Abstract:
A control strategy for supply fans in constant-volume heating, ventilating, and air-conditioning (HVAC) systems that reduces the speed of the fan at part-load conditions is provided. The invention consists of a constant-volume HVAC system, wireless discharge air temperature sensors, wireless hot source and cold source temperature sensors, and a wireless controller coupled to a fan modulation device. The controller includes a finite state machine that switches between a high-temperature control mode and a low-temperature control mode. The controller also includes a calculator that calculates a high temperature setpoint and a low temperature setpoint as a function of the hot source and cold source temperatures. In high temperature control mode, the controller compares the maximum discharge air temperature with the high temperature setpoint, and it commands the fan modulating device so that the maximum discharge air temperature remains close to the high temperature setpoint. In low temperature control mode, the controller compares the minimum discharge air temperature with the low temperature setpoint, and it commands the fan modulating device so that the minimum discharge air temperature remains close to the low temperature setpoint. Alternatively, the controller includes a calculator that computes a largest load as a function of previous fan command, discharge air temperatures, and readings from wireless zone temperature sensors. The controller increases the speed of the fan as the largest load increases, and reduces the speed of the fan as the largest load decreases.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit under Title 35, United States Code §119(e) of U.S. Provisional Application No. 60/644,669 filed on Jan. 18, 2005. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The following invention relates to controls for fans in heating, ventilating, and air-conditioning (HVAC) systems, specifically to controls for converting constant speed fans to variable flow operation while preserving proper zone temperature control. 
     2. Description of Prior Art 
     Modern buildings typically use heating, ventilating, and air-conditioning (HVAC) systems to control indoor temperature, pressure, ventilation rate and other variables. Prior to the oil embargo of the 1970s it was common to design HVAC systems with constant-speed fans and with temperature controls that would re-heat cooled air or mix heated air with cooled air to maintain space temperature. HVAC systems with constant speed fans are called constant-volume systems. 
     There are three common types of constant-volume HVAC systems that serve multiple zones. One of these three is the single-duct re-heat system. These systems have a single supply duct that delivers cooled air to each zone re-heat coil. Re-heat coils add heat to the cooled air to keep the zone space temperature close to a setpoint. In rare cases the supply duct delivers hot air that is re-cooled by zone re-cooling coils. 
     A second type of constant-volume system is the dual-duct constant-volume system. Dual-duct systems deliver heated air and cooled air all the way to each zone terminal unit with separate hot air ducts and cold air ducts. The hot air duct has a heating coil and heating valve that are used to keep the hot air duct temperature close to a setpoint. The cold air duct has a cooling coil and a cooling valve that are used to keep the cold air duct temperature close to a setpoint. Zone terminal units mix the heated air with the cooled air to keep the zone space temperatures close to a setpoint. 
     The third common kind of constant-volume system is the multi-zone system. A multi-zone system is a special kind of dual-duct system where the hot air duct and the cold air duct are short, and are referred to as the hot deck and the cold deck, respectively. The mixing dampers are close to the fan, and are integrated with the hot deck and the cold deck. The mixing dampers for each zone mix heated air from the hot deck with cooled air from the cold deck to keep the zone temperature close to a setpoint. The hot deck and cold deck are packaged with the fan and other components of the system. 
     Constant-volume HVAC systems are inefficient. In states with strict energy codes, such as California, they are effectively prohibited in new construction. For HVAC systems that serve multiple zones, it is now common to use variable-air-volume (VAV) systems. 
     VAV systems have variable-speed fans that are controlled so that the amount of simultaneous heating and cooling or re-heating is significantly reduced. There are two common kinds of VAV systems: single-duct and dual-duct. Single-duct VAV systems supply cooled air to each zone terminal unit, where it is metered with a control damper when cooling is required or re-heated when heating is required. When heating, the amount of cooled air is reduced to a low level by the terminal controls. Dual-duct systems deliver heated air and cooled air all the way to each zone terminal unit with separate air ducts. Dual-duct VAV terminal units have independent dampers that modulate hot airflow rate to heat and modulate cold airflow rate to cool. Unlike the dual-duct constant-volume system, the dual-duct VAV system does very little mixing. Most of the time it supplies a variable amount of hot air when heating and a variable amount of cooled air when cooling. It only mixes air when the amount of heating or cooling is close to zero so that adequate ventilation air is provided. 
     Although constant-volume systems are less common in new construction, there is still a large installed base that serves billions of square feet of commercial buildings. Since they are inefficient, many retrofit strategies have been developed to modify their design and operation in order to make them more efficient. 
     One approach is to convert constant-volume systems to VAV systems. Typical VAV conversions for single-duct or dual-duct constant-volume systems involve replacing the constant-volume terminal units with VAV terminal units, adding terminal controls, adding a supply duct static pressure sensor, adding a variable frequency drive (VFD) to the fan, and adding a controller to regulate the supply duct pressure by modulating the fan speed with the VFD. These conversions are very expensive and intrusive because of the mechanical modifications. The spaces served by the system may have to be evacuated while the construction work is performed. 
     A typical VAV conversion for a multi-zone system involves disabling the hot deck, adding terminal units with control dampers and reheat coils to each zone supply duct, adding terminal controls, adding a supply duct static pressure sensor, adding a variable frequency drive (VFD) to the fan, and adding a controller to regulate the supply duct pressure by modulating the fan speed with the VFD. This approach is very expensive and very intrusive because it requires significant mechanical modifications. 
     Another way to convert constant-volume systems to VAV is to use VAV diffusers. U.S. Pat. No. 6,736,326 to Hunka, U.S. Pat. No. 6,176,777 to Smith et al., and U.S. Pat. No. 5,556,335 to Holyoake all describe VAV diffusers that can be used to convert constant-volume systems to VAV operation. This approach still requires mechanical modifications. Furthermore, all of the diffusers must be replaced for this method to work properly, and most zones have more than one diffuser. 
     For single-duct constant-volume systems, Liu, Claridge, and Turner (Continuous Commissioning Guidebook for Federal Energy Managers, DOE, 2002) add a VFD to reduce the fan speed during after-hours operation. During occupied hours the fan is operated at full speed. This strategy does not save energy for systems that are shut off after hours. Even when there is after-hours operation, this method is not cost effective unless the system is large because the energy savings are limited. 
     For dual-duct constant-volume systems, Liu and Claridge (Converting Dual-Duct Constant-volume Systems to Variable Volume Systems Without Retrofitting the Terminals,  ASHRAE Transactions , Vol. 101, Part 1, 1999, pp. 66-70) describe a means for improving energy performance without retrofitting terminal units. They add a damper to the hot duct and use it to control the pressure in the hot duct. This strategy still requires a mechanical modification, which is intrusive and requires that the system be shut down. It also requires the installation of pressure sensors in the hot air duct and cold air duct. 
     For multi-zone constant-volume systems, Liu, Claridge, and Turner (Continuous Commissioning Guidebook for Federal Energy Managers, DOE, 2002) describe a means for improving the energy performance by adding a VFD to the supply fan and controlling the supply fan speed so that the most-open mixing damper is 95% open to the hot deck in the heating season. In the cooling season their strategy controls the fan speed so that the most-open mixing damper is 95% open to the cold deck. The command to the VFD comes from a Proportional-Integral-Derivative (PID) controller that takes the most-open damper position as input. This strategy requires that position sensors be added to the mixing dampers. Position sensors are expensive and difficult to install. Resistive position sensors are prone to vibration-induced premature failure. This strategy cannot be applied to single-duct constant-volume systems because they do not have mixing dampers. 
     Johnson (Johnson, G. A., 1984, “Retrofit of a Constant Volume Air System for Variable Speed Fan Control,” ASHRAE Transactions, 90(2B), 201-212.) describes a system for retrofitting single-zone constant volume air-handling units to VAV operation. Johnson&#39;s system does not apply to air-handling units that serve multiple zones. It also does not apply to single-zone systems that heat and cool. The units on which it was demonstrated were cooling-only units. Johnson&#39;s approach is to control the supply air temperature to a fixed setpoint with the cooling valve or outdoor air dampers, and then adjust the supply fan speed based on the average zone temperature so that the average zone temperature is maintained at a setpoint. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a control system for reducing the speed of the supply fan of a constant-volume HVAC system comprises the supply fan, a fan modulating device, and a plurality of discharge air temperature (DAT) sensors and a controller that calculates the supply fan speed based on the discharge air temperature sensor readings. The controller causes the fan speed to be reduced when the heating or cooling load is less than the design load. 
     OBJECTS OF THE INVENTION 
     Accordingly, a primary object of the present invention is to provide a control strategy for fans of constant-volume HVAC systems that can improve the energy efficiency at part-load conditions. 
     Another object of the present invention is to provide a control strategy for fans of constant-volume HVAC systems that can improve the energy efficiency at part-load conditions without requiring the installation of mechanical components such as air terminals, dampers or VAV diffusers. 
     Another object of the present invention is to provide a control strategy for fans of constant-volume HVAC systems that is applicable to all kinds of constant-volume HVAC systems. 
     Another object of the present invention is to provide a control strategy for fans of constant-volume HVAC systems that can improve the energy efficiency at part-load conditions and that uses sensors that are easy to install and that are inexpensive. 
     Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims, and detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a portion of a single-duct constant-volume HVAC system. 
         FIG. 2  is a schematic diagram of a portion of a dual-duct constant-volume HVAC system. 
         FIG. 3  shows a state transition diagram of a preferred embodiment for controlling a supply fan of a constant-volume HVAC system. 
         FIG. 4  shows a block diagram of the high temperature control loop of the preferred embodiment. 
         FIG. 5  shows a block diagram of the low temperature control loop of the preferred embodiment. 
         FIG. 6  shows a block diagram of the control calculations of an alternative embodiment. 
         FIG. 7  shows a relationship between the largest normalized load and the fan modulation device command for an alternative embodiment. 
       
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   REFERENCE NUMERALS IN DRAWINGS 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   1 
                   HVAC system 
                   2 
                   single-duct constant-volume 
                 
                 
                     
                     
                     
                   system 
                 
                 
                   3 
                   dual-duct constant-volume system 
                   4 
                   cooling coil 
                 
                 
                   5 
                   supply fan 
                   6 
                   supply air duct 
                 
                 
                   7 
                   re-heat coil 
                   8 
                   discharge air duct 
                 
                 
                   9 
                   discharge air diffuser 
                   10 
                   hot water supply pipe 
                 
                 
                   11 
                   thermostat 
                   12 
                   re-heat valve 
                 
                 
                   13 
                   heating coil 
                   14 
                   cold air duct 
                 
                 
                   15 
                   hot air duct 
                   16 
                   dual duct air terminals 
                 
                 
                   17 
                   mixing dampers 
                   18 
                   actuator 
                 
                 
                   19 
                   discharge air temperature sensor 
                   20 
                   supply fan controller 
                 
                 
                   21 
                   fan modulation device 
                   22 
                   hot source sensor 
                 
                 
                   23 
                   cold source sensor 
                   24 
                   high temperature control 
                 
                 
                     
                     
                     
                   mode 
                 
                 
                   25 
                   low temperature control mode 
                   26 
                   low DAT event 
                 
                 
                   27 
                   high DAT event 
                   28 
                   initialize PID entry function 
                 
                 
                   29 
                   high temperature control loop 
                   30 
                   low temperature control 
                 
                 
                     
                     
                     
                   loop 
                 
                 
                   31 
                   maximum temperature calculator 
                   32 
                   high temperature setpoint 
                 
                 
                     
                     
                     
                   calculator 
                 
                 
                   33 
                   PID calculator 
                   34 
                   minimum fan command 
                 
                 
                   35 
                   maximum command calculator 
                   36 
                   minimum temperature 
                 
                 
                     
                     
                     
                   calculator 
                 
                 
                   37 
                   low temperature setpoint calculator 
                   38 
                   zone temperature sensor 
                 
                 
                   39 
                   zone load calculator 
                   40 
                   largest load calculator 
                 
                 
                   41 
                   fan command calculator 
                   42 
                   summation calculator 
                 
                 
                   43 
                   previous fan command 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the supply fan control system is illustrated in  FIG. 1-5 . HVAC system  1  may be a single-duct re-heat system  2 , a dual-duct constant-volume system  3  or a multi-zone system.  FIG. 1  shows single-duct system  2 . Single-duct systems  2  include a cooling coil  4 , a supply fan  5 , supply air ducts  6 , re-heat coils  7 , discharge air ducts  8 , and discharge air diffusers  9 . Cooling coil  4  is a heat exchanger that carries a cooling fluid such as chilled water or a chilled water and glycol solution. Cooling coil  4  is mounted in supply air duct  6 . Supply fan  5  could be a centrifugal fan or an axial fan. Supply fan  5  is mounted in supply duct  6 . A duct is an elongate sheet metal structure with round or rectangular cross-section designed to transport air. Supply duct  6  contains branches that lead to re-heat coils  7 . Re-heat coil  7  is a heat exchanger that carries heating fluid supplied by a hot water supply pipe  10 . It is mounted between a branch of supply duct  6  and discharge air duct  8 . Discharge air duct  8  is a duct between reheat coil  7  and diffuser  9 . A thermostat  11  in the occupied space adjusts a re-heat valve  12 , which modulates the flow of heating fluid through re-heat coil  7 . Re-heat valve  12  is connected to hot water supply pipe  10  and re-heat coil  7 . 
       FIG. 2  shows dual-duct system  3 . Dual-duct systems  3  include supply fan  5 , supply duct  6 , cooling coil  4 , a heating coil  13 , a cold air duct  14 , a hot air duct  15 , dual duct air terminals  16 , discharge air ducts  8 , and diffusers  9 . Supply fan  5  is mounted in supply duct  6 . Supply duct  6  is connected to cold air duct  14  and hot air duct  15 . Cooling coil  4  is mounted at the beginning of cold air duct  14 . Heating coil  13  is mounted at the beginning of hot air duct  15 . Cold air duct  14  has branches that connect to air terminals  16 . Hot air duct  15  also has branches that connect to air terminals  16 . Air terminals contain mixing dampers  17  that mix hot air from hot air duct  15  with cold air from cold air duct  14 . Mixing dampers  17  are assemblies consisting of movable blades mounted on axles in a frame. Thermostat  11  adjusts actuator  18 , which is connected to mixing dampers  17 . 
     A multi-zone system is a dual-duct system  3  with a short cold air duct  14  and a short hot air duct  15  (referred to as a cold deck and a hot deck, respectively) so that mixing dampers  17  are located close to supply fan  5 . 
     HVAC systems  1  contains other mechanical components not shown in  FIG. 1  and  FIG. 2  such as filters, louvers, and humidifiers, which are used for other functions such as cleaning air, ventilation, and humidification. 
     For single-duct systems  2 , a discharge air temperature sensor  19  is located in discharge air duct  8  to measure air temperature inside discharge air duct  8 . Discharge air temperature sensors  19  are preferably wireless devices that communicate with a supply fan controller  20  using radio frequency communication. Controller  20  is preferably an electronic device comprising in combination a memory, a microprocessor, and a radio. Controller  20  is connected to a fan modulation device  21 . Fan modulation device  21  could be a variable-speed drive, variable inlet guide vanes, a throttling device such as a damper, or a device to adjust the pitch of the fan blades. A hot source sensor  22  is attached to hot water pipe  10  to measure temperature of hot water supply pipe  10 . Hot source sensor  22  is preferably a wireless device that communicates with controller  20  using radio frequency communication. A cold source sensor  23  is located in supply duct  6  to measure air temperature inside supply duct  6 . Cold source sensor  23  is preferably a wireless device that communicates with controller  20  using radio frequency communication. 
     For dual-duct systems  3 , a discharge air temperature sensor  19  is located in discharge air duct  8  to measure air temperature inside discharge air duct  8 . Discharge air temperature sensors  19  are preferably wireless devices that communicate with controller  20  using radio frequency communication. Controller  20  is preferably an electronic device comprising in combination a memory, a microprocessor, a radio, and analog outputs. Controller  20  is connected to a fan modulation device  21 . Fan modulation device  21  could be a variable-speed drive, variable inlet guide vanes, a throttling device such as a damper, or a device to adjust the pitch of the fan blades. A hot source sensor  22  is located in hot air duct to measure air temperature inside hot air duct  15 . Hot source sensor  22  is preferably a wireless device that communicates with controller  20  using radio frequency communication. A cold source temperature sensor  23  is located in cold air duct to measure air temperature inside cold air duct  15 . Cold source temperature sensor  23  is preferably a wireless device that communicates with controller  20  using radio frequency communication. 
       FIG. 3  shows a state transition diagram for controller  20 . The finite state machine has two modes of operation, a high temperature control mode  24  and a low temperature control mode  25 . A low DAT event  26  and a high DAT event  27  are the state transition events linking high temperature control mode  24  and low temperature control mode  25 . Low DAT event  26  occurs when the minimum discharge air temperature becomes less than the low temperature setpoint minus a low temperature offset. High DAT event  27  occurs when the maximum discharge air temperature becomes greater than the high temperature setpoint plus a high temperature offset. High temperature control mode  24  and low temperature control mode  25  both have an initialize PID entry function  28 . High temperature control mode  24  has a high temperature control loop  29  activity. Low temperature control mode  25  has a low temperature control loop  30  activity. 
       FIG. 4  shows a block diagram of high temperature control loop  29 . Outputs of discharge air temperature sensors  19  are inputs to a maximum temperature calculator  31 . Outputs of hot source temperature sensor  22  and cold source temperature sensor  23  are inputs to a high temperature setpoint calculator  32 . Output of maximum temperature calculator  31  and output of high temperature setpoint calculator  32  are inputs of a summation calculator  42 . Output of summation calculator  42  is an input to a PID calculator  33 . Output of PID calculator  33  and output of a minimum fan command  34  are inputs of a maximum command calculator  35 . Output of maximum command calculator  35  is input to fan modulation device  21 . Fan modulation device affects the output of supply fan  5 , which in turn affects discharge air temperatures. 
       FIG. 5  shows a block diagram of low temperature control loop  30 . Outputs of discharge air temperature sensors  19  are inputs to a minimum temperature calculator  36 . Outputs of hot source temperature sensor  22  and cold source temperature sensor  23  are inputs to a low temperature setpoint calculator  37 . Output of minimum temperature calculator  36  and output of low temperature setpoint calculator  37  are inputs of summation calculator  42 . Output of summation calculator  42  is an input to PID calculator  33 . Output of PID calculator  33  and minimum fan command  34  are inputs to maximum command calculator  35 . Output of maximum command calculator  35  is input to fan modulation device  21 . Fan modulation device affects the output of supply fan  5 , which in turn affects discharge air temperatures. 
     OPERATION OF THE PREFERRED EMBODIMENT 
     In operation, controller  20  adjusts fan modulation device  21  to either keep the maximum discharge air temperature close to the high temperature setpoint or to keep the minimum discharge air temperature close to the cold temperature setpoint. 
     When the system is first turned on controller  20  reads values from hot source temperature sensor  22  and cold source temperature sensor  23  and computes the high temperature setpoint and the low temperature setpoint as follows:
 
 T   h,s   =T   c   +F   h ( T   h   −T   c )  (1)
 
 T   c,s   =T   c   +F   c ( T   h   −T   c )  (2)
 
where T h,s  denotes high temperature setpoint, T c  denotes cold source temperature, F h  is a number between zero and one preferably equal to 0.85, T h  denotes hot source temperature, T h,s  denotes high temperature setpoint, and F c  is a number between zero and F h  preferably equal to 0.15.
 
     On startup controller  20  begins operating in high temperature control mode  24  unless one of the two conditions is true: 1) the minimum discharge air temperature is less than the low temperature setpoint and the maximum discharge air temperature is less than the high temperature setpoint, 2) the minimum discharge air temperature is less than the low temperature setpoint and the maximum discharge air temperature is greater than the high temperature setpoint but the difference between the low temperature setpoint and the minimum discharge air temperature is greater than the difference between the maximum discharge air temperature and the high temperature setpoint. If either of these two conditions is true, then controller  20  begins operation in low temperature control mode  25 . 
     If controller  20  begins operating in high temperature control mode  24  and the maximum discharge air temperature is greater than the high temperature setpoint, then PID calculator  33  causes fan modulation device  21  to increase airflow rate through supply fan  5 . In response to the increased flowrate, thermostat  11  causes the maximum discharge air temperature to decrease, preserving the heat transfer rate. Eventually controller  20  adjustments to fan modulation device  21  cause the discharge air temperature to reach the high temperature setpoint. Otherwise fan modulation device  21  receives minimum fan command  34 , causing supply fan  5  to deliver the minimum required airflow. 
     If controller  20  begins operating in low temperature control mode  25  and the minimum discharge air temperature is less than the low temperature setpoint, then PID calculator  33  causes fan modulation device  21  to increase airflow rate through supply fan  5 . In response to the increased airflow rate, thermostat  11  causes the minimum discharge air temperature to increase, preserving the heat transfer rate. Eventually controller  20  adjustments to fan modulation device  21  cause the minimum discharge air temperature to reach the low temperature setpoint. Otherwise fan modulation device  21  receives the minimum fan command  34 , causing supply fan  5  to deliver the minimum required airflow. 
     If while operating in high temperature control mode  24  the low DAT event  26  occurs, then operation switches to low temperature control mode  25 . PID calculator  33  is initialized by initialize PID  28  entry function so that the mode switching is bumpless. This is accomplished by setting the integration term of PID calculator  33  to the value that will make the output of PID calculator  33  equal the last output in high temperature control mode  24 . After PID calculator  33  is initialized, low temperature control loop  30  is executed repeatedly. Low temperature control loop  30  execution involves reading values from discharge air temperatures sensors  19 , computing the minimum discharge air temperature with minimum temperature calculator  36 , reading values from hot source temperature sensor  22  and cold source temperature sensor  23 , computing the low temperature setpoint with low temperature setpoint calculator  37 , subtracting the minimum discharge air temperature from the low temperature setpoint with summation calculator  42 , computing the output of PID calculator  33 , then passing the maximum of the output of PID calculator  33  and minimum fan command  34  to maximum command calculator  35 . Output of maximum command calculator  35  is passed as input to fan modulation device  21 . 
     If while operating in low temperature control mode  25  the high DAT event  27  occurs, then operation switches to high temperature control mode  24 . PID calculator  33  is initialized by initialize PID  28  entry function so that the mode switching is bumpless. This is accomplished by setting the integration term of PID calculator  33  to the value that will make the output of PID calculator  33  equal the last output in low temperature control mode  25 . After PID calculator  33  is initialized, high temperature control loop  29  is executed repeatedly. High temperature control loop  29  execution involves reading the values of discharge air temperature sensors  19 , computing the maximum value with maximum temperature calculator  31 , reading values from hot source temperature sensor  22  and cold source temperature sensor  23 , computing the high temperature setpoint with high temperature setpoint calculator  32 , subtracting the maximum discharge air temperature from the high temperature setpoint with summation calculator  42 , computing the output of PID calculator  33 , then passing the maximum of the output of PID calculator  33  and minimum fan command  29  to maximum command calculator  35 . The output of maximum command calculator  35  is passed as input to fan modulation device  21 . 
     DESCRIPTION OF AN ALTERNATIVE EMBODIMENT 
     An alternative embodiment is illustrated in  FIGS. 1 ,  2 ,  6 , and  7 . The alternative embodiment has the same mechanical components as the preferred embodiment. For single-duct system  2 , these include cooling coil  4 , supply fan  5 , supply air duct  6 , re-heat coils  7 , discharge air ducts  8 , discharge air diffusers  9 , hot water supply pipes  10 , thermostats  11 , and re-heat valves  12 , which are interconnected in the manner described in the description of the preferred embodiment. For a dual-duct system  3 , these include cooling coil  4 , supply fan  5 , supply duct  6 , discharge air ducts  8 , discharge air diffusers  9 , thermostats  11 , heating coil  13 , cold air duct  14 , hot air duct  15 , dual duct air terminals  16 , mixing dampers  17 , and actuators  18 , which are interconnected in the manner described in the description of the preferred embodiment. 
     Discharge air temperature sensors  19  are located in discharge air ducts  8  to measure air temperature inside discharge air ducts  8 . Discharge air temperature sensors  19  are preferably wireless devices that communicate with controller  20  using radio frequency communication. Each zone that has discharge air temperature sensor  19  has a zone temperature sensor  38 . Zone temperature sensors  38  measure temperature in the occupied space of a building. Zone temperature sensors  38  are preferably wireless devices that communicate with controller  20  using radio frequency communications. Controller  20  is preferably an electronic device comprising in combination a memory, a microprocessor, a radio, and analog outputs. Controller  20  is connected to a fan modulation device  21 . Fan modulation device  21  could be a variable-speed drive, variable inlet guide vanes, a throttling device such as a damper, or a device to adjust the pitch of the fan blades. 
       FIG. 6  show a block diagram of the calculations of the alternative embodiment. Controller  19  contains a zone load calculator  39 , a largest load calculator  40 , and a fan command calculator  41 . Outputs from discharge air temperature sensors  19 , zone temperature sensors  38 , and the previous fan command  43  are inputs to zone load calculator  39 . Outputs from zone load calculator  39  are inputs to largest load calculator  40 . Output from largest load calculator  40  is input to fan command calculator  41 . Output of fan command calculator  41  is input to fan modulation device  21  and previous fan command  43 . 
     OPERATION OF AN ALTERNATIVE EMBODIMENT 
     In operation, controller  20  adjusts fan modulation device  21  such that the fan delivers more air when the largest load is larger and delivers the minimum necessary flow when the largest load is zero. 
     When the system is first turned on, previous fan command  43  is initialized to the minimum fan command. Controller  20  reads values from discharge air temperature sensors  19  and zone temperature sensors  38  and previous fan command  43 . For each zone with a discharge air temperature sensor  19  and a zone temperature sensor  38 , zone load calculator  39  computes a calculated load according to the following equation:
 
 L   i   =w   i   ρMFC   p ( T   d,i   −T   z,i )  (3)
 
where L i  denotes a calculated load associated the i th  zone, w i  denotes a weight associated with the i th  zone, ρ denotes air density, M denotes previous fan command  43 , F denotes the supply flow rate when the fan modulation device  21  command is 100%, C p  denotes the specific heat of air at constant pressure, T d,i  denotes discharge air temperature of the i th  zone, and T z,i  denotes zone temperature of the i th  zone.
 
     Largest load calculator  40  computes the largest normalized load by first computing the maximum of the absolute values of the calculated zone loads, then multiplying the maximum absolute zone load by the sign of the calculated zone load with largest absolute value, then dividing that result by a design load. If the sign of the calculated zone load with largest absolute value is negative, indicating a cooling load, then the design load used in the calculation is the design cooling load. If the sign of the calculated zone load with largest absolute value is positive, indicating a heating load, then the design load used in the calculation is the design heating load. 
     Fan command calculator  41  computes the command from controller  20  to fan modulation device  21  using the function shown in  FIG. 7 . The function is linear between a minimum value and a maximum value. The maximum fan command when heating is preferably lower than the maximum fan command when cooling. 
     CONCLUSION, RAMIFICATIONS, AND SCOPE 
     Accordingly, the reader will see that the supply fan control system of this invention has a number of advantages including the following:
         (a) It can greatly improve the energy efficiency of constant-volume HVAC systems.   (b) It does not require the installation of mechanical components such as air terminals, dampers or VAV diffusers.   (c) It is applicable to all kinds of constant-volume HVAC systems.   (d) It uses sensors that are easy to install and that are inexpensive.       

     This disclosure is provided to reveal preferred embodiments of the invention and a best mode for practicing the invention. Having thus described the invention in this way, it should be apparent that various different modifications can be made to the preferred embodiment without departing from the scope and spirit of this disclosure. For instance, the invention can be adapted to operate a single-duct re-heat system with electric re-heat by using a fixed rather than measured hot source temperature. Discharge air temperature sensors  19  could be installed in diffusers  9  instead of discharge air ducts  8 , since discharge air flows through them. Fixed zone temperature values such as 72 degrees Fahrenheit could be used instead of readings from zone temperature sensors  38 . Return air temperature could be used instead of zone temperature. A reading from a supply airflow sensor could be used to compute calculated loads instead of fan modulation device  21  command. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.