Patent Publication Number: US-10775069-B2

Title: HVAC system and corresponding methods of controlling HVAC fan and coil capacity

Description:
FIELD 
     The disclosure herein relates to heating, ventilation, and air-conditioning (“HVAC”) systems and methods, and more particularly to fan capacity and cooling/heating coil capacity controls used in such HVAC systems and methods. Generally, systems and methods described herein are directed to coordinating fan capacity and cooling/heating coil capacity controls in the HVAC systems. 
     BACKGROUND 
     HVAC units and systems typically include cooling/heating coils and discharge fans. The discharge fans may be configured to drive airflow through the cooling/heating coils to control a space temperature, for example in a building, by modulating the airflow. The discharge fans and/or the cooling/heating coils can be controlled to regulate a volume and/or a temperature of the airflow. 
     SUMMARY 
     Methods of regulating a fan capacity of a discharge fan and a discharge air temperature of a cooling/heating coil in a HVAC system are described. The methods as disclosed herein can generally directed to varying the fan capacity and the discharge air temperature of the cooling/heating coil and to coordinating them to, for example, regulate a space temperature. 
     In some embodiments, such as a HVAC system with a two speed discharge fan, the method may include maintaining a discharge fan capacity at a low fan capacity setting and varying a coil discharge air temperature between a low discharge air temperature limit and a high discharge air temperature limit to maintain or modulate a space temperature. When the coil discharge air temperature reaches the low discharge air temperature limit, the discharge fan capacity can be varied between the low fan capacity setting and a high fan capacity setting to maintain or modulate a space temperature while maintaining the coil discharge air temperature at the low discharge air temperature limit. 
     In some embodiments, the method may include when the coil discharge air temperature reaches the high discharge air temperature limit, the discharge fan capacity can be varied between the low fan capacity setting and the high fan capacity setting to maintain the space temperature setpoint while maintaining the coil discharge air temperature at the high discharge air temperature limit. 
     In some embodiments, the method may include when the coil discharge air temperature reaches the low discharge air temperature limit and the discharge fan capacity reaches the high fan capacity setting, the discharge air temperature can be varied between the low discharge air temperature and a first boost discharge air temperature that is lower than the low discharge air temperature limit when, for example, extra cooling is needed. In some embodiments, the method may include when the coil discharge air temperature reaches the high discharge air temperature limit and the discharge fan capacity reaches the high fan capacity setting, the coil discharge air temperature can be varied between the high discharge air temperature limit and a second boost discharge air temperature that is higher than the high discharge air temperature limit when, for example, extra heating is needed. 
     In some embodiments, varying the discharge fan capacity may include selecting a fan speed of a discharge fan among a plurality of speed settings of the fan when a multiple speed discharge fan is used. The discharge fan capacity can be modulated by operating the discharge fan for a variable amount of time at different fan speeds. In some embodiments, varying the discharge fan capacity may include varying a fan speed of a discharge fan when a variable speed discharge fan is used. 
     In some embodiments, the method may include when a variable speed compressor and a variable speed discharge fan are used in the HVAC system, calculating the low fan capacity setting based on an operation speed of the variable speed compressor. 
     Other features and aspects of the systems, methods, and control concepts will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the drawings in which like reference numbers represent corresponding parts throughout. 
         FIG. 1  illustrates an exemplary HVAC system, with which the embodiments as described herein may be practiced. 
         FIG. 2  illustrates a method to control a HVAC system, according to one embodiment. 
         FIG. 3  illustrates an embodiment that can be incorporated into the method as described in  FIG. 2 . 
         FIG. 4  illustrates a method to control a fan capacity of a discharge fan and a coil discharge air temperature, according to one embodiment. 
         FIG. 5  illustrates a method to control a fan capacity of a discharge fan and a coil discharge air temperature, according to another embodiment. 
         FIGS. 6A to 6C  illustrate three fan capacity maps that can be used with the method as described in  FIG. 2  or  FIG. 5 .  FIG. 6A  is directed to a fan capacity map of a two-speed discharge fan.  FIG. 6B  is directed to a fan capacity map of a three-speed discharge fan.  FIG. 6C  is directed to a fan capacity map of a variable speed discharge fan. 
         FIG. 7  illustrates a capacity map of an airflow damper, which may be used with the method as illustrated in  FIG. 2  or  FIG. 5 . 
         FIG. 8  illustrates yet another method to control a fan capacity of a discharge fan and a coil discharge air temperature. 
         FIGS. 9A and 9B  illustrate two embodiments to control a fan capacity of a discharge fan and a coil discharge air temperature in a HVAC system equipped with a variable speed compressor.  FIG. 9A  illustrates a method to control a variable discharge air temperature, when a variable speed fan is used.  FIG. 9B  illustrates a method to control a coil discharge temperature, when a variable speed fan is used. 
         FIGS. 10A and 10B  illustrate fan capacity maps that can be used with the methods as described in  FIGS. 9A and 9B .  FIG. 10A  is directed to a fan capacity map of a cooling mode of the HVAC system.  FIG. 10B  is directed to a fan capacity map of a heating mode of the HVAC system. 
     
    
    
     DETAILED DESCRIPTION 
     Fans and cooling/heating coils of a HVAC units and systems can be regulated to change a volume and a temperature of airflow delivered to a space of a building so as to regulate a temperature of the space. The discharge fans and the cooling/heating coils can be regulated independently or coordinately. Coordinating the regulations of the discharge fans and the cooling/heating coils may help increase efficiency of the HVAC system. 
     Methods to coordinate the regulations of a fan and a cooling/heating coil of a HVAC system are described herein. In some embodiments, the method may include a variable discharge air temperature modulating block that is configured to regulate a discharge air temperature of the cooling/heating coil of the HVAC system between a low discharge air temperature limit and a high discharge air temperature limit to maintain or modulate a space temperature, such as to maintain the space temperature at or about a space temperature setpoint, while maintaining a low fan capacity setting of a discharge fan. The method may also include a low discharge air temperature maintaining block that is configured to maintain the low discharge air temperature limit of the cooling/heating coil, while varying the fan capacity of the discharge fan to maintain or modulate the space temperature. In some embodiments, the method may include a high discharge air temperature maintaining block that is configured to maintain the high discharge air temperature limit of the cooling/heating coil, while varying the fan capacity of the discharge fan to maintain or modulate the space temperature. In some embodiment, the method may include a discharge capacity boost block that is configured to decrease (or elevate) the discharge air temperature of the cooling/heating coil from the low discharge air temperature limit (or the high discharge air temperature limit) so as to boost the cooling (or heating) capacity of the cooling/heating coil of the HVAC system if desired. The methods as described herein may be applicable to a HVAC system that has variable airflow capability, such as a HVAC system including a multiple speed discharge fan (e.g. a two-speed or three-speed discharge fan), as well as a variable speed discharge fan. The methods described herein may also be adapted to modulate a heating capacity and an airflow damper in a Variable Air Volume (VAV) unit. 
     References are made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of the embodiments in which the embodiments may be practiced. It is to be understood that the term used herein are for the purpose of describing the figures and embodiments and should not be regarded as limiting the scope. 
       FIG. 1  illustrates an exemplary HVAC system  100 , with which the methods described herein can be practiced. The HVAC system  100  includes an airflow duct  110  that is configured to direct air in and out of a space  112  through the airflow duct  110 . The airflow duct  110  includes a fan  120  and a cooling/heating coil  130 . The cooling/heating coil  130  can be configured to include a cooling coil  130   a  configured to provide cooling and a heating coil  130   b  configured to provide heating of airflow  132 . The cooling coil  130   a  is equipped with a control valve  190 . 
     A control module  140  includes a main controller  142  that is configured to coordinate a fan control module  144  and a coil control module  146 . The fan control module  144  is configured to modulate the discharge fan  120  of the HVAC system  100 , and the coil control module  146  is configured to modulate the cooling/heating coil  130 . 
     A temperature sensor  148  is positioned near an exit  150  of the cooling/heating coil  130  and is configured to measure a discharge air temperature of the cooling/heating coil  130 . The discharge air temperature measured by the temperature sensor  148  is sent to the main controller  142 . 
     The discharge fan  120  is a fan that can generally be configured to provide a variable fan capacity, such as a multiple speed discharge fan (e.g. a two-speed or three-speed discharge fan) or a variable speed discharge fan. By changing the speed of the discharge fan  120 , the capacity of the discharge fan  120  can be regulated. Generally, increasing the speed of the discharge fan  120  leads to more air being moved through the coil  130 , which correlates to increasing a capacity of the HVAC system  100 ; while reducing the speed of the discharge fan  120  leads to less air being moved through the coil  130 , which correlates to decreasing the capacity of the HVAC system  100 . 
     In operation, the main controller  142  can be configured to receive information, such as temperature measured by the temperature sensor  148 , a space temperature in the space  112 , and/or a space temperature setpoint for the space  112 . The main controller  142  can be configured to use the information to determine, for example, whether to increase or decrease fan speeds (or capacity) of the discharge fan  120 , and/or whether to increase or decrease the discharge air temperature of the cooling/heating coil  130 . The main controller  142  can be configured to instruct the fan control module  144  to regulate the fan speeds (or the capacities) of the discharge fan  120 , and/or the coil control module  146  to modulate the discharge air temperature of the cooling/heating coil  130 . The discharge air temperature of the cooling/heating coil  130  can be regulated, for example, by mechanically regulating the control valve  190 . By regulating the fan capacity and/or the discharge air temperature of the cooling/heating coil  130 , the space temperature in the space  112  can be maintained or changed. The fan capacity and/or the discharge air temperature of the cooling/heating coil  130  can also be modulated according to, for example, a load requirement of the HVAC system  100 . 
     Generally, a higher fan speed or fan capacity of the discharge fan  120  and/or a lower discharge air temperature of the coil  130  are associated with a higher capacity of the HVAC system  100  in a cooling mode; and a lower fan speed or fan capacity of the discharge fan  120  and/or a higher discharge air temperature of the coil  130  are associated with a lower capacity of the HVAC system  100  in a cooling mode. Likewise, a higher fan speed or fan capacity of the discharge fan  120  and/or a higher discharge air temperature of the coil  130  are associated with a higher capacity of the HVAC system in a heating mode; and a lower fan speed or fan capacity of the discharge fan  120  and/or a lower discharge air temperature of the coil  130  are associated with a lower capacity of the HVAC system in a heat mode. 
     It is to be appreciated that the embodiment of the HVAC system  100  is exemplary. Embodiments of the methods described herein can also be used with other HVAC configurations. Generally, the methods described herein can be used with a HVAC system that includes a fan configured to provide a variable capacity and/or a cooling/heating coil configured to provide a variable discharge air temperature. 
     It is to be appreciated that in some embodiments, the HVAC system  100  may be configured to include the heating coil  130   b  or the cooling coil  130   a  only, and consequently only heating or cooling is provided by the HVAC system  100 . It is generally known in the art that methods configured to modulate cooling and heating together can be modified to modulating cooling only or heating only. 
       FIG. 2  illustrates a method  200  that can be executed by a controller of a HVAC system, for example, the main controller  142  of the HVAC system  100  in  FIG. 1 , so that a space temperature can be maintained at a space temperature setpoint or the space temperature can be changed to for example a new space temperature setpoint. The controller can be configured to control a fan and/or a cooling/heating coil of the HVAC system (e.g. the discharge fan  120  and/or the cooling/heating coil  130  in  FIG. 1 ). The method  200  generally includes five control blocks: a variable discharge air temperature modulating block  210 , a cooling discharge air temperature T min  maintaining block  220 , an optional ΔT min  cooling capacity boost block  230 , a heating discharge air temperature T max  maintaining block  240  and an optional ΔT max  heating capacity boost block  250 . 
     The variable discharge air temperature (DAT) modulating block  210  is typically configured to work with both a cooling mode and a heating mode to maintain or modulate the space temperature. The cooling mode is generally referred to as a HVAC operation mode when the discharge air temperature of the cooling/heating coil is lower than the space temperature of a space (e.g. the space  112  in  FIG. 1 ). The heating mode is generally referred to as a HVAC operation mode when the discharge air temperature of the cooling/heating coil is higher than the space temperature of the space. 
     In the variable discharge air temperature modulating block  210 , the controller can determine the discharge air temperature of the cooling/heating coil based on, for example, the space temperature in the space and/or the temperature setpoint in the space. For example, if cooling is needed, the controller can raise a capacity of the cooling coil (such as the cooling coil  130   a  in  FIG. 1 ) and/or reduce a capacity of the heating coil (such as the heating coil  130   b  in FIG.  1 ), so that the discharge air temperature of the cooling/heating coil is configured to be below the space temperature in the space. The controller can regulate the capacity of the cooling coil by, for example, mechanically regulating a control valve (such as the control valve  190  in  FIG. 1 ). The controller can similarly control the capacity of the cooling/heating coil in the heating mode. If more heating is needed, the controller can raise the capacity of the heating coil and/or reduce the capacity of the cooling coil, so that the discharge air temperature of the cooling/heating coil is configured to be above the space temperature in the space. 
     Two discharge air temperature limits T min  and T max  can be set up and used in the variable discharge air temperature modulating block  210 . The T min  may be the low temperature limit that the cooling/heating coil is designed to discharge in a cooling operation. The T max  may be the high temperature limit that the cooling/heating coil is designed to discharge in a heating operation. The T min  and T max  are typically fixed values. The T min  and T max  can be set up by an engineer, for example, based on a designed limit, a capacity limit, a comfort limit of occupancy in the space, and/or safe operation limits. The T min  and T max  may also be provided by a manufacturer. It is noted that in some embodiments, the T min  and the T max  may not be the lowest temperature and the highest temperature that the cooling/heating coil is capable of providing. 
     As illustrated in  FIG. 2 , in the variable discharge air temperature modulating block  210 , the controller can determine whether the HVAC system is operated in the cooling mode or in the heating mode. In the cooling mode, the controller can generally change the discharge air temperature toward the T min  if more cooling capacity is required in the variable discharge air temperature modulating block  210 . Likewise, in the heating mode, the controller can generally change the discharge air temperature toward the T max  if more heating capacity is required in the variable discharge air temperature modulating block  201 . 
     When the discharge air temperature of the cooling/heating coil reaches the T min , which generally is when only the cooling coil is in operation, the controller can be configured to proceed to the cooling discharge air temperature T min  maintaining block  220 . In some embodiments, the controller can be configured to proceed to the cooling discharge air temperature T min  maintaining block  220  when, for example, a designed cooling capacity limit of the cooling/heating coil is reached. The designed cooling capacity limit can be determined, for example, by a HVAC manufacturer and/or space comfort needs. 
     The cooling discharge air temperature T min  maintaining block  220  is configured to maintain or modulate the space temperature while maintaining the discharge air temperature of the heating/cooling coil at or about the T min  in the cooling mode. This can be achieved typically by regulating a fan capacity of a discharge fan configured to move airflow through the heating/cooling coil, such as the discharge fan  120  as shown in  FIG. 1 . The fan capacity can be regulated for example by increasing or decreasing a fan speed of the discharge fan. Generally, as illustrated in  FIG. 2 , if more cooling capacity is required, in the cooling discharge air temperature T min  maintaining block  220 , the fan capacity can be increased. 
     As illustrated in  FIG. 2 , in the heating mode, the controller can generally change the discharge air temperature toward the T max  if more heating capacity is required in the variable discharge air temperature modulating block  210 . 
     When the discharge air temperature of the cooling/heating coil reaches the T max , which generally is when only the heating coil is in operation, the controller proceeds to the heating discharge air temperature T max  maintaining block  240 . 
     The heating discharge air temperature T max  maintaining block  240  is configured to maintain or modulate the space temperature while maintaining the discharge air temperature of the heating/cooling coil at or about the T max  in the heating mode and regulate the fan capacity of the discharge fan configured to move airflow through the heating/cooling coil to maintain space temperature at its setpoint. The fan capacity can be regulated, for example, by increasing or decreasing the fan speed of the discharge fan. Generally, as illustrated in  FIG. 2 , if more heating capacity is required, in the heating discharge air temperature T max  maintaining block  240 , the fan capacity can be increased. 
     In some embodiments, the cooling/heating coil may not reach its maximum capacity in the cooling discharge air temperature T min  maintaining block  220  when the discharge fan reaches its maximum fan speed or capacity, and/or in the heating discharge air temperature T max  maintaining block  240  when the discharge fan reaches its maximum fan speed or capacity. The controller can be configured to optionally proceed to the ΔT min  cooling capacity boost block  230  and/or the A T max  heating capacity boost block  250  in the cooling mode and the heating mode respectively. In the optional ΔT min  cooling capacity boost block  230  and the optional ΔT max  heating capacity boost block  250 , the discharge air temperature limit T min  or T max  of the cooling/heating coil can be configured to be reduced by ΔT min  in the cooling mode or raised by ΔT max  in the heating mode respectively. Reducing the cooling discharge air temperature limit T min  in the cooling mode or increasing the heating discharge air temperature limit T max  in the heating mode respectively can help increase the cooling/heating coil capacity beyond, for example the design limits, when desired, such as for example to maintain the space temperature setpoint in an extreme environmental temperature or to achieve fast cooling or heating. 
     It is appreciated that generally the fan speed is associated with fan capacity. Generally, the higher the fan speed is, the higher the fan capacity and vise verse. 
     ΔT min  and/or ΔT max  can be determined based on information provided, for example, by a manufacturer of the cooling/heating coil. Generally, ΔT min  and/or ΔT max  may be configured so that the cooling/heating coil is capable of providing the discharge air temperature at a low boost temperature limit (T min −ΔT min ) in the cooling mode and/or at a high boost temperature limit (T max +ΔT max ) in the heating mode. In one embodiment, the ΔT min  is about 5° F. and/or ΔT max  is about 20° F. 
     In the ΔT min  cooling capacity boost block  230  and the ΔT max  heating capacity boost block  250 , the controller can be configured to, for example, gradually lower the discharge air temperature toward the low boost temperature limit (T min −ΔT min ) or raise the discharge air temperature toward the high boost temperature limit (T max +ΔT max ) when more extra cooling or heating is required respectively, while maintaining the discharge fan at a high capacity setting. 
     As illustrated in  FIG. 2 , generally in the cooling mode, the controller is configured to proceed from the variable discharge air temperature modulating block  210 , to the cooling discharge air temperature T min  maintaining block  220 , and optionally to the ΔT min  cooling capacity boost block  230  in sequence, while the cooling capacity required by the space generally increases. Generally, in the heating mode, the controller proceeds from the variable discharge air temperature modulating block  210 , to the heating discharge air temperature T max  maintaining block  240 , and optionally to the ΔT max  heating capacity boost block  250  in sequence, while the heating capacity required by the space increases. It is to be appreciated, however, that it may not be necessary for the controller to proceed from one control block to another control block in any particular order. Any of the operation blocks can be independently executed. 
       FIGS. 3, 4, 5, 6A-6C, 7, 8, 9A-9B and 10A-10B  illustrate embodiments that can be incorporated into the method  200  as illustrated in  FIG. 2 . 
       FIG. 3  illustrates that any of the variable discharge air temperature modulating block  210 , the discharge air temperature T min /T max  maintaining blocks  220 / 240 , and/or the optional ΔT min /ΔT max  capacity boost blocks  230 / 250  as illustrated in  FIG. 2  can include a fan capacity regulating block  344  to regulate a fan capacity and a coil DAT regulating block  346  to regulate a coil discharge temperature. 
     The fan capacity regulating block  344  can be configured to include a method to control a fan capacity (or an operational speed) of a discharge fan (e.g. the discharge fan  120  in  FIG. 1 ). The fan capacity regulating block  344  can be executed by a fan controller, such as the fan control module  144  in  FIG. 1 . 
     The coil DAT regulating block  346  can be configured to include a method to control a capacity of a coil (e.g. the cooling/heating coil  130  in  FIG. 1 ). The coil DAT regulating block  346  can be executed by a coil controller, such as the coil control module  146  in  FIG. 1 . By including a method of controlling the fan capacity and a method of controlling the cooling/heating coil in each control block, it is possible to coordinate the control of the discharge fan and the cooling/heating coil in any of the control block (e.g. the variable discharge air temperature modulating block  210 , the discharge air temperature T min /T max  maintaining blocks  220 / 240 , and/or the optional ΔT min /ΔT max  capacity boost blocks  230 / 250  as illustrated in  FIG. 2 ). 
       FIG. 4  illustrates one embodiment of a variable discharge air temperature modulating block  410  (see  210  in  FIG. 2 ), which includes a fan capacity regulating block  444  and a coil DAT regulating block  446 . 
     Generally, in the variable discharge air temperature modulating block  410  as illustrated in  FIG. 4 , the fan capacity is generally maintained at or about a low capacity setting (which is performed by the fan capacity regulating block  444 ), while the discharge air temperature of the coil is regulated to maintain or modulate the temperature of the space (which is performed by the coil DAT regulating block  446 ). 
     The fan capacity regulating block  444  is configured to include a method  450 , which is configured to maintain a discharge fan at or about the low fan capacity (e.g. operate the discharge fan at or about a low fan speed) setting. 
     The method  450  may be adapted to accomodate different types of discharge fans. For example, when a two-speed discharge fan with a high speed setting and a low speed setting is used, the method  450  can be configured to maintain the fan speed at or about the low speed setting. When a three-speed discharge fan with a high speed setting, an intermediate speed setting and a low speed setting is used, the method  450  can be configured to maintain the fan speed at or about the low speed setting. When a variable speed discharge fan with a continuously variable speed from a low speed setting to a high speed setting is used, the method  450  can be configured to maintain the fan speed at or about the low fan speed setting. The low fan speed settings can be provided, for example, by a manufacturer of the HVAC system, or determined based on, for example, ventilation, fan stability, coil effectiveness, sound, agency regulations, and/or machine safety requirements. 
     The coil DAT regulating block  446  is configured to include a method  460 , which is configured to vary a discharge air temperature of a coil (e.g. the heating/cooling coil  130  in  FIG. 1 ) so that a space temperature (such as the space  112  in  FIG. 1 ) can be maintained or modulated. As illustrated in  FIG. 2 , generally the discharge air temperature of the coil may be varied between the T min  and the T max . In the HVAC system  100  as illustrated in  FIG. 1 , the discharge air temperature can be regulated by changing the cooling capacity and/or heating capacity of the cooling coil  130   a  and/or the heating coil  130   b.    
       FIG. 5  illustrates an embodiment of a discharge air temperature T min /T max  maintaining block  520 , which includes a fan capacity regulating block  544  and a coil DAT regulating block  546 . 
     Generally, in the discharge air temperature T min /T max  maintaining block  520  as illustrated in  FIG. 5 , the coil discharge air temperature is generally maintained at or about a designed low (or high) discharge air temperature limit T min  (or the T max ) (which is performed by the coil DAT regulating block  546 ), while the fan capacity is mapped (or modulated) to maintain or modulate the space temperature at or about its setpoint while maintaining the coil discharge air temperature at or about the T min  or the T max  (which is performed by the fan capacity regulating block  544 ). 
     The coil DAT regulating block  546  includes a method  560 , which is configured to maintain a coil discharge air temperature of the coil (such as the cooling/heating coil  130  in  FIG. 1 ). Referring back to  FIG. 2 , the controller proceeds from the variable discharge air temperature modulating block  210  to the discharge air temperature limit T min (or T max ) maintaining block  220  (or  240 ), when the discharge air temperature of the cooling/heating coil, for example, reaches the T min  (or the T max ). In the method  560 , this designed low (T min ) or high discharge air temperature limit (T max ) is maintained by the coil DAT regulating block  546 . 
     The fan capacity regulating block  544  is configured to include a method  550 . The method  550  is configured to modulate a fan capacity (such as the discharge fan  120  in  FIG. 1 ) to maintain or modulate the space temperature while maintaining a discharge air temperature (e.g. the T min /T max ) of the coil. Modulating the fan capacity can be achieved by, for example, a proportional integral (PI) control method. Generally, in the cooling mode, if the discharge air temperature is deviated higher from the T min , the discharge fan may be mapped to provide a lower fan capacity; while if the discharge air temperature is deviated lower from the T min , the discharge fan may be mapped to provide a higher fan capacity. In the heating mode, if the discharge air temperature is deviated higher from the T max , the discharge fan may be mapped to provide a higher fan capacity; while if the discharge air temperature is deviated lower from the T max , the discharge fan may be mapped to provide a lower fan capacity. The term “map” generally means regulate a fan capacity of the discharge fan, which may include determining the operation speed of the discharge fan and/or operation time of the discharge fan at the operation speed. 
     The discharge fan is generally a discharge fan capable of providing a variable fan capacity, such as a fan with a multiple speed settings or a variable speed discharge fan. The capacity of the discharge fan is generally referred to as an airflow volume moved by the discharge fan in a given period of time. The discharge fan can be configured to have a low capacity F low  setting and a high capacity F high  setting. 
     For a two-speed discharge fan with a high speed setting and a low speeding setting, the F low  may be associated with the fan capacity when the discharge fan operates at the low speed setting, while the F high  may be associated with the fan capacity when the discharge fan operates at the high speed setting. The method  550  may be configured to operate the fan at either the high speed setting or the low speed setting. The fan capacity can be regulated by switching the discharge fan between the two speed settings. 
     For a three-speed discharge fan with a high speed setting, an intermediate speed setting and a low speed setting, the F low  may be be associated with the low speed setting, while the F high  may be configured to be associated with the high speed setting. The method  550  may be configured to include an intermediate capacity setting F int , and the intermediate capacity setting F int  may be associated with the intermediate speed setting. The method  550  may be configured to choose the fan speed among the three speed settings for the three-speed discharge fan. 
     For a variable speed discharge fan, the F low  may be associated with the low speed setting of the variable speed discharge fan, and the F high  may be associated with the high speed setting of the variable speed discharge fan. The method  550  may be configured to determine the fan speed between the low speed setting and the high speed setting for the variable speed discharge fan to maintain or modulate the space temperature while maintaining the discharge air temperature of the cooling/heating coil. 
     It is to be appreciated that the fans described herein are exemplary. Other fans configured to provide a variable fan capacity can also be used. 
       FIGS. 6A to 6C  illustrate three embodiments of fan capacity maps that can be used in the method  550  to determine a fan speed of the discharge fan as illustrated in  FIG. 5 . Generally, the fan capacity is defined as an airflow volume moved by the discharge fan in a given period of time (such as for example 20 minutes). The fan capacity may be generally associated with the operation speed of the fan as well as the amount of time the fan operate at the operation speed. The fan capacity maps as illustrated in  FIGS. 6A to 6C  can be used to determine the operation speed and the amount of time the discharge fan should be operated at the operation speed. 
     For a discharge fan that have distinctive speed settings, such as a two speed or three speed discharge fan, generally the higher amount of time the discharge fan is operated at a relatively high speed setting in the given period of time is associated with a higher fan capacity. The higher amount of time the discharge fan is operated at a relatively low speed setting in the given period of time is associated with a lower fan capacity. 
       FIG. 6A  illustrates a fan capacity map for a two-speed discharge fan configured to have a high speed setting and a low speed setting. As illustrated, when the discharge fan is operated at or about the high speed setting constantly, the fan capacity is associated with 100% of a high fan capacity setting. When the discharge fan is operated at or about the low speed setting constantly, the fan capacity is at or about P low-A . P low-A  is a fan capacity relative to the high fan capacity setting when the two-speed discharge fan is operated at the low speed setting (in terms of percentage of the high fan capacity setting). The P low-A  can be measured when the discharge fan is operated at the low speed setting or can be provided by a manufacturer of the discharge fan. To regulate the fan capacity of the discharge fan with two speed setting, the fan can be operated at either the high speed setting or the low speed setting for certain portions of the given period time (e.g. about 20 minutes). The discharge fan can be cycled between the high speed fan setting and the low speed fan setting. 
     In operation, the fan capacity can be mapped to the percentage of time on which the two-speed discharge fan operates at or about the high speed setting (or at or about the low speed setting) in a given period of time. If a higher fan capacity is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the discharge fan can be mapped to operate on the high speed setting for a larger percentage of time. Conversely, if a lower fan capacity is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the discharge fan can be mapped to a lower percentage of time to operate the discharge fan at or about the high speed setting. By changing the percentage of time on which the two-speed discharge fan operates at or about the high speeding setting (or the low speed setting), the fan capacity can be provided as a combined total airflow volume provided by the low speed setting and the high speed setting over the given period of time. In one embodiment, the period of time can be for example about 20 minutes, with the appreciation that other amounts of time can be used. 
       FIG. 6B  illustrates a fan capacity map for a three-speed discharge fan, which is configured to have a high speed setting, an intermediate speed setting and a low speed setting. When the discharge fan is operated at or about the high speed setting constantly, the fan capacity is associated with 100% of a high fan capacity setting. When the discharge fan is operated at or about the low speed setting constantly, the fan capacity is P low-B  which is a fan capacity relative to the fan capacity of the high fan capacity setting (in terms of percentage of the high fan capacity setting). When the discharge fan is operated at or about the intermediate speed setting constantly, the fan capacity is P int-B  which is a fan capacity relative to the fan capacity of the high fan capacity setting (in terms of percentage of the high fan capacity setting). The high speed setting, the intermediate speed setting and the low speed setting can be provided by, for example, a manufacturer of the discharge fan. 
     In operation, the fan capacity can be mapped to a percentage of time on which the three-speed discharge fan operates at or about the high speed setting, the intermediate speed setting or the low speed setting in a given period of time. 
     If a fan capacity required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil is lower than the P int-B , the discharge fan can be operated between the low speed setting and the intermediate speed setting. The fan capacity can be mapped to a percentage of time on which the three-speed discharge fan operates at or about the intermediate speed setting (or the lower speed setting). A higher fan capacity requirement can be mapped to a higher percentage of time on which the three-speed discharge fan operates at or about the intermediate speed setting or a lower percentage of time on which the three-speed discharge fan operates at or about the low speed setting. A lower fan capacity requirement can be mapped to a lower percentage of time on which the three-speed discharge fan operates at or about the intermediate speed setting or a higher percentage of time on which the three speed discharge fan operates at or about the low speed setting. 
     If the fan capacity required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil is higher than the P int-B , the discharge fan can be operated between the intermediate speed setting and the high speed setting. The fan capacity can be mapped to a percentage of time on which the three-speed discharge fan operates at or about the high speed setting (or the intermediate speed setting). A higher fan capacity requirement can be mapped to a higher percentage of time on which the three-speed discharge fan operates at or about the high speed setting or a lower percentage of time on which the three speed discharge fan operates at or about the intermediate speed setting. A lower fan capacity requirement can be mapped to a lower percentage of time on which the three-speed discharge fan operates at or about the high speed setting or a higher percentage of time on which the three speed discharge fan operates at or about the intermediate speed setting. 
       FIG. 6C  illustrates a fan capacity map for a variable speed discharge fan, which is configured to have a continuous variable speed between a low speed setting and a high speed setting. When a continuous variable speed discharged fan is used, a specific fan capacity can be associated with a specific fan speed. As illustrated, the fan capacity required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil can be continuously mapped to a speed at which the variable speed discharge fan can operate. 
     In operation, if a higher fan capacity is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the discharge fan can be mapped to a higher fan speed. If a lower fan capacity is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the discharge fan can be mapped to a lower fan speed. 
     It is to be appreciated that the methods described herein can be used with other multiple speed discharge fan configurations. It is further to be appreciated that the fan capacity maps illustrated in  FIGS. 6A to 6C  are exemplary. The fan capacity maps can be, for example, modified for a particular HVAC system. 
     In some HVAC systems, such as a variable air volume (VAV) unit, an air handler fan speed may be variable to maintain a constant duct static pressure. Airflow may be regulated by an airflow damper, such as an airflow damper as taught in the U.S. Pat. No. 5,741,180. The airflow damper can be modulated by opening up or closing down an airflow path of the airflow damper. The methods and systems as disclosed herein can be adapted to control the airflow damper. For example, the method  550  as illustrated in  FIG. 5  can be adapted to control the airflow damper. Generally, if a higher airflow is required to maintain or modulate the space temperature, the airflow damper can be configured to open up to increase the airflow. If a lower airflow is required to maintain or modulate the space temperature, the airflow damper can be configured to close down to decrease the airflow. 
       FIG. 7  illustrates an airflow damper map that can be used, for example, with the method  550  to control the airflow damper. 
     As illustrated in  FIG. 7 , when the airflow command of the airflow damper is 100% (fully open), the airflow is mapped to a maximum airflow setpoint in the VAV unit (about 48 in the illustrated embodiment). If the airflow command of the airflow damper is 0% (fully closed), the airflow is mapped to the minimum airflow setpoint in the VAV unit (about 20 in the illustrated embodiment). The airflow setpoint is an indication of an amount of airflow volume in the VAV unit. The higher the value, the higher the amount of airflow volume in the VAV is. 
     In operation, if a higher airflow volume is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the airflow damper can be opened up. If a lower airflow volume is required to maintain or modulate the space temperature while maintaining the discharge air temperature of the coil, the discharge fan can be closed down. 
     It is to be appreciated that the embodiments as disclosed herein can be used with other method of regulating fan capacity or airflow volume. 
       FIG. 8  illustrates an embodiment of an optional ΔT min /ΔT max  boost block  830 , which includes a fan capacity regulating block  844  and a coil DAT regulating block  846 . The embodiments can be used with the ΔT max /ΔT max  capacity boost blocks  230 ,  250  as illustrated in  FIG. 2 . 
     The fan capacity regulating block  844  includes a method  850 , which is configured to maintain a fan capacity of a fan at or about a high fan capacity setting. That is, the discharge fan is operated at or about a high speed setting of multiple speeding settings. 
     The coil DAT regulating block  846  includes a method  860 . The method  860  is configured to change a discharge air temperature from a designed coil discharge air temperature limit (e.g. T min  or T max ) by a ΔT min  or ΔT max , so that the coil capacity can be boosted in a cooling mode or a heating mode respectively. In the cooling boost mode, generally a discharge air temperature limit T min  may be lowered by the ΔT min ; in a heating boost mode, generally a discharge air temperature limit T min  may be increased by the ΔT max . In one embodiment, the A T min  and ΔT max  are about 5° F. and about 20° F. respectively. The method  860  can be configured to change the discharge air temperature gradually toward T min −ΔT min  or T max +ΔT max  in the cooling boost mode or the heating boost mode respectively. 
     Some HVAC systems may be equipped with a variable speed compressor. A variable speed compressor can change its operation speeds to provide different compressor loads. In HVAC systems equipped with both a variable speed compressor and a variable speed discharge fan, it may be desirable to modulate fan speeds of the discharge fan according to operation speeds of the variable speed compressor. The method  200  as illustrated in  FIG. 2  can also be used with a HVAC system with a variable speed compressor and a variable speed discharge fan. 
       FIGS. 9A and 9B  illustrate an embodiment of a variable discharge air temperature modulating block  910 , and an embodiment of a discharge air temperature T min /T max  maintaining block  920  that can be used with a HVAC system equipped with both a variable speed compressor and a variable speed discharge fan. 
     As illustrated in  FIG. 9A , the variable discharge air temperature modulating block  910  is configured to include a fan capacity regulating block  944   a  and a coil DAT regulating block  946   a . The fan capacity regulating block  944   a  includes a method  950   a  to maintain the variable speed discharge fan at or about a minimal fan speed to meet, for example, a minimal fan capacity requirement. The coil DAT regulating block  946   a  includes a method  960   a  to regulate a coil discharge air temperature similar to the method  460  as described in  FIG. 4  to, for example, maintain or modulate a space temperature. With the variable speed compressor, the method  960   a  can also be configured to regulate an operation speed of the variable speed compressor. 
     Generally, the higher the operation speed of the variable speed compressor is, the higher a coil capacity. Typically, it is desired to move more air through the coil when the compressor speed is relatively higher. Therefore, when the compressor speed varies during operation, it generally is desired to vary a minimal fan speed of the discharge fan accordingly in the method  950   a.    
     The method  950   a  and the method  960   a  are configured to work together, so that different minimal fan capacity requirements can be determined and maintained at  950   a  according to the operation speed of the variable compressor speed determined at  960   a.    
       FIGS. 10A and 10B  illustrate two embodiments of fan capacity maps of a variable speed discharge fan that can be used with the variable discharge air temperature modulating block  910  and the discharge air temperature T min /T max  maintaining block  920 .  FIG. 10A  is a fan capacity map in a cooling mode and  FIG. 10B  is a fan capacity map in a heating mode. 
     In the cooling mode as illustrated in  FIG. 10A , a region  101   a  can be used with the variable discharge air temperature modulating block  910 , and a region  102   a  can be used with the discharge air temperature T min /T max  maintaining block  920  (see below for more details for the region  102   a ). Line  103   a  illustrates a correlation map between a cooling discharge air temperature and a cooling capacity demand. Generally, in the region  101   a , the cooling discharge air temperature changes between a T max −a and a T min −a when the cooling capacity demand changes. Generally, the higher the cooling capacity demand is, the lower the cooling discharge air temperature. For each discharge air temperature between T max −a to T min −a in the region  101   a , the operation speed of the variable speed compressor can vary from 0% to 100% of a compressor speed setting of the variable speed compressor (cmpr). The compressor speed setting can be a highest operation speed that the compressor is designed to be operated on. Line  104   a  illustrates an association map between the cooling capacity demand and the minimal fan capacity requirement (in terms of rated air flow) when the variable speed compressor works at the 100% of its compressor speed setting (100% cmpr). Line  105   a  illustrates an association map between the cooling capacity demand and the minimal fan capacity requirement (in terms of rated air flow) when the variable speed compressor works at the 0% of its compressor speed setting (0% cmpr). For other operation speeds of the variable speed compressor between the 0% cmpr and 100% cmpr, the minimal fan capacity requirement (and the corresponding minimal fan speed setting) can be mapped into a shaded region  106   a  between the lines  104   a  and  105   a  in region  101   a . Based on the fan capacity map as illustrated in  FIG. 10A , the minimal fan airflow requirement (and the corresponding minimal fan speed setting) can be determined according to the current operation speed of the variable compressor. 
       FIG. 10B  illustrates a fan capacity map in a heating mode. In the heating mode as illustrated in  FIG. 10B , a region  101   b  can be used with the variable discharge air temperature modulating block  910 , and a region  102   b  can be used with the discharge air temperature T min /T max  maintaining block  920  (see below for more details for the region  102   b ). Line  103   b  illustrates a correlation map between a heating discharge air temperature and a heating capacity demand. Generally, in the region  101   b , the heating discharge air temperature ranges between a T min −b and a T max −b when the heating capacity demand changes. Generally, the higher the heating capacity demand is, the higher the heating discharge air temperature. 
     In region  101   b , line  104   b  illustrates an association map between the heating capacity and a fan capacity requirement including a minimal fan capacity requirement when the variable speed compressor works at or about the 100% of its compressor speed setting (100% cmpr). Line  105   b  illustrates an association map between the heating capacity and a minimal fan capacity requirement when the variable speed compressor works at or about its 0% of the compressor speed setting (0% cmpr). For other operation speeds of the variable speed compressor between the 0% cmpr and 100% cmpr, the minimal fan capacity requirement can be mapped into a shaded region  106   b  between the lines  104   b  and  105   b.    
     Regions  101   a  and  101   b  in  FIGS. 10A and 10B  can be used with the methods  950   a  and  960   a  to help determine the minimal fan capacity requirement (or corresponding minimal fan speed setting) in the cooling mode or heating mode, while varying the coil discharge air temperature. 
     The region  101   a  (or region  101   b ) can be used with the method  950   a  and  960   a , where the discharge air temperature of the coil can vary while the fan capacity is generally maintained at or about a speed that is associated to the minimal fan capacity requirement. In the region  101   a  (or region  101   b ), as illustrated by both the line  104   a  (or  104   b ) for the 100% of the compressor operation speed setting and the line  105   a  (or  105   b ) for the 0% of the compressor operation speed setting of the variable compressor speed in  FIG. 10A  (or  FIG. 10B ), the coil discharge air temperature varies between the T min −a and T max −a (or T min −b and T max −b) as the cooling (or heating) capacity varies. For a specific coil discharge air temperature, the compressor speed can vary between the 0% cmpr and 100% cmpr to maintain the specific coil discharge air temperature. As illustrated by the shaded area  106   a  in the cooling mode (or  106   b  in the heating mode), minimal fan capacity requirement can be determined, for example, based on the specific coil discharge air temperature and the compressor operation speed. The fan capacity regulation block  944   a  can, for example, be configured to maintain the discharge fan at or about the fan speed corresponding to the minimal fan capacity. 
     Referring to  FIG. 9B , a discharge air temperature T min /T max  maintaining block  920  is illustrated. The discharge air temperature T min /T max  maintaining block  920  is configured to include a fan capacity regulating block  944   b  that is configured to include a method  950   b  to modulate fan speed of the variable speed discharge fan, and a coil DAT regulating block that is configured to include a method  960   b  to maintain a discharge air temperature of the coil at or about T min  in the cooling mode or at or about T max  in the heating mode. 
     When the operation speed of the variable speed compressor rises in the method  960   b , it is generally desirable to increase a minimal fan capacity requirement in the method  950   b ; and vice versa. 
     Regions  102   a  and  102   b  in  FIGS. 10A and 10B  can be used with the methods  950   b  and  960   b  to determine the minimal fan capacity requirement (or the corresponding minimal fan speed setting) in the cooling mode and the heating mode respectively, while maintaining the coil discharge air temperature. 
     In the region  102   a , as illustrated by the shaded area  107   a  between the line  104   a  for the 100% of the compressor operation speed setting and the line  105   a  for the 0% of the compressor operation speed setting of the variable compressor speed in  FIG. 10A , the minimal fan capacity requirement increases when the cooling capacity demand increases. Accordingly, the minimal fan capacity requirement (or a corresponding minimal fan speed setting) in the method  950   b  in the cooling mode can be mapped to the shaded area  107   a  between the lines  104   a  and  105   a  according to the current variable compressor operation speed relative to the 100% of the compressor operation speed setting. The method  950   b , accordingly, is configured to modulate the variable speed discharge fan between a high fan capacity setting of the variable speed discharge fan (or the corresponding high fan speed setting) and the minimal fan capacity requirement determined based on  FIG. 10A  (or the corresponding minimal fan speed setting). The high fan capacity setting (or the corresponding high fan speed setting) is generally fan capacity when the variable speed discharge fan is operated at or about a high speed setting. 
     In the heating mode as illustrated in the region  102   b  of  FIG. 10B , the method  950   b  can also determine the minimal fan capacity requirement (or corresponding minimal fan speed setting) according to the current variable speed compressor operation speed relative to the 100% of the compressor operation speed setting and the heating capacity demand. The method  950   b  can control the fan capacity (or the fan speed setting) between the minimal fan capacity requirement (or the corresponding minimal fan speed setting) determined (as illustrated by the shaded region  107   b ) and the high fan capacity setting (or the corresponding high fan speed setting). 
     It is to be noted that the fan capacity maps as illustrated in  FIG. 10A  and  FIG. 10B  can also be adapted to control a multiple speed discharge fan by modulating a percentage of time that the discharge fan operates in a particular speed. 
     Generally, the T min  and the T max  as illustrated in  FIG. 2  can be fixed numbers. For example, in one embodiment, the T min  is about 50° F. and the T max  is about 104° F. In some embodiments, the numbers are determined based on a designed limit, a capacity limit, a comfort limit of occupancy in the space, and/or safe operation limits. 
     It is to be noted that the T min  and the T max  do not have to be fixed numbers in some embodiments. In some embodiments, the T min  and the T max  can be calculated based on a space temperature setpoint for a space (such as the space  112  in  FIG. 1 ) and a ratio of the airflow when the fan is operated at or about the high speed setting to the airflow of the discharge fan when the discharge fan is operated at or about the low speed setting (CFM ratio). 
     When PI control is used to maintain the space temperature of the space while maintaining the discharge air temperature of the coil, the low and high discharge air temperature limits in the PI control (T low-Limit  and T high-Limit ) of the coil can be calculated. In one embodiment, calculated T low-Limit =(the temperature setpoint for the space)−(CFM ratio)*(the temperature setpoint for the space−designed coil low discharge air temperature). In one embodiment, calculated T high-Limit =(the temperature setpoint for the space)+(CFM ratio)*(the designed coil high discharge air temperature−the temperature setpoint for the space). The designed low coil discharge air temperatures can be provided, for example, by a manufacturer for a specific cooling/heating coil configuration. For example, in some embodiments, the low and high coil discharge air temperature limits are the coil temperatures when the coil reaches a capacity approximately equal to airflow ratio of minimum airflow to design airflow in the cooling mode and the heating mode respectively. Calculating the discharge air temperature T low-Limit  and T high-Limit  based on the ratio of the high airflow to the low airflow can help ensure that that the variable discharge air temperature control region and the variable fan control region have the same dynamic performance in space temperature control. 
     In some embodiments, such as in a heating VAV unit where a heating coil or electric heat section with an airflow damper is used, the method can also be adapted to control heating capacity and the airflow damper. In some embodiments, the calculated T max  can be used as the high discharge air temperature limit in the heating mode. 
     With regard to the foregoing description, it is to be understood that changes may be made in detail, without departing from the scope of the present invention. It is intended that the specification and depicted embodiments are to be considered exemplary only, with a true scope and spirit of the invention being indicated by the broad meaning of the claims.