Abstract:
A method for protecting an HVAC system of an air handling unit from low temperature outdoor ventilation air. The air handling unit may include an outdoor fresh air region, a return air region, a supply air region, and a damper situated in or adjacent to the fresh air region to regulate the flow of outdoor air into the air handling unit. The air handling unit mixes the outdoor fresh air and the return air to provide a mixed air stream to the HVAC system. In one illustrative embodiment, one or more sensors are used to measure the temperature and flow rate of the air entering or passing through the outdoor fresh air region, the temperature of the air entering or passing through the return air region and the flow rate of the air passing through the supply air region. The temperature of the mixed air stream, which is provided to the HVAC system, is then calculated using a controller or the like. In some cases, when the temperature of the mixed air stream falls below a threshold value, the controller may instruct the damper to close and reduce the fresh outdoor air that is brought into the air handler unit. Various other embodiments and algorithms are disclosed.

Description:
FIELD  
       [0001]     The present invention relates generally to Heating, Ventilation, and Air Conditioning (HVAC) systems, and more particularly to methods and apparatus for regulating the flow of outdoor air to help protect the HVAC systems from low temperature air.  
       BACKGROUND  
       [0002]     Modern buildings have an HVAC system that is used to control the environment of the building&#39;s inside space. In many systems, air from the building&#39;s inside space is drawn into return ducts and provided back to the HVAC system. To meet desired ventilation requirements, many HVAC systems include an exhaust port for exhausting at least some of the return air to the outside environment, and an intake port for bringing in fresh air to the HVAC system. In some cases, a damper is provided that selects how much return air is exhausted and how much outside air is brought into the building. Thus, and depending on the conditions, the air entering the rooms is often a mixture of fresh outdoor air and return air.  
         [0003]     In some systems, an HVAC economizer is provided to act as a first stage of cooling to increase fuel economy of the HVAC system during some cooling cycles. The HVAC economizer may mix cooler outdoor air and warmer return air to provide essentially “free” cooling during some or all cooling cycles. At times of heating or when the outdoor air temperature is unsuitably low, the economizer may automatically enter a “lockout” position. The lockout position may hold the outdoor air damper at a closed position or minimum outdoor airflow setting to prevent the undesirably low temperature air from entering the HVAC system.  
         [0004]     In many HVAC economizer applications, a low temperature controller is installed in the mixed air stream. The low temperature controller limits the amount of cooler outdoor airflow when the mixed air temperature drops below a safe operating low temperature limit of the HVAC system. If the mixed air temperature is too low, it may cause condensation or freezing in the HVAC cooling and/or heating coils, which in many cases, is undesirable. In the case of modular or packaged air handling unit components, it is difficult to install low temperature protection sensors and control. In some cases it may be physically impossible to install an averaging or other sensor in the mixed air stream plenum of many HVAC economizers. In other cases, the economizer outdoor air dampers, airflow station, and controller are fabricated and shipped as an enclosed module, which makes it difficult or impossible to install a mixed air low temperature sensor/controller. Thus, a method and system for determining the mixed air temperature to help protect the HVAC system from undesirably low temperatures would be desirable in these and other applications.  
       SUMMARY  
       [0005]     The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
         [0006]     The present invention relates generally to Heating, Ventilation, and Air Conditioning (HVAC) systems, and more particularly to methods and apparatus for regulating the flow of outdoor air to help protect the HVAC systems from undesirably low temperature air. In one illustrative embodiment, an Air Handling Unit (AHU) may be provided to determine a characteristic of a mixed air stream, wherein the AHU includes at least two flow inputs and a flow output. In order to determine a characteristic of the mixed air stream such as mixed air temperature, predetermined characteristics of the first flow input, predetermined characteristics of the second flow input and predetermined characteristics of the flow output may be measured or otherwise determined. The characteristic of the mixed air stream may then be calculated as a function of these predetermined characteristics, as desired. In some cases, the predetermined characteristics may include air temperature, air flow, air humidity, dew point and/or some other characteristic or characteristics, as desired.  
         [0007]     In one example, the air in the first flow input may be fresh outdoor air, the air in the second flow input may be return air, and the air in the flow output may be a supply air. The AHU may measure or otherwise determine the temperature and air flow of the fresh outdoor air, the temperature of the return air, and the air flow of the supply air. From this, the AHU may calculate the air temperature of the mixed air stream that passes to the HVAC cooling coils of the HVAC system. It is contemplated that various combinations of air temperature and air flow of the fresh outdoor air, return air, supply air may be used to compute the temperature of the mixed air stream, as desired. In some cases, if the temperature of the mixed air stream falls below a predetermined threshold, the AHU may begin to close the damper that controls the air flow of the fresh outdoor air to help increase the temperature of the mixed air stream. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram of an illustrative Air Handling Unit (AHU) having an HVAC system for use with a building;  
         [0009]      FIG. 2  is a flow diagram of an illustrative mixed air temperature calculation method;  
         [0010]      FIG. 3  is a flow diagram of another illustrative mixed air temperature calculation method;  
         [0011]      FIG. 4  is a flow diagram of another illustrative mixed air temperature calculation method;  
         [0012]      FIG. 5  is a flow diagram of another illustrative mixed air temperature calculation method;  
         [0013]      FIG. 6  is a flow diagram of an illustrative calculation of the supply airflow rate;  
         [0014]      FIG. 7  is a flow diagram of an illustrative method for protecting an HVAC system from undesirably low temperatures; and  
         [0015]      FIG. 8  is a legend that defines the parameters used in the flow diagrams of  FIGS. 2-7 . 
     
    
     DETAILED DESCRIPTION  
       [0016]     The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The detailed description and drawings show several embodiments which are meant to be illustrative of the claimed invention.  
         [0017]      FIG. 1  is a schematic diagram of an illustrative air handling unit (AHU)  16  in a building  20 . The building  20  may be a residential, commercial, or any other suitable building, as desired. The AHU  16  may include a heating, ventilation, and air conditioning (HVAC) unit  40 , which in some cases, may include one or more cooling and/or heating coils. In the illustrative embodiment, the AHU  16  includes at least two inputs and one output. A first input may correspond to the fresh outdoor air input  32 . The temperature and/or flow rate of the fresh outdoor air input may be measured or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic, such as humidity, dew point, carbon dioxide level, etc., of the fresh outdoor air input  32  may be measured and/or determined, as desired.  
         [0018]     A second input to the AHU  16  may correspond to the return air input  30 . The return air input  30  may include air that is pulled from the rooms inside of the building  20 . In some cases, some of the return air may be exhausted as shown at  38  through a damper  29 , and some of the return air may be recirculated back into the HVAC  40 . The temperature and/or flow rate of the return air input  30  may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the return air may be measured and/or determined as desired.  
         [0019]     A mixed air stream  34  may correspond to the AHU output. The mixed air stream  34  may be a mixture of the fresh outdoor air input  32  and the return air input  30 . The temperature and/or flow rate of the mixed air stream  34  may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the mixed air stream  34  may be measured and/or determined as desired. The HVAC system  40  may include a fan  18  to induce flow of air through the HVAC  40  and ductwork, as desired, to produce supply air  36  to the building  20 . In some cases, the temperature and/or flow rate of the supply air  36  may be measured and/or otherwise determined by one or more sensors, by computational methods, and/or any other method as desired. Additionally, any other characteristic of the return air may be measured and/or determined as desired.  
         [0020]     In some illustrative embodiments, a damper  28  may be provided to regulate the flow of fresh outdoor air  32  into the building  20 . Likewise, a damper  29  may be provided to regulate the amount of return air that is exhausted  38  from the building  20 . Yet another damper  31  may be provided to regulate the flow of return air  30  to mix with the fresh outdoor air  32 . In many cases, the dampers  28 ,  29  and  31  may be mechanically coupled together so that the dampers  28  and  29  open and close together or in sequence, and damper  31  opens and closes in an opposite manner to dampers  28  and  29 . Thus, when damper  28  is opened to allow more fresh outdoor air into the building, damper  29  also opens to allow a similar amount of return air to be exhausted from the building. In this example, the return air damper  31  may close as the dampers  28  and  29  open. This arrangement may help balance the pressure inside the AHU  16 .  
         [0021]     In some cases, the dampers  28  and  29  and associated duct work may be provided in an economizer, shown generally at  50 . Under some conditions, the economizer  50  may provide a first stage of “free” cooling by mixing cooler fresh outdoor air  32  with the sometimes warmer return air  30  to provide a mixed air stream  34  to the cooling coils of the HVAC system  40 . If the temperature of the mixed air stream  34  is too low, it may cause condensation or freezing in the HVAC cooling and/or heating coils, which in many cases, is undesirable. Thus, the present invention may include a controller  54  that calculates the temperature and possibly other characteristics of the mixed air stream  34  and, by adjusting the damper  28  and sometimes dampers  29  and  31 , limits the low temperature of the mixed air stream  34  that is provided to the HVAC cooling and/or heating coils.  
         [0022]     In some cases, the AHU  16  may also include a heat exchanger generally shown at  52 . The heat exchanger  52  may be adapted to efficiently transfer heat energy between the incoming fresh outdoor air  32  and the exhausted air stream  38 , which may be useful under some operating conditions.  
         [0023]      FIG. 2  is a flow diagram of an illustrative mixed air stream  34  temperature calculation method. To perform the illustrative mixed air stream  34  temperature calculation, sensor data is first acquired, as shown at block  60 . In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow  36  sensor (SplyFlow) is acquired by the controller  54  (see  FIG. 1 ). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow  36  sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream  34 , if desired. Once this data is acquired, and as shown at block  62 , the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Temp     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢             ⁢             ⁢   Air   ⁢           ⁢   Temp   *     (       Supply   ⁢           ⁢   Flow     -     Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow       )         )     *     (       1   /   Supply     ⁢           ⁢   Flow     )             
         [0024]     Note that when other characteristics of the various flow streams are measured, different characteristics of the mixed air flow stream may be calculated. For example, if an outdoor dew point sensor (OAD) and a return air dew point sensor (RAD) are provided, a Mixed Air Dew Point (MAD) value may be determined using a similar illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Dew   ⁢           ⁢   Point     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Dew   ⁢           ⁢   Point   *   Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢           ⁢   Air   ⁢           ⁢   Dew   ⁢           ⁢   Point   *     (       Supply   ⁢           ⁢   Flow     -       Outdoo   ⁢   r     ⁢           ⁢   Air   ⁢           ⁢   Flow       )         )     *     (       1   /   Supply     ⁢           ⁢   Flow     )           
 
 Instead of using dew point sensors, humidity and temperature sensors may be used to calculate the Outdoor Air Dew Point and the Return Air Dew Point, if desired. Also, Mixed Air Carbon Dioxide levels (MACD), as well as many other mixed air parameters may be determined in a similar manner, if desired. 
 
         [0025]     In any event, the illustrative mixed air temperature (MAT) calculation may be used to help protect the HVAC system  40  from low outdoor air  32  temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block  64 . The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow  32  may be reduced, as shown at block  66 . The fresh outdoor airflow  32  may be reduced by, for example, adjusting the damper  28  position to reduce the incoming fresh outdoor airflow  32 . In some cases, the damper  29  position may be equally decreased and the return air damper  31  may be increased to help balance the pressure within the HVAC system  40 .  
         [0026]      FIG. 3  is a flow diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream  34  temperature calculation, sensor data is first acquired, as shown at block  70 . In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and return air flow sensor (RAFlow) is acquired by the controller  54  (see  FIG. 1 ). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and return air flow (RAFlow) may be provided at convenient locations outside of the mixed air flow stream  34 , if desired. Once this data is acquired, and as shown at block  72 , the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Temp     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢             ⁢             ⁢   Air   ⁢           ⁢   Temp   *   Return   ⁢           ⁢   Air   ⁢           ⁢   Flow       )     *     (     1   /     (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢           ⁢   Air   ⁢           ⁢   Fow       )       )             
 The illustrative mixed air temperature (MAT) calculation may then be used to help protect the HVAC system  40  from low outdoor air  32  temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block  74 . The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow  32  may be reduced, as shown at block  76 . The fresh outdoor airflow  32  may be reduced by, for example, adjusting the damper  28  position to reduce the incoming fresh outdoor airflow  32 . In some cases, the damper  29  position may be equally decreased and the return air damper  31  may be increased to help balance the pressure within the HVAC system  40 . 
 
         [0027]      FIG. 4  is a logic diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream  34  temperature calculation, sensor data is first acquired, as shown at block  80 . In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and supply airflow sensor (SplyFlow) is acquired by the controller  54  (see  FIG. 1 ). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and supply airflow sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream  34 , if desired. Once this data is acquired, and as shown at block  82 , the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Temp     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Temp   *     (       Supply   ⁢           ⁢   Flow     -     Return   ⁢           ⁢   Air   ⁢           ⁢   Flow       )       +     Return   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Return   ⁢           ⁢   Air   ⁢           ⁢   Flow       )     *     (       1   /   Supply     ⁢           ⁢   Flow     )             
 The illustrative mixed air temperature (MAT) calculation may then be used to help protect the HVAC system  40  from low outdoor air  32  temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block  84 . The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow  32  may be reduced, as shown at block  86 . The fresh outdoor airflow  32  may be reduced by, for example, adjusting the damper  28  position to reduce the incoming fresh outdoor airflow  32 . In some cases, the damper  29  position may be equally decreased and the return air damper  31  may be increased to help balance the pressure within the HVAC system  40 . 
 
         [0028]      FIG. 5  is a logic diagram of another illustrative mixed air temperature calculation method. To perform the illustrative mixed air stream  34  temperature calculation, sensor data is first acquired, as shown at block  90 . In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and return air exhaust air flow sensor (RAExhaustFlow) is acquired by the controller  54  (see  FIG. 1 ). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), return airflow sensor (RAFlow), and return air exhaust air flow sensor (RAExhaustFlow) may be provided at convenient locations outside of the mixed air flow stream  34 , if desired. Once this data is acquired, and as shown at block  82 , the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Temp     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Return   ⁢           ⁢   Air   ⁢           ⁢   Exhaust   ⁢           ⁢   Flow     +     Return   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Return   ⁢           ⁢   Air   ⁢           ⁢   Flow       )     *     (     1   /     (       Return   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢           ⁢   Air   ⁢           ⁢   Exhaust   ⁢           ⁢   Flow       )       )             
 This function assumes that that the return air exhaust airflow (RAExhaustFlow) is approximately equal to the outdoor airflow (OAFlow), and thus dampers  28  and  29  preferably move together in this illustrative embodiment. Also, return air damper  31  may open and close in an opposite manner to dampers  28  and  29 . 
 
         [0029]     The illustrative mixed air temperature (MAT) calculation may be used to help protect the HVAC system  40  from low outdoor air  32  temperatures. For example, and in one illustrative embodiment, the mixed air temperature (MAT) may be compared to a mixed air temperature threshold temperature, as shown at block  94 . The mixed air temperature threshold may be any temperature in the range of, for example, 32 degrees Fahrenheit to 50 degrees Fahrenheit, but other temperatures may also be used as desired. If the illustrative mixed air temperature (MAT) is less than the mixed air temperature threshold, the fresh outdoor airflow  32  may be reduced, as shown at block  96 . The fresh outdoor airflow  32  may be reduced by, for example, adjusting the damper  28  position to reduce the incoming fresh outdoor airflow  32 . The damper  29  position may equally or substantially equally decrease the return air exhaust airflow (RAExhaustFlow), and the return air damper  31  may increase the return air flow  30  accordingly.  
         [0030]      FIG. 6  is a flow diagram of an illustrative method for determining the supply airflow (SplyFlow)  36 , as shown at block  100 . The illustrative method may be run on a regular basis, such as every second, minute, hour, day, etc., and may be used to set and/or update the supply airflow (SuplyFlow) parameter.  
         [0031]     The supply airflow (SplyFlow)  36  may depend on the type of HVAC system  40  used. Block  110  determines if a constant volume system is used. If a constant volume system is used, control is passed to block  112 . Block  112  determines if an outdoor airflow station is present, which provides a measure of the outdoor air flow (OAFlow). If an outdoor air flow station is present, control is passed to block  114 . Block  114  determines if the damper  28  of the economizer is at the full open (&gt;=100%) position. If the damper  28  is at the full open (&gt;=100%) position, control is passed to block  116 , which updates a MAXFLOW parameter with the outdoor air flow (OAFlow) that is currently measured by the outdoor airflow station. Control is then passed to block  120 , which updates the supply air flow (SplyFlow) with the updated MAXFLOW parameter.  
         [0032]     Referring back to block  114 , if the damper  28  is not at the full open (&gt;=100%) position, control is passed to block  120 , which updates the supply air flow (SplyFlow) with the old MAXFLOW parameter.  
         [0033]     Referring back to block  112 , if an outdoor air flow station is not present in the system, or is otherwise not working, control is passed to block  118 . Block  118  updates the supply air flow (SplyFlow) with the designed flow rate of the system (DsgFlow) multiplied by a Dirty Filter Factor (DirtyFltrFctr). The Dirty Filter Factor (DirtyFltrFctr) may be a value ranging from one (clean) to zero (very dirty), and may provide a measure of the reduction in supply air flow caused by the HVAC filter. From blocks  118  and  120 , control is passed to block  136 , wherein the algorithm is exited.  
         [0034]     Referring back to block  110 , if a constant volume system is not used, the system must be a Variable Air Volume (VAV) system, and control is passed to block  122 . Block  122  determines if the system is a Variable Air Volume (VAV) HVAC system that includes a Supply Flow Station for providing a measure of the supply air flow. If so, control is passed to block  124 . Block  124  receives the current supply air flow (SplyFlow) from the Supply Flow Station. Control is then passed to block  136 , wherein the algorithm is exited.  
         [0035]     Referring back to block  122 , if a Variable Air Volume (VAV) HVAC system  40  is used that does not include a Supply Flow Station, control is passed to block  126 . Block  126  determines if the Variable Air Volume (VAV) HVAC system  40  includes an outdoor air flow Station that provides a measure of the outdoor air flow (OAFlow). If so, control is passed to block  128 . Block  128  determines if the economizer damper  28  is at the full open (&gt;=100%) position. If the damper  28  is not at the full open (&gt;=100%) position, control is passed to block  134 . If the damper  28  is at the fill open (&gt;=100%) position, control is passed to block  130 , which updates the MAXFLOW parameter with the outdoor air flow (OAFlow) currently measured by the outdoor airflow station. Control is then passed to block  134 .  
         [0036]     Block  134  updates the supply air flow (SplyFlow) as a function of the flow capacity signal of the Variable Air Volume system, the minimum air flow setting of the Variable Air Volume system times a dirty filter parameter, and the MAXFLOW parameter. Control is then passed to block  136 , wherein the algorithm is exited.  
         [0037]     Referring back to block  126 , if the Variable Air Volume (VAV) HVAC system  40  does not includes an outdoor air flow Station that provides a measure of the outdoor air flow (OAFlow), control is passed to block  132 . Block  132  calculates the supply air flow (SplyFlow) as a function of the flow capacity signal of the Variable Air Volume system, the minimum air flow setting of the Variable Air Volume system, the MAXFLOW parameter, and a dirty filter parameter. The flow capacity signal Page:  11   [0] typical ranges from 0 to 100%, depending on the cooling demand of the zone terminals. Control is then passed to block  136 , wherein the algorithm is exited.  
         [0038]      FIG. 7  is a flow diagram of an illustrative method of protecting an HVAC system from undesirably low temperatures. It is contemplated that the illustrative method shown in  FIG. 7  may be run on a regular basis, such as every second, minute, hour, day, etc.  
         [0039]     To perform the illustrative method, sensor data is first acquired as shown at block  200 . In the illustrative embodiment, sensor data from an outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow  36  sensor (SplyFlow) is acquired by the controller  54  (see  FIG. 1 ). Note that the outdoor air temperature sensor (OAT), return air temperature sensor (RAT), outdoor airflow sensor (OAFlow), and supply airflow  36  sensor (SplyFlow) may be provided at convenient locations outside of the mixed air flow stream  34 , if desired. Once this data is acquired, and as shown at block  210 , the mixed air stream temperature (MAT) may be determined as a function of these parameters using the illustrative function:  
         Mixed   ⁢           ⁢   Air   ⁢           ⁢   Temp     =       (       Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Temp   *   Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow     +     Return   ⁢             ⁢             ⁢   Air   ⁢           ⁢   Temp   *     (       Supply   ⁢           ⁢   Flow     -     Outdoor   ⁢           ⁢   Air   ⁢           ⁢   Flow       )         )     *     (       1   /   Supply     ⁢           ⁢   Flow     )           
 
 Control is then passed to block  220 . Block  220  determines if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Heat Exchanger Low Limit (HtgExchgLow) or a Low Comfort Limit (LowComfort). If so, control is passed to block  222 . Block  222  limits the outdoor air flow ventilation that is provided. The fresh outdoor airflow  32  may be reduced by, for example, adjusting the economizer damper  28  position to reduce the incoming fresh outdoor airflow  32 . In some cases, the damper  29  position may be equally decreased to help balance the pressure within the HVAC system  40 . Control is then passed to block  224 , which issues a diagnostic warning signal or message. 
 
         [0040]     Referring back to block  220 , if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are not less than the Heat Exchanger Low Limit (HtgExchgLow) or the Low Comfort Limit (LowConfort), control is passed to block  230 . Block  230  determines if the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Safety Limit. If not, control is passed to block  240 , wherein the algorithm is exited.  
         [0041]     If the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) are less than a Safety Limit, then control is passed to block  232 . Block  232  starts a delay timer. Control is then passed to block  234 . Block  234  determines if the Delay Timer has exceeded a timer limit. If the Delay Timer has not exceeded the timer limit, control is passed to block  240 , wherein the algorithm is exited. The timer limit allows the Outdoor Air Temperature (OAT) and the Mixed Air Temperature (MAT) to be less than a Safety Limit for a predetermined period of time, which may help reduce nuisance fan stops, as further described below.  
         [0042]     If the Delay Timer has exceeded the timer limit, control is passed to block  236 . Block  236  stops the fan system and closes the outside air damper  28 . Control is then passed to block  238 , which issues an alarm signal or message. Control is then passed to block  240 , wherein the algorithm is exited.  FIG. 8  is a legend that defines the parameters used in the flow diagrams of  FIGS. 2-7 .  
         [0043]     Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.