the first valve or the second valve while monitoring the actual pressure differential (e.g., as obtained in step  510 ) to drive the actual pressure differential to the pressure differential setpoint (step  512 ), according to some embodiments. In some embodiments, step  512  is performed by the pressure controller  514  using a closed loop pressure based control scheme. Step  512  can include adjusting the second valve to increase or decrease a flowrate of air leaving the space to drive the actual pressure differential toward the pressure differential setpoint. Step  512  can be performed by the pressure controller  314 , the first controller  232   a , and the second controller  232   b.    
     Referring now to  FIG. 16 , a flow diagram of a process  600  for performing a hybrid control scheme is shown, according to some embodiments. Process  600  can include steps  602 - 618  and may be performed by the hybrid controller  312  of the controller  208 . Process  600  may integrate functionality of both the VOC controller  310  (or process  400 ) and the pressure controller  314  (or process  500 ) in a hybrid control scheme. Process  600  can be performed by the control system  300 , or more particularly, by the hybrid controller  312 . 
     Process  600  includes providing a system for a space that includes a first valve through which air enters the space, a second valve through which air leaves the space, a reference pressure sensor, and a space pressure sensor (step  602 ), according to some embodiments. Step  602  can be the same as or similar to step  502  of process  500 . Process  600  also includes determining an actual pressure differential between the reference pressure sensor and the space pressure sensor (step  604 ), according to some embodiments. In some embodiments, step  604  is the same as or similar to step  510  of process  500 . The reference pressure sensor and the space pressure sensor can be provided as a single pressure differential sensor (e.g., the pressure differential sensor  234 ). 
     Process  600  also includes determining an offset flowrate setpoint (step  606 ), obtaining a flowrate from both the first valve and the second valve (step  608 ), determining an actual flowrate between the flowrate of air entering the space and the flowrate of air exiting the space (step  610 ), and adjusting operation of at least one of the first valve or the second valve while monitoring the actual offset flowrate to drive the actual offset flowrate to the offset flowrate setpoint (step  612 ), according to some embodiments. In some embodiments, steps  606 - 614  are the same as or similar to steps  404 - 412  of process  400 . 
     Process  600  includes determining if the actual pressure differential is within a pressure deadband (step  614 ), according to some embodiments. Step  614  can be performed in real-time or concurrently with any of steps  606 - 612 . In some embodiments, the actual pressure differential is the pressure differential between the space pressure and the reference pressure as determined in step  604 . If the actual pressure differential is within the deadband (step  614 , “YES”), process  600  proceeds to step  616  and continues operating the valves. If the actual pressure differential is not within the deadband (step  614 , “NO”), process  600  proceeds to step  616 . 
     Process  600  includes updating the offset flowrate with a new offset flowrate setpoint (step  616 ) in response to the actual pressure differential not being within the deadband (step  614 , “NO”), according to some embodiments. In some embodiments, step  616  is performed by the pressure controller  314 . In some embodiments, after performing step  616 , process  600  returns to step  602  and performs steps  602 - 612  using the new offset flowrate setpoint. 
     Referring now to  FIG. 18 , a portion of the air duct assembly  1  is shown in greater detail, according to some embodiments. Specifically,  FIG. 18  shows the damper assembly  50  in greater detail. The damper assembly  50  can be driven by the actuator  104  to rotate about an axis  56  defined by a shaft  54  of the damper assembly  50 . The damper assembly  50  includes a structural member  52  (e.g., a disk) that is fixedly coupled (e.g., fastened) with the shaft  54 . The damper assembly  50  also includes multiple fingers, radially extending members, rubber members, sealing members, etc., shown as fingers  72 . The fingers  72  are fixedly coupled at an outer edge of the structural member  52  along both a leading edge and a trailing edge of the structural member  52 . The fingers  72  can have a generally arcuate or curved shape and each protrude a varying distance  58  from the outer edge of the structural member  52 . The distance  58  of the fingers  72  may increase and then decrease along the outer edge of the structural member  52 , with a finger  72  that is orthogonal or perpendicular with the axis  56  having a greatest distance  58 . Adjacent fingers  72  may define a space  74  therebetween. 
     When the damper assembly  50  is driven to rotate about the axis  56 , the fingers  72  on the leading edge may be driven into contact with the interior wall  4  at a leading position of the interior wall  4 , and the fingers  72  on the leading edge may be driven into contact with the interior wall  4  at a trailing position of the interior wall  4 . The damper assembly  50  can be driven to rotate about the axis  56  until the fingers  72  on the leading edge and the trailing edge of the structural member  52  fully engage the interior wall  4  (e.g., thereby limiting or preventing airflow through the air duct assembly  1  and substantially “shutting” the air duct assembly  1 ). The damper assembly  50  can also be driven to rotate about the axis  56  to an intermediate position so that the fingers  72  only partially engage the interior wall  4  of the air duct assembly  1 . When the damper assembly  50  is driven to the intermediate position so that the fingers  72  only partially engage the interior wall  4 , the spaces  74  therebetween the fingers  72  can allow air to flow between the fingers  72 . Rotation of the damper assembly  50  incremental amounts can cause the spaces  74  between the fingers  72  to increase or decrease, thereby adjusting a cross-sectional area through which the air flows. In this way, rotation of the damper assembly  50  can result in changes of flowrate through the air duct assembly  1 . 
     As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. 
     “Optional’ or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. 
     Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising’ and “comprises,” means “including but not limited to,” and is not intended to exclude, for example, other additives, components, integers or steps. “Exemplar” means “an example of” and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, bur for explanatory purposes. 
     Disclosed are components that can be used to perform the disclosed methods, equipment and systems. These and other components are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc., of these components are disclosed that while specific reference of each various individual and collective combination and permutation of these may not be explicitly disclosed, each is specifically contemplated and described herein, for all methods, equipment and systems. This applies to all aspects of this application including, but not limited to, steps in disclosed methods. Thus, if there ae a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the disclosed methods. 
     It should further be noted that any patents, applications and publications referred to herein are incorporated by reference in their entirety.