Abstract:
Demand ventilation protocols can address the issue of sensor failure while still providing desirable levels of energy conservation. An occupancy indicator such as a sensor can be monitored. If the sensor reading is determined to be incorrect, unexpected or otherwise erroneous, the ventilation system can provide an amount of fresh air sufficient for adequate ventilation without over-ventilating the building.

Description:
TECHNICAL FIELD 
     The invention relates generally to ventilation and more particularly to demand control ventilation. More specifically, the invention pertains to sensor failure within demand control ventilation. 
     BACKGROUND 
     Many buildings are designed to be as airtight as possible in order to save energy that would otherwise be expended on heating and cooling. It has been discovered, however, that such buildings can suffer from an excessive build up of indoor air contaminants from a variety of sources. A possible solution to indoor air quality issues is to ventilate. 
     A number of studies have indicated that buildings should be ventilated with specific amounts of outside air to counter this potential build up of indoor air contaminants. Consequently, the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) has established indoor air standards that have been adopted by many building codes and design engineers. In particular, ASHRAE standards dictate that buildings should be ventilated with differing amounts of outside air based on a number of factors such as potential pollutant levels and occupancy levels. 
     A number of strategies have previously been provided in answering the need for ventilation. One solution is a fixed air strategy in which a building&#39;s HVAC system permits a fixed amount of outside air to enter the building at all times through either passive or active ventilation. While this solution can provide ample fresh air, energy may be wasted heating or cooling fresh air during times when the building is unoccupied or only lightly occupied. 
     Another strategy is to base ventilation rates on estimated, or projected, occupancy. In particular, unnecessary ventilation may be reduced by estimating or guessing times when a building or a portion of a building such as a room will be less heavily occupied and therefore providing a reduced level of ventilation at such times. This strategy may require manual adjustments of ventilation levels in response to varying occupancy estimates and furthermore may result in incorrect levels of ventilation when occupancy estimates are in error. 
     Many HVAC systems now include energy recovery ventilators, or air-to-air heat exchangers, in order to capture some of the thermal energy that would otherwise be lost due to exchange of stale indoor air for fresh outdoor air. For example, during a cooling system, the exiting stale (but still cool) air is used to remove at least some heat from the incoming fresh (but relatively warmer) air. During a heating season, the exiting (but still warm) air is used to add at least some thermal energy to the incoming fresh (but relatively colder) air. In some instances, these energy recovery ventilators operate constantly and therefore add load to the HVAC system by over ventilating when ventilation needs are reduced. 
     A useful strategy is known as demand ventilation, in which the amount of fresh air provided to a building or a portion of a building is varied to accommodate actual demand. While this strategy can address many of the shortcomings of other ventilation schemes such as those discussed above, the strategy inherently relies upon some reliable indication of building occupancy levels. 
     In some cases, building occupancy levels can be indicated by a variety of different sensors. Unfortunately, sensors can fail. In some systems, a failed sensor or a reading that indicates a failed sensor causes the ventilation system to default to a full ventilation mode in which the ventilation damper or dampers revert to a fully open position. 
     While this strategy ensures at least adequate ventilation, this can result in excessive ventilation, thereby wasting energy that has been used to heat or cool the air within the building. A need remains, therefore, for demand ventilation protocols that address the issue of sensor failure while still providing for desirable levels of energy conservation. 
     SUMMARY 
     The invention provides methods of ventilating an environment utilizing demand control ventilation. In particular, the invention provides methods of recognizing and adapting to sensor failure within demand control ventilation systems. 
     Accordingly, an illustrative embodiment of the present invention pertains to a method of controlling ventilation of an environment. An occupancy indicator that indicates an occupancy level of the environment is monitored. A set amount of fresh air may be provided to the environment if the occupancy indicator is erroneous or otherwise in error. 
     In some instances, providing a set amount of fresh air to the environment may involve providing an amount of fresh air that is less than a maximum amount of fresh air that could be provided. In some cases, the occupancy indicator may be deemed to be erroneous if it returns an unexpected value, or if the occupancy indicator indicates a sensor failure. In some embodiments, the occupancy indicator can include a sensor reading that is at least substantially proportional to environment occupancy, or the total number of people present in a particular space. 
     Another illustrative embodiment of the present invention pertains to a method of controlling the ventilation of an environment. An indication of occupancy of the environment is monitored. In some instances, monitoring an indication of occupancy can include monitoring a carbon dioxide sensor. In other cases, a motion sensor may be monitored. 
     A relative amount of fresh air that is provided to the environment may be modified in response to the indication of occupancy. In some cases, modifying the relative amount of fresh air that is provided to the environment can include at least partially opening or at least partially closing a damper. In other cases, a damper cycle rate can be increased or decreased. 
     If the indication of occupancy is determined to be in error or is otherwise erroneous, a set amount of fresh air can be provided to the environment. In some embodiments, providing a set amount of fresh air to the environment may include partially opening the damper to a position that is determined as a function of maximum environment occupancy, or perhaps as a function of occupancy history. In some instances, the set amount of fresh air that is provided to the environment can represent an amount of fresh air that is less than a maximum amount of fresh air. 
     In some instances, the indication of occupancy can be a voltage that is proportional to an occupancy level. In some cases, an excessively low or minimum voltage can be interpreted as an erroneous indication of occupancy. In some cases, an excessively high or maximum voltage can be interpreted as an erroneous indication of occupancy. 
     Another illustrative embodiment of the present invention pertains to a method of adjusting a damper that is configured to ventilate an environment. A carbon dioxide detector can be monitored. The damper may be adjusted to a set position upon sensing an erroneous reading from the carbon dioxide detector. In some instances, the set position can be a damper position that is less than fully opened. 
     Yet another illustrative embodiment of the present invention pertains to a controller that is configured to control ventilation of an environment. The controller may be adapted to monitor an indication of occupancy and to subsequently modify a relative amount of fresh air provided to the environment in response to the indication of occupancy. In some embodiments, the controller may be adapted to monitor an indication of occupancy that can be a voltage that is proportional to occupancy. In some instances, the controller is configured to monitor an indication of occupancy that can include a carbon dioxide sensor and/or a motion sensor. The controller may be configured to modify the relative amount of fresh air provided to the environment by at least partially opening or at least partially closing a damper. 
     In some instances, the controller may be adapted to provide a set amount of fresh air to the environment if the indication of occupancy is erroneous. The controller may be adapted to determine the set amount of fresh air as a function of maximum environment occupancy. In some instances, the set amount of fresh air can represent an amount of fresh air that is less than a maximum amount of fresh air. 
     The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures, Detailed Description and Examples which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a controller in accordance with an embodiment of the present invention; 
         FIG. 2  is a flow diagram showing an illustrative method that may be implemented by the controller of  FIG. 1 ; 
         FIG. 3  is a flow diagram showing an illustrative method that may be implemented by the controller of  FIG. 1 ; 
         FIG. 4  is a flow diagram showing an illustrative method that may be implemented by the controller of  FIG. 1 ; and 
         FIG. 5  is a flow diagram showing an illustrative method that may be implemented by the controller of  FIG. 1 . 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized. 
     The invention pertains generally to ventilation systems such as demand control ventilation systems.  FIG. 1  in particular illustrates a controller  10  that may be adapted to control at least particular aspects of a demand control ventilation system. Controller  10  can include an occupancy sensor control  12  having software and/or hardware that is adapted to receive an occupancy signal from an occupancy sensor  14 . The occupancy signal may be provided to the controller  10  via one or more wires, a network such as ModBus, LON or some other network protocol, an optical and/or wireless link, or in any other suitable manner. In some embodiments, more than one occupancy sensor may be provided, particularly if the environment to be monitored is large. 
     Occupancy sensor  14  can be or can include any suitable sensor that can provide a signal that is proportional to or otherwise indicative of an occupancy level. In some embodiments, occupancy sensor  14  can include a carbon dioxide sensor, as the amount of carbon dioxide present in an indoor environment can be proportional to the number of carbon dioxide-exhaling humans present within the environment. In other embodiments, occupancy sensor  14  can include a motion sensor. 
     Controller  10  also includes a fresh air control  16  that includes software and/or hardware that is adapted to send and receive signals from a fresh air source  18 . In some embodiments, fresh air source  18  can include a ventilation system, a demand control ventilation system or one or more dampers within a ventilation system. In particular embodiments, fresh air source  18  may include one or more dampers that are moveable between a fully closed position, a fully open position and one or more intermediate positions therebetween. 
     Fresh air control  16  can send signals to fresh air source  18  such as instructions pertaining to damper opening and closing. For example, fresh air control  16  can instruct fresh air source  18  to fully open one or more dampers, to fully close one or more dampers, or to move one or more dampers to an intermediate position that is either less open or more open than the position the one or more dampers were in prior to receiving such instructions. In some cases, fresh air control  16  can receive signals such as confirmation signals from fresh air source  18 . 
     A user interface  20  can be in communication with controller  10  and may be used to provide any necessary informational messages. Examples of suitable messages include error messages, as will be discussed in greater detail hereinafter. 
     Controller  10  may also include a memory block  22  that includes software and/or hardware that contains the programming necessary to operate controller  10 . This programming may include, for example, information on how to translate occupancy levels into corresponding ventilation requirements. 
     ASHRAE standard 62-2001, for example, provides a number of suggested ventilation levels based on occupancy and room usage. For example, office space is generalized as having a maximum occupancy of 7 people per one thousand square feet, and needing a minimum of 20 cubic feet per minute (CFM) of fresh air per person. Reception areas, however are estimated as having a maximum occupancy of 60 people per one thousand square feet, and requiring a minimum of 15 CFM of fresh air per person. Differing requirements are provided for a variety of different space usages. 
     Turning now to  FIG. 2 , there is illustrated a method that can be carried out by controller  10  ( FIG. 1 ). At block  24 , occupancy sensor control  12  ( FIG. 1 ) monitors occupancy sensor  14  ( FIG. 1 ). At decision block  26 , controller  10  determines if the reading provided by occupancy sensor  14  is within an expected range. The expected range may be static, or it may vary depending on time and/or day of week. 
     To illustrate, memory block  22  ( FIG. 1 ) can be programmed with or may learn that an office space, for example, tends to contain a larger number of people between 9 am and 5 pm Monday through Friday and a lesser number or even no people during evenings and or weekends. In this scenario, a reading at 11 am Tuesday, for example, indicating that no people are present may be interpreted as unexpected and thus may indicate a sensor failure. Conversely, a reading at 11 pm Tuesday indicating that the office is full of people may similarly be interpreted as an error. 
     In some embodiments, occupancy sensor  14  ( FIG. 1 ) may return a signal that corresponds to a numerical value. Controller  10  ( FIG. 1 ) can be programmed with or can learn typical ranges for this numerical value. If occupancy sensor  14  returns a numerical value that is outside this range, the occupancy indicator can be interpreted to be incorrect, invalid, out of range or otherwise erroneous. In some cases, occupancy sensor  14  can return a voltage, current and/or frequency that is at least partially proportional to occupancy level. In such cases, controller  10  may be programmed with or can learn an expected range for this voltage, current and/or frequency. If occupancy sensor  14  returns a voltage, current and/or frequency that is outside of this expected range, the occupancy indicator can be interpreted as erroneous. 
     Returning to  FIG. 2 , if at decision block  26  controller  10  ( FIG. 1 ) determines that the occupancy reading is within an expected range, control reverts back to block  24  where occupancy sensor control  12  ( FIG. 1 ) continues to monitor occupancy sensor  14  ( FIG. 1 ). However, if the occupancy reading is not within an expected range, control passes to block  28 . At block  28 , controller  10  calculates a set amount of fresh air that should be provided by fresh air source  18  ( FIG. 1 ). Fresh air control  16  ( FIG. 1 ) then provides the appropriate instructions to fresh air source  18  as to how far to open the one or more dampers present within fresh air source  18 . In some instances, a set amount of fresh air can be provided by changing a cycle rate of the one or more dampers included in fresh air source  18 . 
     The set amount of fresh air can be calculated by, for example, using the ASHRAE minimum ventilation standards in combination with information pertaining to maximum building occupancy. In some instances, this can provide a set ventilation level that is sufficient yet represents an amount of fresh air that is less than could be provided if fresh air source  18  ( FIG. 1 ) were fully opened. In some cases, historical occupancy levels can be used to determine the set ventilation level. In some embodiments, the set amount of fresh air may be calculated using the ASHRAE minimum ventilation standards in combination with occupancy levels immediately before occupancy sensor  14  is determined to be providing an unexpected, erroneous or otherwise incorrect signal. 
       FIG. 3  shows another illustrative method that may be carried out by controller  10  ( FIG. 1 ). At block  30 , occupancy sensor control  12  ( FIG. 1 ) monitors occupancy sensor  14  ( FIG. 1 ). At decision block  32 , controller  10  determines if the reading provided by occupancy sensor  14  is valid, or is within an expected range as was discussed with respect to  FIG. 2 . If the reading provided by occupancy sensor  14  is determined to be valid, control passes to decision block  34 , where fresh air control  16  ( FIG. 1 ) determines if a ventilation change is necessitated by the valid reading provided by occupancy sensor  14 . 
     If a change is required, control passes to block  36 , at which point fresh air control  16  ( FIG. 1 ) instructs fresh air source  18  ( FIG. 1 ) to provide a different level of ventilation by, for example, either further opening or further closing one or more dampers. Control then returns to block  30  and occupancy sensor  14  ( FIG. 1 ) is monitored. 
     However, if at block  32  the reading provided by occupancy sensor  14  ( FIG. 1 ) is deemed to not be valid, i.e. is unexpected, out of range or otherwise apparently incorrect, control passes to block  38 . At block  38 , controller  10  ( FIG. 1 ) determines what the set amount of fresh air should be, based on, for example, ASHRAE ventilation standards, and either the building&#39;s maximum occupancy, occupancy immediately before sensor failure, or some other indication of present occupancy. 
     Control may pass to optional block  40 , where controller  10  ( FIG. 1 ) may provide an error message through user interface  20  ( FIG. 1 ) in order to communicate that occupancy sensor  14  ( FIG. 1 ) may have failed. Any suitable error message may be provided, including relatively more technical messages pertaining to sensor failure or less technical messages such as instructing someone to arrange for a service call. 
       FIG. 4  shows another illustrative method that may be carried out by controller  10  ( FIG. 1 ). At block  42 , occupancy sensor control  12  ( FIG. 1 ) monitors occupancy sensor  14  ( FIG. 1 ), which in this case is a carbon dioxide sensor. The carbon dioxide sensor measures carbon dioxide content of the air within the building and, in the illustrative embodiment, returns a voltage, current and/or frequency that is at least somewhat proportional to the carbon dioxide concentration. Because humans exhale predictable amounts of carbon dioxide, carbon dioxide concentration can be used as a reasonable indication of the number of people present in an environment as well as an indication of how well the environment within the building is being ventilated. 
     At decision block  44 , controller  10  determines if the reading from the carbon dioxide sensor (occupancy sensor  14  of  FIG. 1 ) is erroneous. As discussed previously with respect to the earlier Figures, a reading may be deemed to be erroneous if it is out of range, unexpected or otherwise apparently in error. For example, and in some embodiments, a carbon dioxide sensor may return a voltage that is between 0 and 10 volts, depending on concentration. 
     A minimum reading, say of zero volts, may be deemed to be in error as such a reading would presumably indicate a carbon dioxide concentration that is below that of ambient outside air. Conversely, a maximum reading, say of ten volts, may be deemed to be in error as such a reading would presumably indicate a carbon dioxide concentration that is substantially higher than might be obtainable if fresh air source  18  ( FIG. 1 ) is operable. If the carbon dioxide sensor produces some other type of output signal, such as a current or frequency, similar thresholds may be determined and used, as desired. 
     If the sensor reading is determined to be valid, control reverts to block  42  where occupancy sensor control  12  ( FIG. 1 ) continues to monitor the carbon dioxide sensor (occupancy sensor  14  of  FIG. 1 ). However, if the sensor reading is not valid, control passes to block  46 , where fresh air control  16  ( FIG. 1 ) instructs fresh air source  18  ( FIG. 1 ) to adjust one or more dampers to a set position. The set position is a function of what a set amount of fresh air should be. This is determined as discussed with respect to the previous Figures. 
       FIG. 5  shows another illustrative method that may be carried out by controller  10  ( FIG. 1 ). At block  48 , occupancy sensor control  12  ( FIG. 1 ) monitors occupancy sensor  14  ( FIG. 1 ), which in this case is a carbon dioxide sensor as discussed with respect to  FIG. 4 . Control passes to decision block  50 , where controller  10  determines if the sensor reading is erroneous. This may be determined as discussed with respect to  FIG. 4 . 
     If the sensor reading is valid, control passes to decision block  52 . At decision block  52 , controller  10  determines if the sensor reading necessitates a change in the amount of fresh air that is being provided to the environment within the building. If no ventilation change is required, control reverts back to block  48 , where occupancy sensor control  12  ( FIG. 1 ) continues to monitor the carbon dioxide sensor (occupancy sensor  14  in  FIG. 1 ). 
     If a ventilation change is required, control passes to block  54 , at which point fresh air control  16  instructs fresh air source  18  ( FIG. 1 ) to provide a different level of ventilation by, for example, either further opening or further closing one or more dampers. Control then returns to block  48  and the carbon dioxide sensor (occupancy sensor  14  in  FIG. 1 ) is monitored. 
     Returning now to decision block  50 , if the carbon dioxide sensor reading is determined to be erroneous, control passes to block  56 , where fresh air control  16  ( FIG. 1 ) instructs fresh air source  18  ( FIG. 1 ) to adjust one or more dampers to a set position. The set position is a function of what a set amount of fresh air should be. This is determined as discussed with respect to the previous Figures. 
     The invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the invention can be applicable will be readily apparent to those of skill in the art upon review of the instant specification.