Abstract:
In an embodiment, a circuit for providing a mixed air temperature signal is provided. The circuit has a temperature input to an economizer controller, an air temperature sensor, a false air temperature device, and a switching device. The switching device has a switch and a switch actuating device. The switch connects the temperature input to the air temperature sensor when the switch is in a first state. The switch connects the temperature input to the false air temperature device when the switch is in a second state. The switch actuating device places the switch in the first state when a mechanical cooling signal is not sent. The switch actuating device places the switch in the second state when the mechanical cooling signal is sent.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/219,306, filed on Mar. 19, 2014. U.S. patent application Ser. No. 14/219,306 claims priority from U.S. Provisional Patent Application No. 61/897,068 entitled MIXED AIR SENSOR BYPASS, filed on Oct. 29, 2013. U.S. patent application Ser. No. 14/219,306 and U.S. Provisional Patent Application No. 61/897,068 are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to HVAC controllers and, more particularly, to control of HVAC economizers. 
       BACKGROUND 
       [0003]    A function of a Heating, Ventilation, and Air Conditioning (HVAC) unit is to cool an enclosed space, usually a building. A typical unit can perform two types of cooling, free cooling and mechanical cooling. In free cooling, the unit mixes cooler outdoor air with return air from the building. In mechanical cooling, mechanical components in the unit operate to condition air flowing through the unit. In particular, an evaporator coil absorbs heat from the air flowing past it. 
         [0004]    Some units are controlled by an economizer controller, also called an economizer control system. During free cooling, the controller may control the unit based on mixed air, the mixture of outdoor air and return air received by the unit. The controller adjusts the relative amounts of outdoor air and return air, attempting to keep the temperature of the resulting mixed air at a mixed air set-point. 
         [0005]    Technically, the term “mixed air” refers to air received by the unit that has not passed the evaporator coil. In contrast, the term “supply air” refers to the air after it passes the evaporator coil. Without mechanical cooling, mixed air temperature and supply air temperature are interchangeable. During mechanical cooling, the evaporator coil reduces the supply air temperature below the mixed air temperature. 
         [0006]    In practice, due to space requirements, the temperature of the mixed air is often measured by a sensor located after the evaporator coil. In a typical HVAC unit, the outdoor air and the return air do not mix sufficiently until after the air passes the evaporator coil. If the mixed air temperature sensor were placed before the evaporator coil, either the outdoor air temperature or the return air temperature would dominate the temperature measured by the sensor. Therefore, to accurately measure the mixed air temperature, the mixed air temperature sensor is located after the evaporator coil. 
         [0007]    Despite its name then, the “mixed air” temperature sensor really measures the temperature of supply air. The position of the mixed air temperature sensor creates an issue when free cooling and mechanical cooling are performed together. The evaporator coil tends to cool the air below the mixed air set-point. In response to the low mixed air temperature, the controller attempts to warm the mixed air. As a result, the controller signals the unit to stop using outdoor air to cool the building. 
         [0008]    It would be desirable if a solution existed that would allow the unit to continue to receive outdoor air when free cooling is available and the unit is performing mechanical cooling. It would further be desirable if the solution could be implemented with only minimal modifications to an existing HVAC unit and HVAC controller. 
       SUMMARY 
       [0009]    In an embodiment, a circuit for providing a mixed air temperature signal is provided. The circuit has a temperature input to an economizer controller, an air temperature sensor, a false air temperature device, and a switching device. The switching device has a switch and a switch actuating device. The switch connects the temperature input to the air temperature sensor when the switch is in a first state. The switch connects the temperature input to the false air temperature device when the switch is in a second state. The switch actuating device places the switch in the first state when a mechanical cooling signal is not sent. The switch actuating device places the switch in the second state when the mechanical cooling signal is sent. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  depicts a HVAC unit and a HVAC controller which may benefit from an exemplary embodiment of the present invention; 
           [0012]      FIG. 2  depicts communications between a HVAC control system and a HVAC unit in accordance with the prior art; 
           [0013]      FIG. 3  is a wiring diagram in accordance with an exemplary embodiment of the present invention; and 
           [0014]      FIG. 4  depicts communications between a HVAC control system and a HVAC unit in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following discussion, numerous specific details are set forth to provide a thorough explanation. However, such specific details are not essential. In other instances, well-known elements have been illustrated in schematic or block diagram form. Additionally, for the most part, specific details within the understanding of persons of ordinary skill in the relevant art have been omitted. 
         [0016]    With reference to  FIG. 1 , depicted is a Heating, Ventilation, and Air Conditioning (HVAC) system  100  which may benefit from an exemplary embodiment of the present invention. HVAC system  100  comprises HVAC unit  102  and economizer controller  104 . Generally speaking, unit  102  informs controller  104  of the cooling needs of the building, and controller  104  instructs unit  102  on how to respond to those cooling needs. 
         [0017]    System  100  and unit  102  are described herein with the common term “HVAC,” but only an air conditioning function of system  100  and unit  102  is discussed. It is not essential that system  100  or unit  102  be able to perform heating or ventilation. HVAC system  100  and HVAC unit  102  may have heating and ventilation functions, but those functions are optional. 
         [0018]    Unit  102  contains mechanical components which, among other things, can draw air into unit  102 , condition the air, and discharge the air into the building. Unit  102  also contains temperature sensors for air temperatures related to conditioning the air. Unit  102  may be referred to as a Roof-Top Unit (RTU). However, unit  102  is not necessarily located on a rooftop. Controller  104  may be referred to as an economizer controller or a unit controller. 
         [0019]    Unit  102  has economizer  106 . Economizer  106  comprises outdoor air damper  108  and return air damper  110 . Outdoor air damper  108  can receive air from outside the building, and return air damper  110  can receive air returned from inside the building. Outdoor air damper  108  and return air damper  110  may each be opened, to receive air from their respective sources, or closed, to keep out air from their respective sources. The mix of air from outdoor air damper  108 , if any, and return air damper  110 , if any, is called the mixed air. 
         [0020]    Unit  102  has blower  112 . Blower  112  circulates air through unit  102 , bringing in air from economizer  106  and discharging the air into the building. 
         [0021]    Unit  102  has evaporator coil  114 . During mechanical cooling, evaporator coil  114  absorbs heat from the air moving across it. Thus, the mixed air is considerably cooler after it moves past evaporator coil  114 . 
         [0022]    In free cooling, unit  102  obtains cool outdoor air from outdoor air damper  108  and uses that air to cool the building. Free cooling is preferable to mechanical cooling, because free cooling does not use energy to mechanically condition air. Free cooling uses energy to operate economizer  106  and blower  112 , but the cooler air is obtained for free rather than being produced by unit  102 . Free cooling therefore uses substantially less energy. 
         [0023]    A purpose of controller  104  is to control free cooling. During free cooling, controller  104  attempts to keep the mixed air temperature at a mixed air set-point. The mixed air set-point may be set by a user, but is commonly 55 degrees Fahrenheit. Mixed air temperature sensor  116  senses the temperature of the mixed air. Controller  104  may read mixed air temperature sensor  116  from mixed air temperature (MAT) signal line  118 . 
         [0024]    To control the temperature of the mixed air, controller  104  adjusts economizer  106 . When controller  104  determines the mixed air temperature is above the mixed air set-point, controller  104  instructs unit  102  to fully open outdoor air damper  108 . When controller  104  determines the mixed air temperature is below the mixed air set-point, controller  104  instructs unit  102  to gradually close outdoor air damper  108  in an effort to raise the mixed air temperature. 
         [0025]    Return air damper  110  may open and close inversely with outdoor air damper  108 . As outdoor air damper  110  opens by an amount, return air damper  110  closes by the same amount. When outdoor air damper  108  closes by an amount, return air damper  110  opens by the same amount. When outdoor air damper  108  is fully open, return air damper  110  is fully closed, and vice versa. 
         [0026]    Free cooling is only available when the outdoor air temperature, as measured by an outdoor air sensor on unit  102 , is at or below an outdoor air set-point. When free cooling is unavailable, controller  104  instructs unit  102  to meet cooling demands through mechanical cooling. Unless otherwise specified, this discussion assumes free cooling is available. 
         [0027]    In addition to MAT signal line  118 , three other signal lines  120 ,  122 , and  124  are shown between unit  102  and controller  104 . Unit  102  and controller  104  will ordinarily have other signal lines in addition to those shown. The use of signal lines  118 ,  120 ,  122 , and  124  will be illustrated with reference to  FIG. 2 . Unit  102  uses Y1 demand signal line  120  to inform controller  104  of an initial, or “stage 1,” cooling demand. Unit  102  uses Y2 demand signal line  122  to inform controller  104  of an additional, or “stage 2,” cooling demand. Controller  104  uses Y1 response signal line  124  to instruct controller  104  to begin mechanical cooling. 
         [0028]    This discussion presents controller  104  as a controller which controls the mixed air temperature during free cooling. However, controller  104  reads the mixed air temperature from mixed air temperature sensor  116 , and mixed air temperature sensor  116  really measures the temperature of supply air. Thus, controller  104  could also be called a controller which controls the supply air temperature during free cooling. Controller  104  could also be called a controller which does not distinguish between mixed air and supply air. 
         [0029]    With reference to  FIG. 2 , depicted is an exemplary exchange  200  of signals between unit  102  and controller  104 . In signal  202 , unit  102  informs controller  104  that the building has a demand for cooling. Signal  202  may be referred to as a “Y1” signal, a “Y1 demand” signal, a “stage 1 demand” signal, or an “initial cooling demand” signal. Signal  202  may be sent over Y1 demand signal line  120 . As an example, unit  102  may send signal  202  by sending 24V across the terminals of Y1 demand signal line  120  for the duration of the building&#39;s initial cooling demand. 
         [0030]    In response to signal  202 , controller  104  reads the mixed air temperature being provided from the mixed air temperature sensor in communication  202 . Controller  104  may read the mixed air temperature from MAT signal line  118 . When the mixed air temperature is above the mixed air set-point, controller  104  instructs unit  102  in signal  206  to fully open outdoor air damper  108 . To save energy, controller  104  does not yet instruct unit  102  to begin mechanical cooling. 
         [0031]    If the outdoor air received through outdoor air damper  108  is insufficient to cool the building, unit  102  informs controller  104  in signal  208  that the building&#39;s cooling demand has not been satisfied. Signal  208  may be referred to as a “Y2” signal, a “Y2 demand” signal, a “stage 2 demand” signal, or an “additional cooling demand” signal. Signal  208  may be sent over Y2 demand signal line  122 . Similar to signal  202 , unit  102  may send signal  208  by sending 24V across the terminals of Y2 demand signal line  122  for the duration of the additional cooling demand. 
         [0032]    Controller  104  may then instruct unit  102  in signal  210  to begin mechanical cooling. Signal  210  may be referred to as a “Y1” signal, a “Y1 response” signal, a “Y1-O” signal, or a “mechanical cooling” signal. Signal  210  may be sent over Y1 response signal line  124 . As an example, controller  104  may send signal  210  by sending 24V across the terminals of Y1 response signal line  124  for the time unit  102  is instructed to perform mechanical cooling. 
         [0033]    When unit  102  begins mechanical cooling, an error occurs due to the position of mixed air temperature sensor  116  within unit  102 . In unit  102 , mixed air temperature sensor  116  is downstream of evaporator coil  114 , as is typical in packaged rooftop HVAC units due to space requirements. Evaporator coil  114  absorbs heat, producing cool air which is ordinarily well below the mixed air set-point. 
         [0034]    Mixed air temperature sensor  116  senses the temperature of this cool air. Unit  102  continues to monitor the mixed air temperature over MAT signal line  118 . In signal  212 , sent over MAT signal line  118 , unit  102  informs controller  104  that the mixed air temperature is below the mixed air set-point. In signal  214 , controller  104  responds by instructing unit  102  to close outdoor air damper  108 . Because the mixed air temperature is likely to remain below the mixed air set-point, outdoor air damper  108  eventually fully closes. 
         [0035]    Due to space requirements, mixed air temperature sensor  116  cannot be easily placed upstream of evaporator coil  114 . However, closing outdoor air damper  108  is an inefficient result, because free cooling can reduce the amount of necessary mechanical cooling. It would be preferable to keep outdoor air damper  108  open when unit  102  is performing mechanical cooling while free cooling is available. 
         [0036]    With reference to  FIG. 3 , depicted is a wiring diagram  300  showing a possible implementation of a solution. Controller  104 , mixed air temperature sensor  116 , MAT signal line  118 , and Y1 response signal line  124  are shown. Circuit  302  is the circuit which controller  104  reads the mixed air temperature from. Controller  104  has MAT terminals  304 A and  304 B. MAT terminals  304 A and  304 B are the mixed air temperature input to controller  104 . Controller  104  reads the voltage drop across terminals  304 A and  304 B as the mixed air temperature. 
         [0037]    A relay  306  and a resistor  308  have been introduced. Relay  306  has an inductor  306 A and a switch  306 B. Inductor  306 A is placed on Y1 response signal line  124 . When controller  104  sends signal  210  on Y1 response signal line  124 , signal  210  energizes relay  306 . 
         [0038]    Switch  306 B controls the temperature input received by controller  104  on MAT signal line  118 . When relay  306  is not energized, switch  306 B is in position  306 B 1 . Controller  104  receives a temperature from mixed air temperature sensor  116  as is conventional. Mixed air temperature sensor  116  creates a voltage drop across terminals  304 A and  304 B. The voltage drop represents the mixed air temperature. 
         [0039]    When relay  306  is energized, switch  306 B is in position  306 B 2 . In position  306 B 2 , relay  306  replaces the input of mixed air temperature sensor  116  to controller  104  with resistor  308 . Resistor  308  produces a voltage drop across terminals  304 A and  304 B. The voltage drop mimics a mixed air temperature at or above the mixed air set-point. Thus, while relay  306  is energized, controller  104  receives a false mixed air temperature from resistor  308 , rather than the actual mixed air temperature from mixed air temperature sensor  116 . In an embodiment, the mimicked mixed air temperature is 70 degrees Fahrenheit, but any mixed air temperature at or above the mixed air set-point is sufficient. 
         [0040]    The circuit in wiring diagram  300  may be easily implemented in an existing unit  102  and controller  104 . One need only insert relay  306  in lines  124  and  118 , and add resistor  308  to circuit  302 . The existence of relay  306  and resistor  308  is transparent to unit  102  and controller  104 . 
         [0041]    With reference to  FIG. 4 , depicted is an exchange  400  of signals between unit  102  and controller  104  in accordance with wiring diagram  300 . Also participating in exchange  400  is relay  306 . 
         [0042]    Exchange  400  is identical to exchange  200  in  FIG. 2  until controller  104  sends signal  210  to unit  102 , instructing unit  102  to perform mechanical cooling. On its way to unit  102 , signal  210  energizes relay  306 . While relay  306  intercepts unit signal  210 , signal  210  continues to unit  102  and causes unit  102  to begin mechanical cooling. 
         [0043]    Because relay  306  is energized, switch  306 B moves from position  306 B  1  to position  306 B 2 . Controller  104  receives signal  402 , the false mixed air temperature from resistor  308 . Signal  402  indicates to controller  104  that the mixed air temperature is above the mixed air set-point. In response, controller  104  sends signal  404 , instructing unit  102  to fully open outdoor air damper  108 . 
         [0044]    Some controllers do not account for mechanical cooling when controlling free cooling. In other words, these controllers control free cooling identically whether or not the unit is also performing mechanical cooling. Relay  306  and resistor  308  are most useful for these controllers. The false mixed air temperature from resistor  308  modifies the behaviour of a controller without any modification to the controller itself. 
         [0045]    Other controllers can account for mechanical cooling when controlling free cooling. Relay  306  and resistor  308  can also be used with these controllers. The controllers receive the false mixed air temperature, and the controllers can also adjust their behaviour according to the mechanical cooling. 
         [0046]    It is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of various embodiments.