US20030181158A1 - Economizer control - Google Patents

Economizer control Download PDF

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US20030181158A1
US20030181158A1 US10353947 US35394703A US2003181158A1 US 20030181158 A1 US20030181158 A1 US 20030181158A1 US 10353947 US10353947 US 10353947 US 35394703 A US35394703 A US 35394703A US 2003181158 A1 US2003181158 A1 US 2003181158A1
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Prior art keywords
air
damper
outside
recited
sensor
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US10353947
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Michael Schell
Rod Valenta
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Edwards Systems Technology Inc
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Edwards Systems Technology Inc
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F3/00Air-conditioning systems in which conditioned primary air is supplied from one or more central stations to distributing units in the rooms or spaces where it may receive secondary treatment; Apparatus specially designed for such systems
    • F24F3/044Systems in which all treatment is given in the central station, i.e. all-air systems
    • F24F3/0442Systems in which all treatment is given in the central station, i.e. all-air systems with volume control at a constant temperature
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F11/00Control or safety arrangements
    • F24F11/0001Control or safety arrangements for ventilation
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F11/00Control or safety arrangements
    • F24F11/30Control or safety arrangements for purposes related to the operation of the system, e.g. for safety or monitoring
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F11/00Control or safety arrangements
    • F24F11/0001Control or safety arrangements for ventilation
    • F24F2011/0002Control or safety arrangements for ventilation for admittance of outside air
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F11/00Control or safety arrangements
    • F24F11/0001Control or safety arrangements for ventilation
    • F24F2011/0006Control or safety arrangements for ventilation using low temperature external supply air to assist cooling
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F2110/00Control inputs relating to air properties
    • F24F2110/20Humidity
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24FAIR-CONDITIONING, AIR-HUMIDIFICATION, VENTILATION, USE OF AIR CURRENTS FOR SCREENING
    • F24F2110/00Control inputs relating to air properties
    • F24F2110/50Air quality properties
    • F24F2110/65Concentration of specific substances or contaminants
    • F24F2110/70Carbon dioxide
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/70Efficient control or regulation technologies
    • Y02B30/76Centralised control
    • Y02B30/767Centralised control of air distribution systems

Abstract

An economizer control for controlling air quality. The economizer control includes a sensor that senses characteristics of air, a damper located relative to the sensor so that the damper can control air flow of outside air and re-circulated air to the sensor, and a controller in communication with the sensor and the damper. The controller controls the opening and closing of the damper according to conditions sensed by the sensor.

Description

    PRIORITY
  • This application claims the benefit of U.S. Provisional Application No. 60/352,857, filed on Feb. 1, 2002. The aforementioned application is incorporated herein by reference.[0001]
  • FIELD OF THE INVENTION
  • The present invention relates generally to an improved economizer control of HVAC (Heating, Ventilation and Air Conditioning) systems. More particularly, the present invention relates to an improved economizer HVAC system control resulting in increased energy efficiency and a more comfortable environment than current economizer controls provide, while also ensuring good indoor air quality. [0002]
  • BACKGROUND OF THE INVENTION
  • A unitary heating, ventilation and air conditioning [Hereafter unitary HVAC] system is one type of HVAC system that is deemed “unitary” because it is generally configured in an integrated manner so that this one system provides heating, cooling and air movement in a single package. This unitary HVAC system can easily be placed on the rooftop of a building. In a typical system, air intake dampers are adjusted to provide a fixed outside air component to air circulated through the system. The amount of outside air required is usually determined by determining the design occupancy of the space and multiplying this times a cfm/person (cfm=cubic feet per minute) recommended ventilation rate required by local codes and standards. Typically, most spaces require 15 cfm per person. This amount of outside air is determined by a design engineer and adjusted by the contractor that actually performs installation of the system. Typically, this adjustment of the outside air intake results in a 20% to 30% mix of outside air together with recirculated air. [0003]
  • Another common type of HVAC system is the unit ventilator. Unit ventilators are a smaller version of unitary air handling equipment that have been designed to serve a single space. Rather than being rooftop mounted, these devices are typically mounted through a wall and are popular in applications such as servicing the HVAC needs of school classrooms and hotel rooms. These devices, like the unitary HVAC, are also integrated units designed to provide heating, cooling and ventilation. The unit ventilator, however, services the heating and cooling requirements for a more limited area than a unitary system can. Unit ventilators are popular because they have a lower initial cost than a centralized system and they allow for specific control of a single zone. Unit ventilator systems can be operated continuously or on an as needed basis for both heating and cooling. [0004]
  • Most HVAC systems are only capable of being controlled based on temperature and occupancy (manual turn off/on or timed operation). Humidity control has not been historically a consideration and most manufacturers do not have strategies for dealing with humidity. Finally, accurate, dependable and low cost humidity sensors have not been available on the marketplace that have the low-cost, low maintenance and long life characteristics demanded by these applications. [0005]
  • The combination of all the factors above have resulted in the increased manifestation of unwanted growth of mold, mildew and other bacterial entities in indoor spaces that can compromise the quality of indoor air and the health of building occupants. The presence of excessive moisture can also result in the deterioration of physical components of a building including drywall, ceilings and wooden structural components. [0006]
  • SUMMARY OF THE INVENTION
  • It is therefore a feature and advantage of the present invention to provide an economizer control for controlling the air quality for a space. In one embodiment of the invention, the economizer control includes a sensor that senses characteristics of air, a damper located relative to the sensor so that the damper can control air flow of outside air and re-circulated air to the sensor, and a controller in communication with the sensor and the damper. The controller controls the opening and closing of the damper according to conditions sensed by the sensor. [0007]
  • In another embodiment of the invention a method for controlling an economizer includes the steps of sensing characteristics of air; and controlling a damper to control the air flow of outside air and re-circulated air in accordance with the sensed characteristics of the air. [0008]
  • In an alternate embodiment of the invention, a system for controlling an economizer includes a means for sensing characteristics of air located in a mixed air section of an air handler, and a means for controlling a damper to control the air flow of outside air and re-circulated air in accordance with sensed characteristics of the air. [0009]
  • There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. [0010]
  • In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. [0011]
  • As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. [0012]
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 is an illustration of one embodiment of the invention. [0013]
  • FIG. 2 is an illustration of another embodiment of the invention. [0014]
  • FIG. 3 is a chart showing the results of a study for economizers serving a retail space. [0015]
  • FIG. 4 is a graph illustrating a pattern of CO2 buildup. [0016]
  • FIG. 5 is a flow diagram showing an embodiment of economizer control. [0017]
  • FIG. 6 is a graph of enthalpy per pound of dry air. [0018]
  • FIG. 7 is a graph of enthalpy per pound of dry air with a shaded region showing a 50% loss with a change of 10 degrees.[0019]
  • DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
  • One embodiment of the invention includes the use of system supply air pressure to move an air sample to an air return sensor, thereby enabling use of same sensors for measurement of one or more air quality parameters for both supply air and return air measurement using pressure to drive reciprocating sampling. One such embodiment of the invention is generally illustrated in FIG. 1. [0020]
  • FIG. 1 illustrates an HVAC system or air handler having an economizer control in accordance with one embodiment of the invention. The HVAC system or air handler includes a fan [0021] 100, heating coil 102 and cooling coil 104. A supply air duct 106 leads to the space or building to be supplied air. A return air duct 108 leads from the space being supplied air back to a mixed air chamber 110 through a return air damper 112. An exterior air damper 114 is provided between the mixed air chamber 110 and exterior air. In this embodiment of the invention, an actuator 116 is provided to open and close return air damper 112 and exterior air damper 114. A controller/sensor 118 is located between supply air duct 106 and return air duct 108. Based on the positions of return air damper 112 and exterior air damper 114, controller/sensor 118 will receive exterior air, return air or a mix of exterior air and return air through conduits 120. In some instances, return air damper 112 may be closed so that return air cannot enter mixed air chamber 110. In this case pressure relief damper 122 opens to exhaust the return air.
  • Control/sensor [0022] 118 controls fan 100, heating coil 102 and cooling coil 104. Return air damper 112 and exterior air damper 114 are also controlled by control/sensor 118 using actuator 116. In one embodiment of the invention a thermostat/CO2 sensor 124 relays information to controller/sensor 118 to control the characteristics of air being supplied.
  • In another embodiment of the invention as depicted in FIG. 2, the HVAC system or air handler includes a fan [0023] 200, heating coil 202 and cooling coil 204. A supply air duct 206 leads to the space or building to be supplied air. A return air duct 208 leads from the space being supplied air back to a mixed air region 210 through a return air damper 212. An exterior air damper 214 is provided between the mixed air chamber 210 and exterior air. In this embodiment of the invention, an actuator 216 is provided to open and close return air damper 212 and exterior air damper 214. A controller 218 is located between supply air duct 206 and return air duct 208. In some instances, return air damper 212 may be closed so that not enough return air enters mixed air chamber 210. In this case pressure relief damper 222 opens to exhaust the return air.
  • Controller [0024] 218 controls fan 200, heating coil 202 and cooling coil 204. Return air damper 212 and exterior air damper 214 are also controlled by controller 218 using actuator 216. In one embodiment of the invention, a sensor 224, which could be a temperature/absolute humidity sensor, receives outside air, return air or mixed air based on the positioning of return air damper 212 and exterior air damper 214. The sensor 224 then relays information to controller 218 to control the dampers for proper airflow.
  • The invention as described above is capable of configuration in several embodiments for use in several different HVAC system economizer control applications. One such application concerns the improved economizer control of unitary equipment. Another embodiment of the present invention results in the improved control of other HVAC systems such as unit ventilators, to control conditions within a classroom or hotel room. The present invention may take the form of a device integrated into a unit ventilator or similar system, or as a wall or surface mounted control. [0025]
  • One or more of sensors for smoke and measurement of other air quality parameters including temperature control rely on imbedded micro-processors for their measurement and control functions. One embodiment of the present invention enables the integration of many or all of these functions into a single device that can share microprocessing power enabling multi-parameter sensing which results in an increased understanding of what is happening in a building, and thereby provides better control of the indoor building environment. [0026]
  • The present invention enables the use of CO2 level measurements in return and supply air to calculate and set the outside air damper position. The current invention also facilitates input and control of remote CO2 sensors. [0027]
  • Carbon dioxide (CO2) is one of the common constituents of the air in our atmosphere. The concentration of CO2 in our atmosphere is typically 380 to 400 parts per million. Due to the natural tendency of gas molecules to readily diffuse and equalize in air, worldwide levels tend to remain relatively constant and generally within the aforementioned range of concentrations. Because the outside CO2 level is relatively constant, the outside CO2 level can be used as a reference value for outside air. [0028]
  • CO2 is also produced by humans at a relatively constant and predictable rate based on a given activity level. An individual exhales approximately 40,000 PPM of CO2 with each breath. A more active individual contributes even more CO2 to a given space. Since people are the most significant contributor of indoor CO2, the concentration level of indoor CO2 is a good indicator of occupancy within a space. For example, doubling the number of people in a space will also double the amount of CO2 produced in the same space. [0029]
  • An inside measurement of CO2 concentration levels provides a dynamic means to measure the number of people occupying a space (contributing CO2) and the amount of low concentration of outside air being drawn into the space to provide fresh air and dilute contaminants. As a result, CO2 can be used to measure and control the amount of outside air that is provided to the space. [0030]
  • Because the CO2 contribution is very predictable based on a common activity level, the measure of CO2 is directly related to the cfm/person of outside air delivered to the space. [0031]
  • When CO2 control is applied to a unitary air handling system, CO2 can be measured in the return air before the air intake dampers and/or in the space. The CO2 sensor is then used to modulate the outside air damper to deliver the proper amount of outside air for the occupancy of the space. Typically this approach allows the damper to be adjusted below the fixed position typically set assuming “design occupancy”. Energy savings are realized here because a portion of outside air does not have to be conditioned. Yet the required ventilation rates established on a per person basis can still be maintained thus ensuring acceptable indoor air quality. A CO2 sensor modulates the air intake damper between an upper limit (typically 20-30% outside air) and a lower limit of about 5% outside air. [0032]
  • Another benefit is that the invention enables use of H2O level measurements in return and supply air to calculate differential enthalpy and/or to control to dewpoint conditions. [0033]
  • FIG. 3 is a chart showing the results of a study for economizers serving a retail space. FIG. 3 shows the relative HVAC cost using no economizer, a 55 drybulb economizer system, a 65 drybulb economizer system, an enthalpy economizer system and a differential enthalpy economizer system. The study revealed that drybulb economizer systems showed HVAC cost savings over a system without an economizer system. The study revealed that both enthalpy based economizer systems showed greater cost savings than the drybulb based systems and that the differential enthalpy system provided the most HVAC cost savings. [0034]
  • One embodiment of the invention includes a sensor to be affixed to a building or one of the interior walls of a space (such as a building or a room) to measure temperature, absolute humidity and CO2 levels. The aforementioned air quality parameter measurements are used to 1) sense occupancy within the space (CO2)2) control outside air ventilation in the space (CO2) 3) maintain humidity levels below the point where moisture related damage may occur when the room is unoccupied and 4) to maintain temperatures when the room is occupied. This particular embodiment is particularly useful to monitor and control spaces like but not limited to hotel rooms where occupancy is unpredictable and current control schemes strictly operate the system in an on/off mode. [0035]
  • The invention enables use of a CO2 level sensor for occupancy determination within a space. The invention determines occupancy within a space by comparing CO2 concentration levels over a period of time and detecting patterns in the variations of the concentration levels that are typical of one or more people in the space. Such a sensor could also be combined with a simple occupancy sensor that could indicate initial occupancy and the CO2 sensor could measure and control for on-going occupancy. [0036]
  • The invention also could be coupled with a simple occupancy sensor to provide a baseline for initial occupancy of the space and where the invention measures occupancy continuously and controls the HVAC system to maintain the proper ventilation rate accordingly. [0037]
  • The invention enables use of a CO2 sensor in the measurement and control of ventilation rates on a per person basis. CO2 levels in a space increase to a predictable level in a predictable exponential manner to a CO2 level that corresponds to a given ventilation rate per person in the space. Conventional CO2 control and measurement approaches generally must wait for CO2 levels to reach the peak or leveling off point of CO2 concentrations (called the equilibrium level) before ventilation rates can be accurately predicted. FIG. 4 shows this pattern of CO2 buildup and leveling off depending on the ventilation rate. The present invention, however, uses a predictive algorithm to look at the rate of rise of CO2 and predict where the leveling point or equilibrium level was and allowing a prediction of the current ventilation rate. This method of measuring ventilation overcomes the traditional problem of waiting for CO2 levels to build up and level off, instead providing a real time indication of the ventilation rate within the space. The calculation is based on the rate of change of CO2 over a fixed period of time ranging from one minute or less to every 15 minutes or more. Variables of the predictive algorithm include the human activity level anticipated in the space, the typical design densities expected, and the mathematical function for the exponential buildup of CO2 concentration levels within a space. The invention allows for calculation and control on a real-time basis of the actual ventilation rate/person to the space. This could be used both as a control parameter and a display parameter to occupants of the space. Such a parameter could also be indicated on a display in a graphical format which would indicate if a space was over or under ventilated or ventilated just right. It would provide a much more relevant indication of ventilation as compared to providing just a CO2 concentration. [0038]
  • In the present invention, sensors can be placed in the return air to compare differential conditions by adjusting dampers to allow a proper measurement of exterior air to determine if outside temperature and humidity conditions (combined measurement is often called enthalpy) are sufficient to utilize outside air for free cooling. If sensors are provided outside, the sensors would not last long due to the extreme conditions they are exposed to. Another embodiment of the present invention includes an absolute humidity sensor and temperature sensor in the mixed air section of the air handler or air handling system. In this arrangement, the sensors continually monitor the conditions of the mixture of return air and supply air. [0039]
  • One such control strategy, as depicted in FIG. 5, controls an economizer as illustrated in FIG. 2. In step [0040] 500, the economizer is in cooling mode. In step 502 the controller 218 periodically opens the exterior dampers to a consistent position above the current or minimum airflow setting. In step 504, the temperature detected by sensor 224 is transmitted to controller 218 to make a determination as to whether the temperature is within an acceptable range for economizer control (e.g. between 55 F. and the return air temperature (70 F.)). If the temperature is not within an acceptable range the exterior air intake damper 214 is returned to a minimum air flow position in step 506. If the temperature is within an acceptable temperature range, the absolute humidity is determined in step 508 from sensor 224. If the absolute humidity concentration is within an acceptable range, the external air intake damper 214 continues to open in step 510. Otherwise, the exterior air intake damper 214 returns to a minimum air flow position.
  • Another variation of this approach is designed to only continue opening of the damper if temperature is acceptable and the absolute humidity level remains the same or is dropped. If the temperature and or the absolute humidity levels increase, the dampers or outside airflow rate will go back to the minimum position. If outside air is used for cooling, CO2 control of the dampers can be overridden by the economizer control. [0041]
  • The sensor in the mixed air may also be used to sense and control the latent heat and sensible heat cooling characteristics of an air handling system by determining the moisture level of air before it enters the cooling coil. The ability for the coil to cool or remove humidity from the air can be controlled by a number of factors including: controlling the velocity of air through the coils, controlling the temperature of liquid in the coils, or staging the operation of a multiple combination of coils to achieve the desired performance level. This allows for much better control of humidity levels than current control methods provide. [0042]
  • Humidity buildup in buildings in summer or any time in humid climates can occur when equipment is placed on a setback or off cycle during evening and weekends. As long as the temperature remains high, humidity concentrations are not a concern. However, when a system is activated and cooling begins, the temperature will be reduced, significantly reducing the moisture holding capacity of the air. The result is that condensation will occur on the coldest parts of the building (such as on slab on grade floors, around cooling ducts). This condensation can lead to mold and mildew contamination that can affect air quality. [0043]
  • The conditions necessary to cultivate a hospitable environment for bacterial or fungal growth include warm temperatures (temperatures of 60° to 90° ), availability of source of nutrients (dust, dirt and organic human byproducts), and the presence of bacterial or mold spores (they come from outside but are everywhere), all of which are readily available in most indoor environments. The missing ingredient that makes it all work is and enzyme solvent, namely water. When conditions in a building reach a point where water condenses on a cold surface in a building the final ingredient to a self-starting science project has been added. Once this growth starts, the contamination will continue to survive regardless of the future presence of water. [0044]
  • Air has a limited capacity to hold water vapor based on its temperature. As illustrated in FIGS. 6 and 7, if the air temperature decreases just 10° F., the air loses half of its ability to hold moisture. Once air becomes saturated, it will condense on the coldest surfaces. It is exactly the same effect that occurs to a cold drink held in the humid summer air. [0045]
  • In a building it's a bit more complicated but the same principle applies. When a room is unoccupied and the system is turned off, the room heats up and humid air from outside enters the room. Because the air is warm it can hold lots of moisture. But when the cooling system turns on, the air conditioning quickly cools the room to the set-point temperature. Unfortunately the system is unable to dehumidify the room as fast as it cools the room and as a result, water begins to form on the coldest surfaces in the room. The coldest surfaces of a room can include areas such as those on and around the air conditioner, slab-on-grade floors (in the carpets) and on bathroom fixtures. Water vapor that has seeped into walls and furniture within the space also reaches the condensation point and begins to break down the material. Humid air that has seeped behind the walls through electrical outlets and other pathways now starts to condense inside the walls on the rapidly cooling drywall further helping its deterioration and allowing unseen microbes to grow unhindered. [0046]
  • The best way to avoid all these problems is to never allow water vapor levels to build up in a space so that condensation can occur when the room reaches its cooling set-point. In one embodiment of the present invention absolute humidity is measured in the space so as not to allow absolute concentrations of water vapor to exceed a certain threshold that will result in condensation occurring at normal daytime cooling temperatures. [0047]
  • The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirits and cope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. [0048]

Claims (21)

    What is claimed is:
  1. 1. An economizer control comprising:
    a sensor that senses characteristics of air;
    a damper located relative to said sensor so that said damper can control air flow of outside air and re-circulated air to said sensor; and
    a controller in communication with said sensor and said damper, said controller opening and closing said damper according to conditions sensed by said sensor.
  2. 2. The economizer control as recited in claim 1 wherein said damper comprises:
    an outside damper that allows outside air to enter a mixed air section of an air handler when said outside damper is open, and prevents outside air from entering the mixed air section when said outside damper is closed.
  3. 3. The economizer control as recited in claim 1 where said damper comprises:
    an inside damper that allows re-circulated air to enter a mixed air section of an air handler when said inside damper is open, and that prevents re-circulated air to enter the mixed air section when said inside damper is closed.
  4. 4. The economizer control as recited in claim 1 wherein said sensor measures temperature, enthalpy and occupancy level.
  5. 5. The economizer control as recited in claim 1 wherein said sensor comprises:
    a temperature sensor, an absolute humidity control sensor and a CO2 sensor.
  6. 6. The economizer control as recited in claim 1 wherein said controller adjusts said damper to limit the amount of outside air entering a mixed air section of an air handler when said sensor senses a humidity level above a predetermined range.
  7. 7. The economizer control as recited in claim 1 wherein said controller adjusts said damper to limit the amount of outside air entering a mixed air section of an air handler when said sensor senses a temperature outside a predetermined range.
  8. 8. A method for controlling an economizer comprising the steps of:
    sensing characteristics of air; and
    controlling a damper to control the air flow of outside air and re-circulated air in accordance with the sensed characteristics of the air.
  9. 9. The method as recited in claim 8 wherein said step of controlling the damper comprises the steps of:
    opening an outside damper to allow outside air enter a mixed air section of an air handler; and
    closing the outside damper to prevent outside air from entering the mixed air section.
  10. 10. The method as recited in claim 8 wherein said step of controlling the damper comprises the steps of:
    opening an inside damper to allow re-circulated air to enter a mixed air section of an air handler; and
    closing the inside damper to prevent re-circulated to enter the mixed air section.
  11. 11. The method as recited in claim 8 wherein the step of sensing characteristics of air comprises the step of sensing the temperature, enthalpy and occupancy level.
  12. 12. The method as recited in claim 8 wherein the step of sensing characteristics of air comprises the step of sensing the temperature, absolute humidity and CO2 level of the air.
  13. 13. The method as recited in claim 8 wherein the step of controlling the damper comprises the step of adjusting the damper to limit the amount of outside air entering a mixed air section of an air handler when a humidity level above a predetermined range is sensed.
  14. 14. The method as recited in claim 8 wherein the step of controlling the damper comprises the step of adjusting the damper to limit the amount of air entering a mixed air section of an air handler when a temperature outside a predetermined range is sensed.
  15. 15. A system for controlling an economizer comprises:
    a means for sensing characteristics of air located in a mixed air section of an air handler;
    a means for controlling a damper to control the air flow of outside air and re-circulated air in accordance with sensed characteristics of the air.
  16. 16. The system as recited in claim 15 wherein said means for controlling the damper comprises:
    a means for opening an outside damper to allow outside air enter a mixed air section of an air handler; and
    a means for closing the outside damper to prevent outside air from entering the mixed air section.
  17. 17. The system as recited in claim 15 wherein said means for controlling the damper comprises:
    a means for opening an inside damper to allow re-circulated air to enter a mixed air section of an air handler; and
    a means for closing the inside damper to prevent re-circulated air from entering the mixed air section.
  18. 18. The system as recited in claim 15 wherein said means for sensing characteristics of air comprises a means for sensing temperature, enthalpy and occupancy level of the air.
  19. 19. The system as recited in claim 15 wherein said means for sensing characteristics of air comprises a means for sensing the temperature, absolute humidity and CO2 level of the air.
  20. 20. The system as recited in claim 15 wherein said means for controlling the damper comprises a means for adjusting the damper to limit the amount of outside air entering a mixed air section of an handler when a humidity level above a predetermined range is sensed.
  21. 21. The system as recited in claim 15 wherein said means for controlling the damper comprises a means for adjusting the damper to limit the amount of air entering a mixed air section of an handler when a temperature outside a predetermined range is sensed.
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