Patent Publication Number: US-11644215-B2

Title: Systems and methods for flow control in an HVAC system

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/447,813 filed Jun. 20, 2019, the entire disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     The present disclosure relates generally to building control systems and more particularly to the field of building management systems. A building management system (BMS) is, in general, a system of devices configured to control, monitor, and manage equipment in or around a building or building area. A BMS can include, for example, an HVAC system, a security system, a lighting system, a fire alerting system, any other system that is capable of managing building functions or devices, or any combination thereof. 
     A BMS and associated devices may be responsible for controlling flow of fluid in an HVAC system. For example, heated or chilled fluid may be provided through a heating or cooling coil to provide heating or air conditioning for a building space. Some previous systems and methods for controlling flow may operate inefficiently and waste energy. Systems and methods that can limit energy waste are generally desired. 
     SUMMARY 
     One implementation of the present disclosure is a method for operating a valve that controls flow of liquid through a coil in an HVAC system. The method includes receiving a first temperature measurement associated with an inlet of the coil, receiving a second temperature measurement associated with an outlet of the coil, calculating a difference between the first temperature measurement and the second temperature measurement, determining that the difference between the first temperature measurement and the second temperature measurement is below a threshold, and adjusting a setpoint associated with the valve. 
     Another implementation of the present disclosure is an HVAC system. The HVAC system includes a coil that facilitates heating or cooling, a valve that controls flow of a liquid through the coil, a pump that provides the liquid at an inlet of the valve, an actuator that controls a position of the valve, and a controller with a processor and a memory. The memory of the controller includes a control application that, when executed by the controller, causes the controller to receive a first temperature measurement associated with an inlet of the coil, receive a second temperature measurement associated with an outlet of the coil, calculate a difference between the first temperature measurement and the second temperature measurement, determine that the difference between the first temperature measurement and the second temperature measurement is below a threshold, and adjust a setpoint associated with the valve. 
     Yet another implementation of the present disclosure is a flow control device for use in an HVAC system. The device includes a valve that controls flow of a liquid through a coil and an actuator that controls a position of the valve. The actuator includes a processor and a memory. The memory of the actuator includes a control application that, when executed by the actuator, causes the actuator to receive a first temperature measurement associated with an inlet of the coil, receive a second temperature measurement associated with an outlet of the coil, calculate a difference between the first temperature measurement and the second temperature measurement, determine that the difference between the first temperature measurement and the second temperature measurement is below a threshold, and adjust a setpoint associated with the valve. 
     Those skilled in the art will appreciate this 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 
         FIG.  1    is a drawing of a building equipped with a building management system (BMS) and an HVAC system, according to some embodiments. 
         FIG.  2    is a schematic of a waterside system which can be used as part of the HVAC system of  FIG.  1   , according to some embodiments. 
         FIG.  3    is a block diagram of an airside system which can be used as part of the HVAC system of  FIG.  1   , according to some embodiments. 
         FIG.  4    is a block diagram of a BMS which can be used in the building of  FIG.  1   , according to some embodiments. 
         FIG.  5    is a block diagram of an example flow control system associated with the BMS of  FIG.  4   , according to some embodiments. 
         FIG.  6    is a flow diagram of a flow control process associated with the example system of  FIG.  5   , according to some embodiments. 
         FIG.  7    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    without using a pulse generation feature, according to some embodiments. 
         FIG.  8    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    using a pulse generation feature, according to some embodiments. 
         FIG.  9    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    without using a change-limiting feature, according to some embodiments. 
         FIG.  10    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    using a change-limiting feature is shown, according to some embodiments. 
         FIG.  11    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    without using a reevaluation feature, according to some embodiments. 
         FIG.  12    is a series of graphs showing behavior of a system that attempts to impose a limit on the temperature change across a coil associated with the system of  FIG.  5    using a reevaluation feature, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Referring generally to the FIGURES, systems and methods for flow control in an HVAC system are shown, according to some embodiments. The systems and methods described herein are used to maintain a desired temperature change across a heating or cooling coil. This functionality drives energy savings and improved performance of the flow control system and HVAC system as a whole. A control application is configured to adjust a setpoint based on a temperature difference between an inlet and an outlet of a heating or cooling coil. Moreover, various features can be added to the control application to improve performance of the control system. These features may include one or more of a pulse generation feature, a change-limiting feature, and a reevaluation feature. 
     Building Management System 
     Referring now to  FIGS.  1 - 4   , an example building management system (BMS) and HVAC system in which the systems and methods of the present disclosure can be implemented are shown, according to an example embodiment. Referring particularly to  FIG.  1   , a perspective view of a building  10  is shown. Building  10  is served by a BMS. A BMS is, in general, a system of devices configured to control, monitor, and manage equipment in or around a building or building area. A BMS can include, for example, an HVAC system, a security system, a lighting system, a fire alerting system, any other system that is capable of managing building functions or devices, or any combination thereof. 
     The BMS that serves building  10  includes an HVAC system  100 . HVAC system  100  can include a plurality of HVAC devices (e.g., heaters, chillers, air handling units, pumps, fans, thermal energy storage, etc.) configured to provide heating, cooling, ventilation, or other services for building  10 . For example, HVAC system  100  is shown to include a waterside system  120  and an airside system  130 . Waterside system  120  can provide a heated or chilled fluid to an air handling unit of airside system  130 . Airside system  130  can use the heated or chilled fluid to heat or cool an airflow provided to building  10 . An example waterside system and airside system which can be used in HVAC system  100  are described in greater detail with reference to  FIGS.  2  and  3   . 
     HVAC system  100  is shown to include a chiller  102 , a boiler  104 , and a rooftop air handling unit (AHU)  106 . Waterside system  120  can use boiler  104  and chiller  102  to heat or cool a working fluid (e.g., water, glycol, etc.) and can circulate the working fluid to AHU  106 . In various embodiments, the HVAC devices of waterside system  120  can be located in or around building  10  (as shown in  FIG.  1   ) or at an offsite location such as a central plant (e.g., a chiller plant, a steam plant, a heat plant, etc.). The working fluid can be heated in boiler  104  or cooled in chiller  102 , depending on whether heating or cooling is required in building  10 . Boiler  104  can add heat to the circulated fluid, for example, by burning a combustible material (e.g., natural gas) or using an electric heating element. Chiller  102  can place the circulated fluid in a heat exchange relationship with another fluid (e.g., a refrigerant) in a heat exchanger (e.g., an evaporator) to absorb heat from the circulated fluid. The working fluid from chiller  102  and/or boiler  104  can be transported to AHU  106  via piping  108 . 
     AHU  106  can place the working fluid in a heat exchange relationship with an airflow passing through AHU  106  (e.g., via one or more stages of cooling coils and/or heating coils). The airflow can be, for example, outside air, return air from within building  10 , or a combination of both. AHU  106  can transfer heat between the airflow and the working fluid to provide heating or cooling for the airflow. For example, AHU  106  can include one or more fans or blowers configured to pass the airflow over or through a heat exchanger containing the working fluid. The working fluid can then return to chiller  102  or boiler  104  via piping  110 . 
     Airside system  130  can deliver the airflow supplied by AHU  106  (i.e., the supply airflow) to building  10  via air supply ducts  112  and can provide return air from building  10  to AHU  106  via air return ducts  114 . In some embodiments, airside system  130  includes multiple variable air volume (VAV) units  116 . For example, airside system  130  is shown to include a separate VAV unit  116  on each floor or zone of building  10 . VAV units  116  can include dampers or other flow control elements that can be operated to control an amount of the supply airflow provided to individual zones of building  10 . In other embodiments, airside system  130  delivers the supply airflow into one or more zones of building  10  (e.g., via supply ducts  112 ) without using intermediate VAV units  116  or other flow control elements. AHU  106  can include various sensors (e.g., temperature sensors, pressure sensors, etc.) configured to measure attributes of the supply airflow. AHU  106  can receive input from sensors located within AHU  106  and/or within the building zone and can adjust the flow rate, temperature, or other attributes of the supply airflow through AHU  106  to achieve setpoint conditions for the building zone. 
     Referring now to  FIG.  2   , a block diagram of a waterside system  200  is shown, according to an example embodiment. In various embodiments, waterside system  200  can supplement or replace waterside system  120  in HVAC system  100  or can be implemented separate from HVAC system  100 . When implemented in HVAC system  100 , waterside system  200  can include a subset of the HVAC devices in HVAC system  100  (e.g., boiler  104 , chiller  102 , pumps, valves, etc.) and can operate to supply a heated or chilled fluid to AHU  106 . The HVAC devices of waterside system  200  can be located within building  10  (e.g., as components of waterside system  120 ) or at an offsite location such as a central plant. 
     In  FIG.  2   , waterside system  200  is shown as a central plant having a plurality of subplants  202 - 212 . Subplants  202 - 212  are shown to include a heater subplant  202 , a heat recovery chiller subplant  204 , a chiller subplant  206 , a cooling tower subplant  208 , a hot thermal energy storage (TES) subplant  210 , and a cold thermal energy storage (TES) subplant  212 . Subplants  202 - 212  consume resources (e.g., water, natural gas, electricity, etc.) from utilities to serve the thermal energy loads (e.g., hot water, cold water, heating, cooling, etc.) of a building or campus. For example, heater subplant  202  can be configured to heat water in a hot water loop  214  that circulates the hot water between heater subplant  202  and building  10 . Chiller subplant  206  can be configured to chill water in a cold water loop  216  that circulates the cold water between chiller subplant  206  building  10 . Heat recovery chiller subplant  204  can be configured to transfer heat from cold water loop  216  to hot water loop  214  to provide additional heating for the hot water and additional cooling for the cold water. Condenser water loop  218  can absorb heat from the cold water in chiller subplant  206  and reject the absorbed heat in cooling tower subplant  208  or transfer the absorbed heat to hot water loop  214 . Hot TES subplant  210  and cold TES subplant  212  can store hot and cold thermal energy, respectively, for subsequent use. 
     Hot water loop  214  and cold water loop  216  can deliver the heated and/or chilled water to air handlers located on the rooftop of building  10  (e.g., AHU  106 ) or to individual floors or zones of building  10  (e.g., VAV units  116 ). The air handlers push air past heat exchangers (e.g., heating coils or cooling coils) through which the water flows to provide heating or cooling for the air. The heated or cooled air can be delivered to individual zones of building  10  to serve the thermal energy loads of building  10 . The water then returns to subplants  202 - 212  to receive further heating or cooling. 
     Although subplants  202 - 212  are shown and described as heating and cooling water for circulation to a building, it is understood that any other type of working fluid (e.g., glycol, CO2, etc.) can be used in place of or in addition to water to serve the thermal energy loads. In other embodiments, subplants  202 - 212  can provide heating and/or cooling directly to the building or campus without requiring an intermediate heat transfer fluid. These and other variations to waterside system  200  are within the teachings of the present invention. 
     Each of subplants  202 - 212  can include a variety of equipment configured to facilitate the functions of the subplant. For example, heater subplant  202  is shown to include a plurality of heating elements  220  (e.g., boilers, electric heaters, etc.) configured to add heat to the hot water in hot water loop  214 . Heater subplant  202  is also shown to include several pumps  222  and  224  configured to circulate the hot water in hot water loop  214  and to control the flow rate of the hot water through individual heating elements  220 . Chiller subplant  206  is shown to include a plurality of chillers  232  configured to remove heat from the cold water in cold water loop  216 . Chiller subplant  206  is also shown to include several pumps  234  and  236  configured to circulate the cold water in cold water loop  216  and to control the flow rate of the cold water through individual chillers  232 . 
     Heat recovery chiller subplant  204  is shown to include a plurality of heat recovery heat exchangers  226  (e.g., refrigeration circuits) configured to transfer heat from cold water loop  216  to hot water loop  214 . Heat recovery chiller subplant  204  is also shown to include several pumps  228  and  230  configured to circulate the hot water and/or cold water through heat recovery heat exchangers  226  and to control the flow rate of the water through individual heat recovery heat exchangers  226 . Cooling tower subplant  208  is shown to include a plurality of cooling towers  238  configured to remove heat from the condenser water in condenser water loop  218 . Cooling tower subplant  208  is also shown to include several pumps  240  configured to circulate the condenser water in condenser water loop  218  and to control the flow rate of the condenser water through individual cooling towers  238 . 
     Hot TES subplant  210  is shown to include a hot TES tank  242  configured to store the hot water for later use. Hot TES subplant  210  can also include one or more pumps or valves configured to control the flow rate of the hot water into or out of hot TES tank  242 . Cold TES subplant  212  is shown to include cold TES tanks  244  configured to store the cold water for later use. Cold TES subplant  212  can also include one or more pumps or valves configured to control the flow rate of the cold water into or out of cold TES tanks  244 . 
     In some embodiments, one or more of the pumps in waterside system  200  (e.g., pumps  222 ,  224 ,  228 ,  230 ,  234 ,  236 , and/or  240 ) or pipelines in waterside system  200  include an isolation valve associated therewith. Isolation valves can be integrated with the pumps or positioned upstream or downstream of the pumps to control the fluid flows in waterside system  200 . In various embodiments, waterside system  200  can include more, fewer, or different types of devices and/or subplants based on the particular configuration of waterside system  200  and the types of loads served by waterside system  200 . 
     Referring now to  FIG.  3   , a block diagram of an airside system  300  is shown, according to an example embodiment. In various embodiments, airside system  300  can supplement or replace airside system  130  in HVAC system  100  or can be implemented separate from HVAC system  100 . When implemented in HVAC system  100 , airside system  300  can include a subset of the HVAC devices in HVAC system  100  (e.g., AHU  106 , VAV units  116 , duct  112 , duct  114 , fans, dampers, etc.) and can be located in or around building  10 . Airside system  300  can operate to heat or cool an airflow provided to building  10  using a heated or chilled fluid provided by waterside system  200 . 
     In  FIG.  3   , airside system  300  is shown to include an economizer-type air handling unit (AHU)  302 . Economizer-type AHUs vary the amount of outside air and return air used by the air handling unit for heating or cooling. For example, AHU  302  can receive return air  304  from building zone  306  via return air duct  308  and can deliver supply air  310  to building zone  306  via supply air duct  312 . In some embodiments, AHU  302  is a rooftop unit located on the roof of building  10  (e.g., AHU  106  as shown in  FIG.  1   ) or otherwise positioned to receive both return air  304  and outside air  314 . AHU  302  can be configured to operate exhaust air damper  316 , mixing damper  318 , and outside air damper  320  to control an amount of outside air  314  and return air  304  that combine to form supply air  310 . Any return air  304  that does not pass through mixing damper  318  can be exhausted from AHU  302  through exhaust damper  316  as exhaust air  322 . 
     Each of dampers  316 - 320  can be operated by an actuator. For example, exhaust air damper  316  can be operated by actuator  324 , mixing damper  318  can be operated by actuator  326 , and outside air damper  320  can be operated by actuator  328 . Actuators  324 - 328  can communicate with an AHU controller  330  via a communications link  332 . Actuators  324 - 328  can receive control signals from AHU controller  330  and can provide feedback signals to AHU controller  330 . Feedback signals can include, for example, an indication of a current actuator or damper position, an amount of torque or force exerted by the actuator, diagnostic information (e.g., results of diagnostic tests performed by actuators  324 - 328 ), status information, commissioning information, configuration settings, calibration data, and/or other types of information or data that can be collected, stored, or used by actuators  324 - 328 . AHU controller  330  can be an economizer controller configured to use one or more control algorithms (e.g., state-based algorithms, extremum seeking control (ESC) algorithms, proportional-integral (PI) control algorithms, proportional-integral-derivative (PID) control algorithms, model predictive control (MPC) algorithms, feedback control algorithms, etc.) to control actuators  324 - 328 . 
     Still referring to  FIG.  3   , AHU  302  is shown to include a cooling coil  334 , a heating coil  336 , and a fan  338  positioned within supply air duct  312 . Fan  338  can be configured to force supply air  310  through cooling coil  334  and/or heating coil  336  and provide supply air  310  to building zone  306 . AHU controller  330  can communicate with fan  338  via communications link  340  to control a flow rate of supply air  310 . In some embodiments, AHU controller  330  controls an amount of heating or cooling applied to supply air  310  by modulating a speed of fan  338 . 
     Cooling coil  334  can receive a chilled fluid from waterside system  200  (e.g., from cold water loop  216 ) via piping  342  and can return the chilled fluid to waterside system  200  via piping  344 . Valve  346  can be positioned along piping  342  or piping  344  to control a flow rate of the chilled fluid through cooling coil  334 . In some embodiments, cooling coil  334  includes multiple stages of cooling coils that can be independently activated and deactivated (e.g., by AHU controller  330 , by BMS controller  366 , etc.) to modulate an amount of cooling applied to supply air  310 . 
     Heating coil  336  can receive a heated fluid from waterside system  200  (e.g., from hot water loop  214 ) via piping  348  and can return the heated fluid to waterside system  200  via piping  350 . Valve  352  can be positioned along piping  348  or piping  350  to control a flow rate of the heated fluid through heating coil  336 . In some embodiments, heating coil  336  includes multiple stages of heating coils that can be independently activated and deactivated (e.g., by AHU controller  330 , by BMS controller  366 , etc.) to modulate an amount of heating applied to supply air  310 . 
     Each of valves  346  and  352  can be controlled by an actuator. For example, valve  346  can be controlled by actuator  354  and valve  352  can be controlled by actuator  356 . Actuators  354 - 356  can communicate with AHU controller  330  via communications links  358 - 360 . Actuators  354 - 356  can receive control signals from AHU controller  330  and can provide feedback signals to controller  330 . In some embodiments, AHU controller  330  receives a measurement of the supply air temperature from a temperature sensor  362  positioned in supply air duct  312  (e.g., downstream of cooling coil  334  and/or heating coil  336 ). AHU controller  330  can also receive a measurement of the temperature of building zone  306  from a temperature sensor  364  located in building zone  306 . 
     In some embodiments, AHU controller  330  operates valves  346  and  352  via actuators  354 - 356  to modulate an amount of heating or cooling provided to supply air  310  (e.g., to achieve a setpoint temperature for supply air  310  or to maintain the temperature of supply air  310  within a setpoint temperature range). The positions of valves  346  and  352  affect the amount of heating or cooling provided to supply air  310  by cooling coil  334  or heating coil  336  and may correlate with the amount of energy consumed to achieve a desired supply air temperature. AHU controller  330  can control the temperature of supply air  310  and/or building zone  306  by activating or deactivating coils  334 - 336 , adjusting a speed of fan  338 , or a combination of both. 
     Still referring to  FIG.  3   , airside system  300  is shown to include a building management system (BMS) controller  366  and a client device  368 . BMS controller  366  can include one or more computer systems (e.g., servers, supervisory controllers, subsystem controllers, etc.) that serve as system level controllers, application or data servers, head nodes, or master controllers for airside system  300 , waterside system  200 , HVAC system  100 , and/or other controllable systems that serve building  10 . BMS controller  366  can communicate with multiple downstream building systems or subsystems (e.g., HVAC system  100 , a security system, a lighting system, waterside system  200 , etc.) via a communications link  370  according to like or disparate protocols (e.g., LON, BACnet, etc.). In various embodiments, AHU controller  330  and BMS controller  366  can be separate (as shown in  FIG.  3   ) or integrated. In an integrated implementation, AHU controller  330  can be a software module configured for execution by a processor of BMS controller  366 . 
     In some embodiments, AHU controller  330  receives information from BMS controller  366  (e.g., commands, setpoints, operating boundaries, etc.) and provides information to BMS controller  366  (e.g., temperature measurements, valve or actuator positions, operating statuses, diagnostics, etc.). For example, AHU controller  330  can provide BMS controller  366  with temperature measurements from temperature sensors  362  and  364 , equipment on/off states, equipment operating capacities, and/or any other information that can be used by BMS controller  366  to monitor or control a variable state or condition within building zone  306 . 
     Client device  368  can include one or more human-machine interfaces or client interfaces (e.g., graphical user interfaces, reporting interfaces, text-based computer interfaces, client-facing web services, web servers that provide pages to web clients, etc.) for controlling, viewing, or otherwise interacting with HVAC system  100 , its subsystems, and/or devices. Client device  368  can be a computer workstation, a client terminal, a remote or local interface, or any other type of user interface device. Client device  368  can be a stationary terminal or a mobile device. For example, client device  368  can be a desktop computer, a computer server with a user interface, a laptop computer, a tablet, a smartphone, a PDA, or any other type of mobile or non-mobile device. Client device  368  can communicate with BMS controller  366  and/or AHU controller  330  via communications link  372 . 
     Referring now to  FIG.  4   , a block diagram of a building management system (BMS)  400  is shown, according to an example embodiment. BMS  400  can be implemented in building  10  to automatically monitor and control various building functions. BMS  400  is shown to include BMS controller  366  and a plurality of building subsystems  428 . Building subsystems  428  are shown to include a building electrical subsystem  434 , an information communication technology (ICT) subsystem  436 , a security subsystem  438 , a HVAC subsystem  440 , a lighting subsystem  442 , a lift/escalators subsystem  432 , and a fire safety subsystem  430 . In various embodiments, building subsystems  428  can include fewer, additional, or alternative subsystems. For example, building subsystems  428  can also or alternatively include a refrigeration subsystem, an advertising or signage subsystem, a cooking subsystem, a vending subsystem, a printer or copy service subsystem, or any other type of building subsystem that uses controllable equipment and/or sensors to monitor or control building  10 . In some embodiments, building subsystems  428  include waterside system  200  and/or airside system  300 , as described with reference to  FIGS.  2  and  3   . 
     Each of building subsystems  428  can include any number of devices, controllers, and connections for completing its individual functions and control activities. HVAC subsystem  440  can include many of the same components as HVAC system  100 , as described with reference to  FIGS.  1 - 3   . For example, HVAC subsystem  440  can include a chiller, a boiler, any number of air handling units, economizers, field controllers, supervisory controllers, actuators, temperature sensors, and other devices for controlling the temperature, humidity, airflow, or other variable conditions within building  10 . Lighting subsystem  442  can include any number of light fixtures, ballasts, lighting sensors, dimmers, or other devices configured to controllably adjust the amount of light provided to a building space. Security subsystem  438  can include occupancy sensors, video surveillance cameras, digital video recorders, video processing servers, intrusion detection devices, access control devices (e.g., card access, etc.) and servers, or other security-related devices. 
     Still referring to  FIG.  4   , BMS controller  366  is shown to include a communications interface  407  and a BMS interface  409 . Interface  407  can facilitate communications between BMS controller  366  and external applications (e.g., monitoring and reporting applications  422 , enterprise control applications  426 , remote systems and applications  444 , applications residing on client devices  448 , etc.) for allowing user control, monitoring, and adjustment to BMS controller  366  and/or subsystems  428 . Interface  407  can also facilitate communications between BMS controller  366  and client devices  448 . BMS interface  409  can facilitate communications between BMS controller  366  and building subsystems  428  (e.g., HVAC, lighting security, lifts, power distribution, business, etc.). 
     Interfaces  407 ,  409  can be or include wired or wireless communications interfaces (e.g., jacks, antennas, transmitters, receivers, transceivers, wire terminals, etc.) for conducting data communications with building subsystems  428  or other external systems or devices. In various embodiments, communications via interfaces  407 ,  409  can be direct (e.g., local wired or wireless communications) or via a communications network  446  (e.g., a WAN, the Internet, a cellular network, etc.). For example, interfaces  407 ,  409  can include an Ethernet card and port for sending and receiving data via an Ethernet-based communications link or network. In another example, interfaces  407 ,  409  can include a Wi-Fi transceiver for communicating via a wireless communications network. In another example, one or both of interfaces  407 ,  409  can include cellular or mobile phone communications transceivers. In one embodiment, communications interface  407  is a power line communications interface and BMS interface  409  is an Ethernet interface. In other embodiments, both communications interface  407  and BMS interface  409  are Ethernet interfaces or are the same Ethernet interface. 
     Still referring to  FIG.  4   , BMS controller  366  is shown to include a processing circuit  404  including a processor  406  and memory  408 . Processing circuit  404  can be communicably connected to BMS interface  409  and/or communications interface  407  such that processing circuit  404  and the various components thereof can send and receive data via interfaces  407 ,  409 . Processor  406  can be implemented as a general purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), a group of processing components, or other suitable electronic processing components. 
     Memory  408  (e.g., memory, memory unit, storage device, etc.) can include one or more devices (e.g., RAM, ROM, Flash memory, hard disk storage, etc.) for storing data and/or computer code for completing or facilitating the various processes, layers and modules described in the present application. Memory  408  can be or include volatile memory or non-volatile memory. Memory  408  can include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described in the present application. According to an example embodiment, memory  408  is communicably connected to processor  406  via processing circuit  404  and includes computer code for executing (e.g., by processing circuit  404  and/or processor  406 ) one or more processes described herein. 
     In some embodiments, BMS controller  366  is implemented within a single computer (e.g., one server, one housing, etc.). In various other embodiments BMS controller  366  can be distributed across multiple servers or computers (e.g., that can exist in distributed locations). Further, while  FIG.  4    shows applications  422  and  426  as existing outside of BMS controller  366 , in some embodiments, applications  422  and  426  can be hosted within BMS controller  366  (e.g., within memory  408 ). 
     Still referring to  FIG.  4   , memory  408  is shown to include an enterprise integration layer  410 , an automated measurement and validation (AM&amp;V) layer  412 , a demand response (DR) layer  414 , a fault detection and diagnostics (FDD) layer  416 , an integrated control layer  418 , and a building subsystem integration later 420. Layers  410 - 420  can be configured to receive inputs from building subsystems  428  and other data sources, determine optimal control actions for building subsystems  428  based on the inputs, generate control signals based on the optimal control actions, and provide the generated control signals to building subsystems  428 . The following paragraphs describe some of the general functions performed by each of layers  410 - 420  in BMS  400 . 
     Enterprise integration layer  410  can be configured to serve clients or local applications with information and services to support a variety of enterprise-level applications. For example, enterprise control applications  426  can be configured to provide subsystem-spanning control to a graphical user interface (GUI) or to any number of enterprise-level business applications (e.g., accounting systems, user identification systems, etc.). Enterprise control applications  426  can also or alternatively be configured to provide configuration GUIs for configuring BMS controller  366 . In yet other embodiments, enterprise control applications  426  can work with layers  410 - 420  to optimize building performance (e.g., efficiency, energy use, comfort, or safety) based on inputs received at interface  407  and/or BMS interface  409 . 
     Building subsystem integration layer  420  can be configured to manage communications between BMS controller  366  and building subsystems  428 . For example, building subsystem integration layer  420  can receive sensor data and input signals from building subsystems  428  and provide output data and control signals to building subsystems  428 . Building subsystem integration layer  420  can also be configured to manage communications between building subsystems  428 . Building subsystem integration layer  420  translate communications (e.g., sensor data, input signals, output signals, etc.) across a plurality of multi-vendor/multi-protocol systems. 
     Demand response layer  414  can be configured to optimize resource usage (e.g., electricity use, natural gas use, water use, etc.) and/or the monetary cost of such resource usage in response to satisfy the demand of building  10 . The optimization can be based on time-of-use prices, curtailment signals, energy availability, or other data received from utility providers, distributed energy generation systems  424 , from energy storage  427  (e.g., hot TES  242 , cold TES  244 , etc.), or from other sources. Demand response layer  414  can receive inputs from other layers of BMS controller  366  (e.g., building subsystem integration layer  420 , integrated control layer  418 , etc.). The inputs received from other layers can include environmental or sensor inputs such as temperature, carbon dioxide levels, relative humidity levels, air quality sensor outputs, occupancy sensor outputs, room schedules, and the like. The inputs can also include inputs such as electrical use (e.g., expressed in kWh), thermal load measurements, pricing information, projected pricing, smoothed pricing, curtailment signals from utilities, and the like. 
     According to an example embodiment, demand response layer  414  includes control logic for responding to the data and signals it receives. These responses can include communicating with the control algorithms in integrated control layer  418 , changing control strategies, changing setpoints, or activating/deactivating building equipment or subsystems in a controlled manner. Demand response layer  414  can also include control logic configured to determine when to utilize stored energy. For example, demand response layer  414  can determine to begin using energy from energy storage  427  just prior to the beginning of a peak use hour. 
     In some embodiments, demand response layer  414  includes a control module configured to actively initiate control actions (e.g., automatically changing setpoints) which minimize energy costs based on one or more inputs representative of or based on demand (e.g., price, a curtailment signal, a demand level, etc.). In some embodiments, demand response layer  414  uses equipment models to determine an optimal set of control actions. The equipment models can include, for example, thermodynamic models describing the inputs, outputs, and/or functions performed by various sets of building equipment. Equipment models can represent collections of building equipment (e.g., subplants, chiller arrays, etc.) or individual devices (e.g., individual chillers, heaters, pumps, etc.). 
     Demand response layer  414  can further include or draw upon one or more demand response policy definitions (e.g., databases, XML files, etc.). The policy definitions can be edited or adjusted by a user (e.g., via a graphical user interface) so that the control actions initiated in response to demand inputs can be tailored for the user&#39;s application, desired comfort level, particular building equipment, or based on other concerns. For example, the demand response policy definitions can specify which equipment can be turned on or off in response to particular demand inputs, how long a system or piece of equipment should be turned off, what setpoints can be changed, what the allowable set point adjustment range is, how long to hold a high demand setpoint before returning to a normally scheduled setpoint, how close to approach capacity limits, which equipment modes to utilize, the energy transfer rates (e.g., the maximum rate, an alarm rate, other rate boundary information, etc.) into and out of energy storage devices (e.g., thermal storage tanks, battery banks, etc.), and when to dispatch on-site generation of energy (e.g., via fuel cells, a motor generator set, etc.). 
     Integrated control layer  418  can be configured to use the data input or output of building subsystem integration layer  420  and/or demand response layer  414  to make control decisions. Due to the subsystem integration provided by building subsystem integration layer  420 , integrated control layer  418  can integrate control activities of the subsystems  428  such that the subsystems  428  behave as a single integrated supersystem. In an example embodiment, integrated control layer  418  includes control logic that uses inputs and outputs from a plurality of building subsystems to provide greater comfort and energy savings relative to the comfort and energy savings that separate subsystems could provide alone. For example, integrated control layer  418  can be configured to use an input from a first subsystem to make an energy-saving control decision for a second subsystem. Results of these decisions can be communicated back to building subsystem integration layer  420 . 
     Integrated control layer  418  is shown to be logically below demand response layer  414 . Integrated control layer  418  can be configured to enhance the effectiveness of demand response layer  414  by enabling building subsystems  428  and their respective control loops to be controlled in coordination with demand response layer  414 . This configuration may advantageously reduce disruptive demand response behavior relative to conventional systems. For example, integrated control layer  418  can be configured to assure that a demand response-driven upward adjustment to the setpoint for chilled water temperature (or another component that directly or indirectly affects temperature) does not result in an increase in fan energy (or other energy used to cool a space) that would result in greater total building energy use than was saved at the chiller. 
     Integrated control layer  418  can be configured to provide feedback to demand response layer  414  so that demand response layer  414  checks that constraints (e.g., temperature, lighting levels, etc.) are properly maintained even while demanded load shedding is in progress. The constraints can also include setpoint or sensed boundaries relating to safety, equipment operating limits and performance, comfort, fire codes, electrical codes, energy codes, and the like. Integrated control layer  418  is also logically below fault detection and diagnostics layer  416  and automated measurement and validation layer  412 . Integrated control layer  418  can be configured to provide calculated inputs (e.g., aggregations) to these higher levels based on outputs from more than one building subsystem. 
     Automated measurement and validation (AM&amp;V) layer  412  can be configured to verify that control strategies commanded by integrated control layer  418  or demand response layer  414  are working properly (e.g., using data aggregated by AM&amp;V layer  412 , integrated control layer  418 , building subsystem integration layer  420 , FDD layer  416 , or otherwise). The calculations made by AM&amp;V layer  412  can be based on building system energy models and/or equipment models for individual BMS devices or subsystems. For example, AM&amp;V layer  412  can compare a model-predicted output with an actual output from building subsystems  428  to determine an accuracy of the model. 
     Fault detection and diagnostics (FDD) layer  416  can be configured to provide on-going fault detection for building subsystems  428 , building subsystem devices (i.e., building equipment), and control algorithms used by demand response layer  414  and integrated control layer  418 . FDD layer  416  can receive data inputs from integrated control layer  418 , directly from one or more building subsystems or devices, or from another data source. FDD layer  416  can automatically diagnose and respond to detected faults. The responses to detected or diagnosed faults can include providing an alert message to a user, a maintenance scheduling system, or a control algorithm configured to attempt to repair the fault or to work-around the fault. 
     FDD layer  416  can be configured to output a specific identification of the faulty component or cause of the fault (e.g., loose damper linkage) using detailed subsystem inputs available at building subsystem integration layer  420 . In other example embodiments, FDD layer  416  is configured to provide “fault” events to integrated control layer  418  which executes control strategies and policies in response to the received fault events. According to an example embodiment, FDD layer  416  (or a policy executed by an integrated control engine or business rules engine) can shut-down systems or direct control activities around faulty devices or systems to reduce energy waste, extend equipment life, or assure proper control response. 
     FDD layer  416  can be configured to store or access a variety of different system data stores (or data points for live data). FDD layer  416  can use some content of the data stores to identify faults at the equipment level (e.g., specific chiller, specific AHU, specific terminal unit, etc.) and other content to identify faults at component or subsystem levels. For example, building subsystems  428  can generate temporal (i.e., time-series) data indicating the performance of BMS  400  and the various components thereof. The data generated by building subsystems  428  can include measured or calculated values that exhibit statistical characteristics and provide information about how the corresponding system or process (e.g., a temperature control process, a flow control process, etc.) is performing in terms of error from its setpoint. These processes can be examined by FDD layer  416  to expose when the system begins to degrade in performance and alert a user to repair the fault before it becomes more severe. 
     Flow Control 
     Referring now to  FIG.  5   , a block diagram of an example flow control system  500  is shown, according to some embodiments. System  500  generally involves controlling the flow of chilled fluid through a cooling coil to provide a desired amount of air conditioning to a building space. System  500  is shown to include a chiller  502 , a pump  504 , a valve  512 , an actuator  514 , and a cooling coil  520 . These components may be similar to chiller  102 , pumps  234 , actuator  354 , valve  346 , and coil  334  as described above, for example. System  500  is also shown to include a flow sensor  510  that provides a flow measurement to actuator  514  in addition to a temperature sensor  522  that provides a temperature measurement associated with the inlet of coil  520  to actuator  514  and a temperature sensor  524  that provides a temperature measurement associated with the outlet of coil  520  to actuator  514 . System  500  is also shown to include a controller  530  that provides a setpoint and possibly other data to actuator  514  and also receives data from actuator  514  (e.g., temperature data, position data, flow data). Controller  530  may be similar to AHU controller  330  and/or BMS controller  366  as described above. Sensors  510 ,  522 , and  524  may provide measurements to controller  530  instead of or in addition to providing measurements to actuator  514 . 
     Actuator  514  may be configured to execute a control application  516  in order to control the flow of chilled fluid through cooling coil  520  by moving valve  512  between an open position and a closed position. Control application  516  can be developed using a variety of programming languages such as MATLAB, C, Python, Java, etc. Actuator  514  may include a processing circuit with at least one processor and a memory that executes control application  516  and maintains data associated with system  500 . It will be appreciated that control application  516  may also be executed by controller  530  and/or in accordance with control logic executed by controller  530 . For example, controller  530  may be responsible for controlling a fan such as fan  338  described above that blows air over coil  520  to provide air conditioning to a building space. 
     Control application  516  can be configured to determine a setpoint that ensures that a difference between temperature measurements generated by sensor  522  and sensor  524  remains above a threshold. The setpoint may be a position setpoint (e.g., valve position), a flow setpoint, a power setpoint (e.g., power output by coil  520 ), or another type of setpoint. The difference between temperature measurements generated by sensor  522  and sensor  524  may be referred to as a temperature change (ΔT) across coil  520 . This functionality allows system  500  to maintain efficiency by preventing operation within a “waste zone” wherein the marginal gain in heat transfer achieved by increasing the flow of fluid through coil  520  is relatively low. As a result, power consumed by chiller  502  and pump  504  may be conserved by operating system  500  in accordance with the desired setpoint. In general, as the flow rate through coil  520  increases, the slope of the rate of heat transfer associated with coil  520  decreases significantly. The slope of the change in ΔT also decreases significantly when increasing flow thorough coil  520 . Accordingly, limiting the flow rate through coil  520  can be advantageous. Further, if not enough flow is provided through coil  520 , then the power output by coil  520  may be less than desired, and inadequate cooling of a building space may result. 
     Control application  516  may generally use feedback control to adjust a setpoint based on the ΔT across coil  520 . For example, in order to improve efficiency of system  500 , it may be desirable to have a minimum ΔT of about 15 degrees Fahrenheit. In this example, if the difference between temperatures measurements generated by sensor  522  and  524  falls below this threshold, control application  516  may adjust a setpoint. This functionality can generally be described by the equation 
                 S   ⁢     P     n   ⁢   e   ⁢   w         =     S   ⁢   P   *     min   ⁡   (         Δ   ⁢   T       Δ   ⁢     T   min         ,   1     )         ,         
where SP new  is a new setpoint, SP is a current setpoint, ΔT is the temperature change across coil  520 , and ΔT min  is the desired minimum temperature change across coil  520 .
 
     In addition to changing a setpoint based on the ΔT across coil  520 , control application  516  may be configured with additional features for improved efficiency of the flow control system. One of these features may be a pulse generation feature, where control application  516  only evaluates the ΔT at periodic intervals. This functionality helps prevent control application  516  from making erroneous control decisions when the system is not at steady state conditions. The periodic interval may be close to, but not less than, the time constant of coil  520  in order to prevent control application  516  from evaluating the ΔT when the system is not operating at steady state conditions. For each pulse, control application may also be configured to determine if the ΔT is too high. Further, a change-limiting feature may be implemented within control application  516  in order to prevent setpoint changes that are too drastic. For example, if a disturbance such as loss of airflow is introduced in system  500 , the ΔT across the coil may change dramatically. Without a change-limiting feature, control application may drastically change a setpoint, such as changing a position setpoint associated with valve  512  from 100% open to 5% open. This can result in undesirable effects such as insufficient flow through coil  520  and thereby inadequate cooling of a building space. However, the change-limiting feature may prevent setpoint changes greater than a certain threshold (e.g., 30%) from occurring to limit these drastic changes. Moreover, the threshold may change based on the current setpoint. For example, if the current valve position setpoint is 80% open, then the threshold may be set at 30%. However, if the current valve position setpoint is only 60% open, then the threshold may be increased to 50%. This functionality can provide more desirable system behavior and improved efficiency. 
     Additional features may be included with control application  516  besides the pulse generation feature and the change-limiting feature described above. A reevaluation feature may be included to prevent the ΔT from rising too far above the desired ΔT min  after a system change, as this phenomenon may also indicate system inefficiency such as inadequate power output by coil  520 . For example, if a system disturbance such as loss of airflow is causes the ΔT to rise above the desired ΔT min  by a certain threshold (e.g., 10%), the reevaluation feature may adjust a setpoint to lower the ΔT and bring it closer to the desired ΔT min . Further, control application  516  may include logic to determine if a setpoint received from an external device (e.g., controller  530 ) should be used instead of the setpoint determined by control application  516  based on the ΔT across coil  520 . For example, control application  516  may be configured to use the lesser of two setpoints or the greater of two setpoints. Moreover, an absolute value feature can be implemented within control application  516  such that the same logic is applicable to both heating and cooling applications. For example, the ΔT may be −15 degrees Fahrenheit for cooling applications, and control application  516  may simply treat this as positive 15 degrees Fahrenheit. Additionally, logic may be implemented to detect a system change and to detect whether the system is operating at steady state conditions. For example, a system change may be detected if the ΔT changes by more than a threshold in a certain period of time (e.g., 5 degrees Fahrenheit in 30 seconds), and the system may only be considered operating at steady state if a setpoint has not changed by a certain threshold over a certain period of time (e.g., +/−3% over 10 minutes). The period of time may be equal to the pulse generation period as described above or a multiple of the pulse generation period, for example. 
     It will be appreciated that system  500  as shown in  FIG.  5    is intended to be an example and the control techniques described herein are applicable to a variety of different systems. For example, chiller  502  may be replaced with a boiler (e.g., boiler  104 ) and heated fluid may be circulated through coil  520  to provide heating to a building space. Moreover, system  500  may include more than one pump, more than one coil, etc. Coil  520  or a similar component may generally be a component of a variety of different types of heat exchangers such as shell and tube heat exchangers, plate heat exchangers, and double pipe heat exchangers. The heat exchangers may have a variety of different flow configurations such as countercurrent flow, crossflow, concurrent flow, and hybrid flow. The heat exchangers may be part of a larger HVAC device such as AHU  106  as described above. Moreover, while not explicitly shown in  FIG.  5   , system  500  may generally include one or more fans that blow air over coil  520  in order to provide heating or cooling for a building space (e.g., fan  338 ). The fluid circulated through coil  520  may be water or another type of fluid. Flow sensor  510 , valve  512 , and actuator  514  may be components of a pressure-independent control valve configured to maintain a flow setpoint independent of pressure applied at the inlet of valve  512 . Actuator  514  may also operate in accordance with a position setpoint for valve  512  and/or a power setpoint associated with coil  520 . 
     Referring now to  FIG.  6   , a process  600  for controlling the flow of fluid through a coil in an HVAC system is shown, according to some embodiments. Process  600  may be performed by actuator  514  when executing control application  516  as part of the example system  500  described above, for example. Process  600  may also be performed by different devices such as controller  530 . Process  600  can be used to improve efficiency of a flow control system by reducing energy waste in a heating or cooling process. Process  600  can generally be used to maintain a desired temperature change across a heating or cooling coil by controlling the flow of fluid though the heating or cooling coil. Process  600  can be used to conserve energy while still providing adequate heating and cooling to a building space. 
     Process  600  is shown to include receiving a first temperature measurement associated with an inlet of a coil (step  602 ). For example, the first temperature measurement may be received by actuator  514  from temperature sensor  522 . Process  600  is also shown to include receiving a second temperature measurement associated with an outlet of the coil (step  604 ). For example, the second temperature measurement may be received by actuator  514  from temperature sensor  524 . Actuator  514  may also receive a flow measurement associated with a valve. For example, the flow measurement may be associated with valve  512  and received by actuator  514  from flow sensor  510 . Process  600  may also involve receiving a flow setpoint, a position setpoint, and/or a power setpoint. For example, actuator  514  may receive one or more setpoints form controller  530 . It will be appreciated that additional flow measurements, temperature measurements, and other types of sensor data may be received in order to make control decisions for a system such as system  500 . For example, controller  530  and/or actuator  514  may receive data related to chiller  502  and pump  504  in addition to data related to air flow across the coil such as fan status, fan speed, and air temperature. This data may also be received by different devices such as higher-level controllers, a local server, a remote computing system (e.g., cloud system), and the like. 
     Process  600  is also shown to include calculating a difference between the first temperature measurement and the second temperature measurement (step  606 ). For example, actuator  514  may determine the ΔT across the coil by calculating a difference between temperature readings from sensor  522  and sensor  524 . The ΔT across the coil may be calculated at a periodic interval such as a periodic interval that is less than or equal to the time constant of the coil. Process  600  may include implementing the pulse generation feature as described above to ensure that the ΔT across the coil is only calculated while the system is at steady state conditions. 
     Process  600  is also shown to include adjusting a setpoint associated with the valve if the difference is below a threshold (step  608 ). For example, the threshold may be ΔT min  as described above and may be equal to about 15 degrees Fahrenheit. In this case, if the ΔT calculated in step  608  is less than 15 degrees Fahrenheit, the setpoint may be adjusted. The setpoint may be a flow setpoint, a positon setpoint, a power setpoint, or another type of setpoint. As discussed above, adjusting the setpoint may include determining a new setpoint by multiplying a current setpoint by a ratio of the ΔT and the threshold (e.g., ΔT min ). Process  600  may also include determining that the ΔT across the coil is above the threshold by more than a second threshold amount (e.g., more than 10% above ΔT min ) such as by implementing the reevaluation feature as described above. Responsive to such a determination, process  600  may include adjusting the setpoint until the ΔT is above the threshold by less than the second threshold amount (e.g., less than 10% above ΔT min ). Moreover, the change-limiting feature may be implemented in process  600  such that adjusting the setpoint includes adjusting the setpoint by no more than a threshold amount (e.g., 30%). As discussed above, this threshold used to implement the change-limiting feature may vary depending on a current value of the setpoint. Process  600  may further include operating a chiller, a boiler, a pump, and/or other equipment of the HVAC system in accordance with the setpoint. For example, demand on chiller  502  may be reduced and/or pump  504  may consume less energy as a result of process  600 . 
     Referring now to  FIGS.  7 - 12   , a variety of graphs demonstrating advantages of the systems and methods described herein are shown, according to various embodiments. These graphs generally show flow, temperature, power, and position as related to a system such as system  500  described above. Similar graphs are shown for systems that do not implement features such as the pulse generation feature, the change-limiting feature, and the reevaluation feature as described above, and systems that do implement such features. The graphs demonstrate how these systems adjust setpoints in order to maintain a desired ΔT across a coil and how they react to different system changes such as disturbances. It can be seen that various features of control application  516  as described above provide more desirable flow control, thereby resulting in improved efficiency of the system as a whole. It can be assumed that the desired ΔT across the coil is about 15 degrees Fahrenheit for the graphs. It will be appreciated that control application  516  may operate in different modes such as position control mode, flow control mode, and power control mode. As such, the setpoints shown in  FIGS.  7 - 12    may not all be applicable at the same time. For example, the system may only change the flow setpoint, the position setpoint, the power setpoint, or any combination thereof. However, example setpoints are illustrated for each of flow, power, and position. The power setpoint as described below may generally be a target power output associated with the coil. 
     Referring specifically to  FIG.  7   , a series of graphs  700  showing behavior of a system that attempts to impose a limit on the temperature change across a coil without using a pulse generation feature is shown, according to some embodiments. Graph  710  depicts flow of a fluid through a coil such as coil  520  described above. This flow can be controlled by actuator  514  by moving the position of a valve such as valve  512  described above. Graph  710  shows that the flow setpoint  712  is about 0.7 gallons per minute and, as a result, the actual flow  711  through the coil is also about 0.7 gallons per minute. Graph  740  depicts the positon of the valve between a fully-open position (100%) and a fully-closed position (0%). It can be seen that the position setpoint  741  and the actual position  742  for the valve remain at about 30% open in accordance with the flow setpoint  712 . 
     Graph  720  depicts temperatures in degrees Fahrenheit associated with the coil. The temperature at the inlet of the coil  721  (e.g., as measured by sensor  522 ) as well as the temperature at the outlet of the coil  722  (e.g., as measured by sensor  524 ) can both be seen. From graph  720 , it can be seen that the ΔT across the coil remains at about 35 degrees Fahrenheit, which is well above the desired level of 15 degrees Fahrenheit. Graph  730  depicts power output of the coil measured in British thermal units per hour, including the power setpoint  732  and the actual power output of the coil  731 . It can be seen from graph  730  that the coil outputs about 12,000 BTUs per hour, which is below the target of 25,000 BTUs per hour because insufficient flow is provided through the coil. When the ΔT is evaluated at each and every time step such as in graphs  700 , the system may exhibit steady state error and non-optimal results. Moreover, the flow may be lower or higher than it needs to be, as is the case in graphs  700 . 
     Referring specifically to  FIG.  8   , a series of graphs  800  showing behavior of a system that attempts to impose a limit on the temperature change across a coil using a pulse generation feature is shown, according to some embodiments. As shown in graph  810 , the flow setpoint  812  and the actual flow  811  begin near about 5 gallons per minute, which is relatively high. As shown in graph  820 , this excess flow results in a ΔT between the inlet temperature  821  and the outlet temperature  822  of about 10 degrees Fahrenheit, which is below the desired level of 15 degrees Fahrenheit. Accordingly, the system is operating inefficiently. However, after about 360 seconds, a pulse occurs and the system lowers the flow setpoint  812  in an effort to raise the ΔT. Another pulse occurs at about 720 seconds, and the system again lowers the flow setpoint  812  in an effort to raise the ΔT. As shown in graph  840 , the valve position setpoint  841  and the actual valve position  842  are also adjusted along with the flow. After the second pulse, the system successfully achieves a ΔT that is about in line with the desired level of 15 degrees Fahrenheit. As shown in graph  830 , the system generally achieves the power output  831  of about 25,000 BTUs per hour, which is consistent with the power setpoint  832 . 
     Referring specifically to  FIG.  9   , a series of graphs  900  showing behavior of a system that attempts to impose a limit on the temperature change across a coil without using a change-limiting feature is shown, according to some embodiments. As shown in graph  910 , the flow setpoint  912  and the actual flow  911  begin at about 5 gallons per minute. However, after about 50 seconds, the system experiences a loss of airflow. The loss of airflow may be due to equipment failure such as failure of fan  338  described above. However, a variety of disturbances may occur and cause changes within the system. Once the loss of airflow occurs and upon evaluation of the ΔT at a pulse that occurs at about 360 seconds, the system drastically lowers the position setpoint  942  and thereby the actual valve position  941  from about 100% open to only about 5% open as shown in graph  940 . As a result, as shown in graph  920 , the ΔT between the inlet temperature  921  and the outlet temperature  922  rises sharply above the desired threshold. Moreover, as shown in graph  930 , the power output  931  falls to nearly zero, well below the setpoint  932  of about 25,000 BTUs per hour. The system experiences a loss of power output mostly due to the loss of airflow, however the ΔT rising well above the desired level results in further inefficiency. 
     Referring specifically to  FIG.  10   , a series of graphs  1000  showing behavior of a system that attempts to impose a limit on the temperature change across a coil using a change-limiting feature is shown, according to some embodiments. The system of graphs  1000  does include the pulse generation feature as described above. Similar to graph  910 , graph  1010  shows that the flow setpoint  1012  and the actual flow  1011  begin near about 5 gallons per minute. Likewise, the valve position setpoint  1041  and the actual valve position  1042  being near 100% open. However, as can be seen in graph  1030 , the power output  1031  falls well below the setpoint  1032  as a result of a loss of airflow that occurs after about 100 seconds. Similarly, the ΔT between the inlet temperature  1021  and the outlet temperature  1022  falls well below the desired threshold. In an effort to raise the ΔT, the system lowers the position setpoint  1041 . However, due to the change-limiting feature, the system only lowers the position setpoint  1041  to about 30% open. As a result, the ΔT does not rise sharply above the desired level. Rather, the ΔT rises to just about the desired level, and the system achieves improved efficiency as a result of the change-limiting feature. As shown in graph  1030 , while the power output  1031  remains well below the setpoint  1032 , it does remain above zero. 
     Referring specifically to  FIG.  11   , a series of graphs  1100  showing behavior of a system that attempts to impose a limit on the temperature change across a coil without using a reevaluation feature is shown, according to some embodiments. As shown in graph  1110 , the system begins with a flow setpoint  1112  and actual flow  1111  near about 5 gallons per minute. Similarly, as shown in graph  1140 , the valve position setpoint  1141  and the actual valve position  1142  begin near the fully-open position. However, after about 50 seconds, the system experiences a loss of airflow as reflected in graph  1130  by the drop in power output  1131  below the setpoint  1132  of nearly 25,000 BTUs per hour. From graph  1120 , it can also be seen that the ΔT between the inlet temperature  1121  and the outlet temperature  1122  falls to nearly zero after the loss of airflow. After a pulse occurs at about 360 seconds, the system lowers the flow setpoint  1141  to about 30% open. The system again lowers the position setpoint  1141  after a second pulse that occurs at about 720 seconds. At about 1000 seconds, the airflow returns and the ΔT rises sharply above the desired level. The power output  1131  increases as well. However, since the ΔT is now above the minimum threshold, the system does not adjust any setpoints. As a result, the system operates ineffectively, and the power output  1131  remains well below the target setpoint  1132 . 
     Referring specifically to  FIG.  12   , a series of graphs  1200  showing behavior of a system that attempts to impose a limit on the temperature change across a coil using a reevaluation feature is shown, according to some embodiments. Similar to graphs  1100 , the system begins with a flow setpoint  1212  and actual flow  1211  near about 5 gallons per minute. Similarly, as shown in graph  1240 , the valve position setpoint  1241  and the actual valve position  1242  begin near the fully-open position. However, after about 50 seconds, the system experiences a loss of airflow as reflected in graph  1230  by the drop in power output  1231  below the setpoint  1232  of nearly 25,000 BTUs per hour. From graph  1220 , it can also be seen that the ΔT between the inlet temperature  1221  and the outlet temperature  1222  falls to nearly zero after the loss of airflow. After a pulse occurs at about 360 seconds, the system lowers the position setpoint  1241  to about 30% open. The system again lowers the position setpoint  1241  after a second pulse that occurs at about 720 seconds. At about 1000 seconds, the airflow returns and the ΔT rises sharply above the desired minimum threshold. The power output  1231  increases as well. However, since this system includes the reevaluation feature as described above, it recognizes that it needs to adjust in order to lower the ΔT back towards the desired level of about 15 degrees Fahrenheit. A shown in graph  1240 , the system raises the position setpoint  1241  at periodic intervals until the ΔT returns to about the desired level. With these changes, the power output  1231  also increases until it reaches about the desired level. Accordingly, the reevaluation feature provides improved efficiency and performance of the flow control system. 
     Configuration of Exemplary Embodiments 
     The construction and arrangement of the systems and methods as shown in the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.). For example, the position of elements can be reversed or otherwise varied and the nature or number of discrete elements or positions can be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. The order or sequence of any process or method steps can be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions can be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present disclosure. 
     The present disclosure contemplates methods, systems and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure can be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
     Although the figures show a specific order of method steps, the order of the steps may differ from what is depicted. Also two or more steps can be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps.