Abstract:
A system and a method for detecting and diagnosing faults in heating, ventilating and air conditioning (HVAC) equipment is described. The system comprises a sensor; a classifier modelling a normal behaviour of the HVAC equipment in situ in the installed operation environment, the classifier having a plurality of classifier parameters for computing a classifier score using an input data based on a measured value from the sensor, the plurality of classifier parameters being created during a training phase of the system using the input data during the training phase; and a decision module for comparing the classifier score to a decision threshold, the decision threshold being set during the training phase.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a heating, ventilating and air conditioning (HVAC) equipment, and more particularly to an HVAC fault detection and diagnosis (FDD) system. 
       BACKGROUND OF THE INVENTION 
       [0002]    HVAC systems are widely used in residential and commercial buildings, and as such consume a large amount of energy throughout the world. So, even small inefficiencies over all HVAC systems results in significant waste of energy. In 2004, the National Energy Management Institute (NEMI) published a residential HVAC market research which estimated that out of about 100 million furnaces in the US, less than 20 million were serviced each year. Improperly maintained HVAC systems, even as simple as a dirty filter, can lead to 10 to 20% loss in efficiency, which results in increased cost for the homeowner while producing unnecessary green house gas into the environment. For example, a report on climate change by the Yukon Government estimated that an improperly maintained furnace produces an extra 2.1 tonnes of carbon dioxide annually. 
         [0003]    While governments have attempted to address this problem by providing incentives for homeowners to replace low-efficiency furnaces (less than 80% efficiency) with high-efficiency furnaces (90% or more efficiency), the incentives are often insufficient to entice a homeowner to purchase a costly high-efficiency furnace. For example, the Canadian government&#39;s ecoENERGY program offered rebates toward a high-efficiency furnace that fell well below the actual cost of a high-efficiency furnace. However, the benefits of a new high-efficiency furnace are negated if it is not maintained properly. 
         [0004]    A cost effective solution consists in maintaining an HVAC system at a high level of efficiency. However, despite governments&#39; efforts to educate the public about the benefits of a well maintained HVAC system, the majority of homeowners do not perform regular maintenance. The main reason is that maintenance plans are expensive and homeowners have no guidance on the level of maintenance required on their HVAC system. Consequently, the majority of homeowners choose to service the equipment only when it fails. By this time, the HVAC system may have been underperforming for years. 
         [0005]    Several attempts have been made to address this problem by employing some form of fault detection and diagnostics (FDD). Automated FDD concerns the processes of automatically detecting faults with physical systems and diagnosing their causes. 
         [0006]    One such attempt is disclosed in U.S. Pat. No. 6,658,372 to Abraham et al. The FDD system described in U.S. Pat. No. 6,658,372 monitors various functions of the furnace and generates data for diagnosis. To prevent misdiagnosis, a history of the data is stored and can be used to assess similar problems. However, this approach requires time consuming intervention and assessment by an experienced technician making it costly to diagnose large numbers of HVACs. Moreover, the surrounding environment or equipment may significantly change over time, thus rendering historical data marginally useful. 
         [0007]    FDD methods can be classified into two broad categories, model-based methods and data-based methods. The two categories differ by the knowledge used to diagnose the cause of faults, although both may use simulation models and measurement data. Model-based methods use explicit prior knowledge of HVAC systems to identify the differences between model simulation results and actual operation measurements. Models are detailed mathematical descriptions of the HVAC equipment, and are configured using design information and component manufacturers&#39; data, and then fine-tuned to match the actual performance of the HVAC equipment by using data measured during functional tests. Data-based methods are derived models from process history data, and learn the relationship between driving conditions, device construction, and normal operating states from data. 
         [0008]    Model-based methods are further divided into quantitative and qualitative modeling methods. Quantitative models are based on mathematical relationship derived from the underlying physical knowledge, and rely on explicit mathematical models of a system to detect and diagnose faults. By understanding the physical relationships of an HVAC system, mathematical equations to represent each component of the system can be developed and solved to simulate its steady and transient behaviour of the systems. In contrast, qualitative modeling uses rule-based methods developed based on prior knowledge. Qualitative rule relationships are employed to detect and diagnose faults instead of quantitative mathematical equations. The rules are derived from expert knowledge, process history data and quantitative models simulation data. Expert knowledge is normally summarized to a database in the form of if-then-statements. 
         [0009]    Producing such physical models for practical HVAC equipment, either analytically or via simulation, is usually very difficult due to the complex thermo-dynamical relationship between external driving conditions and the details of HVAC equipment fabrication. Existing models, have had limited success in producing accurate estimates of expected operating states as a function of driving condition. In addition, although models are commonly based on physical principles, they may not properly fit the process data, and cannot explain systematic variation. This is the driving motivation behind the use of data-based models. 
         [0010]    Automatic model-based FDD systems have also been attempted in various forms. One such form is the use of a model based FDD system to determine the performance of the HVAC system. For example, in U.S. Pat. No. 6,223,544 to Seem, fault detection is achieved by employing a finite state machine. Upon transition, it compares the actual performance of the HVAC system to that of the FSM model. Another example is the HVAC monitoring system disclosed in U.S. Pat. No. 6,385,510 to Hoog et al., which continuously monitors the general condition and efficiency of the HVAC system. The monitored values are then compared against optimum values for an HVAC system of the size and capacity being monitored by means of industry standard tables and equations. In both systems, a disadvantage is that the model/industry standard HVAC system is not customized to account the actual environment where the HVAC system operates. Moreover, the model/industry standard HVAC system does not consider the conditions particular to the HVAC installation and other external conditions such as the weather. This may lead to inaccurate diagnosis and false alarms. 
         [0011]    Data-based methods are subdivided into black-box and gray-box methods. They differ on the physical meaning of their model parameters. Black-box methods use non-physical based relationship to represent the characteristics of a system. Model parameters do not represent actual physical properties. Black box methods often employ techniques such as linear or multiple linear regression, statistical or neural network classification, and fuzzy logic. In a gray-box model, the model parameters are determined based on physical principles. Parameter estimation techniques are often used to obtain those parameters from measurement data. Gray-box modeling typically requires higher-level of expertise than black-box modeling to form the model parameters and estimate parameter values. Data-based methods are completely driven by recorded measurement data, yet do not always generate good process insight. 
         [0012]    There have also been prior attempts at data-based FDD systems that learn the expected behaviour of a particular appliance (e.g. HVAC equipments). In U.S. Pat. No. 5,706,191 to Bassett et al., an appliance interface apparatus (AIM) is designed to allow various home appliances to communicate with one another and to accept commands remotely in managing the residence autonomously. AIM monitors various sensors to ensure that the home appliances, for example, gas furnace is functioning properly. This is achieved by first “learning” the expected behaviour of the particular appliance (e.g. offset between a thermostat setting and the return air temperature, delays in ignition sequences). Thereafter, AIM watches for deviations from such expected values. One of the disadvantages of this system is that the system only learns simple expected values without truly creating a comprehensive input-to-output mapping among all the parameters. Therefore, without an overall, comprehensive picture of all the parameters, a non-optimal parameter may skew the outlook of the system, resulting in false alarms. 
         [0013]    In U.S. Pat. No. 7,444,251 to Nikovski et al., a locally weighted regression model is generated using training data from healthy HVAC equipment. This allows us to predict the value of observed state variables under normal operation of the HVAC equipment, given the input driving conditions. Second, using prediction residuals from the regression model and observed values of the state variables, they train classifiers to distinguish between three fault conditions of operation of the equipment (normal, overcharged and undercharged). It is trained using data collected from faulty HVAC equipment in abnormal conditions. Once learned, the regression model is used to detect signal readings that deviate significantly from the expected values for normal operation. Once a faulty state has been detected, the classifier establishes the specific type of fault condition. One of the disadvantages of this system is that the regression model is generated using training data that are not particular to the environment where the HVAC system is installed. The black-box methods used in this approach are trained using data collected in a laboratory from normal and abnormal condition, and will provide a lower accuracy because they do not account or personalize for the specific HVAC installation and environment. Further, the techniques employed for locally weighted regression and for classification do not allow for automated on-line incremental learning to update or refine models without retraining from the start using all available training data. Training a locally weighted regression model with laboratory data is generally less tolerant to deviations from the pre-set conditions. As such, it is regular practice to include tolerances that contribute to the inefficiencies of the system. 
         [0014]    Therefore, there is a need for a low-cost HVAC FDD system that accurately detects degradation in performance and automatically notifies the homeowner of actions to take in correcting the problem. Further, there is a need to generate data-driven FDD models that are personalized for an HVAC installation, using data collected from the specific equipment under different conditions in situ. There is further a need for on-line incremental learning to adapt such black-box models over time with a comprehensive input-output mapping among all parameters. 
       SUMMARY OF THE INVENTION 
       [0015]    According to an aspect of the present invention there is provided a system for detecting and diagnosing faults in heating, ventilating and air conditioning (HVAC) equipment. The system comprises a sensor, a classifier modelling a normal behaviour of the HVAC equipment in situ in the installed operation environment, and a decision module for comparing the classifier score to a decision threshold, the decision threshold being set during the training phase. The classifier has classifier parameters for computing a classifier score using an input data based on a measured value from the sensor. The classifier parameters are created during a training phase of the system using the input data during the training phase. 
         [0016]    Preferably, the system further comprises a second classifier modelling a fault condition of the HVAC equipment, the second classifier has classifier parameters for computing a second classifier score using the input data. The classifier parameters are created during the training phase using the fault condition of the HVAC equipment detected during a monitoring phase of the system. 
         [0017]    Preferably, the system further comprises a communication module for communicating the classifier score to a remote server when the classifier score is below the decision threshold and for receiving instructions from the remote server; and a control module for controlling the HVAC equipment based on the instructions from the remote server. 
         [0018]    Preferably, the system further comprises a filtering and converting module for filtering and converting the measured value from the sensor into digital data, the digital data being the input data to the classifier. 
         [0019]    Preferably, during a transient state operation, the digital data is further input into a windowing module for extracting invariant and discriminate features, the windowing module outputs the input data for the classifier. 
         [0020]    Preferably, the system further comprises a performance degradation estimator for estimating degradation in performance of the HVAC equipment based on the classifier score and the input data. 
         [0021]    Preferably, the performance degradation estimator estimates degradation in efficiency. 
         [0022]    Preferably, the system further comprises a memory for storing the plurality of classifier parameters, the decision threshold, the measured value and the input data used during the training phase. 
         [0023]    Preferably, the communication module includes a wireless transceiver for communicating with the remote server. 
         [0024]    Preferably, the system further comprises a thermostat bypass relay to enable and disable overriding of the control module by a local switch or remote server. 
         [0025]    Preferably, the sensor is selected from the group consisting of: a flue gas temperature sensor, a return air temperature sensor, a return air humidity level sensor, a supply air temperature sensor, a supply air flow sensor, a supply air carbon monoxide level sensor, an outside temperature sensor, an outside humidity level sensor, a room temperature sensor, a room humidity level sensor and a combination thereof. 
         [0026]    According to another aspect of the present invention there is provided a method for detecting and diagnosing faults in heating, ventilating and air conditioning (HVAC) equipment, comprising: measuring an input data from a sensor; inputting the input data from the sensor into a classifier modelling a normal behaviour of the HVAC equipment in situ in the installed operation environment, the classifier having a plurality of classifier parameters determined during a training phase of the classifier; calculating a classifier score using the plurality of classifier parameters and the input data; comparing the classifier score to a decision threshold set during the training phase; and communicating the classifier score to a remote server. 
         [0027]    Preferably, the training phase comprises: sampling the sensor and estimating the plurality of classifier parameters; calculating the classifier score using the plurality of classifier parameters and the initial data until the classifier score is constant over a pre-defined number of successive repetitions or until a maximum number of repetitions are performed; and setting the decision threshold based on the plurality of classifier parameters. 
         [0028]    Preferably, the method further comprises diagnosing the HVAC equipment to determine a fault condition; storing information related to the fault condition; creating a second classifier modelling the fault condition, the second classifier having a second plurality of classifier parameters based on the stored information; training the second classifier comprising: sampling the sensor; estimating the second plurality of classifier parameters based on the stored information related to the fault condition; calculating the classifier score using the second plurality of classifier parameters and the sampled data from the second sensor until the classifier score is constant over a pre-defined number of successive repetitions or until a maximum number of repetitions are performed; and setting the decision threshold based on the plurality of classifier parameters; and monitoring the HVAC equipment with the classifier modelling the normal behaviour and the classifier modelling the fault condition of the HVAC equipment. 
         [0029]    Preferably, the stored information comprises the measured data, the input data, the classifier score, the classifier parameters, and the decision threshold related to the fault condition. 
         [0030]    Preferably, inputting comprises filtering and converting the measured data from the sensor into digital data, the digital data being the input data to the classifier. 
         [0031]    Preferably, wherein, during a transient state operation, inputting further comprises extracting invariant and discriminate features from the digital data, the digital data being the input data to the classifier. 
         [0032]    Preferably, the method further comprises estimating degradation in performance of the HVAC equipment based on the input data and the classifier score. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein: 
           [0034]      FIG. 1  is a high-level system diagram depicting the use of the intelligent FDD system in accordance with an embodiment of the present invention; 
           [0035]      FIG. 2  is a block level diagram of the FDD system in accordance with an embodiment of the present invention; 
           [0036]      FIG. 3  is a detailed block level diagram of the classification and decision modules of the FDD system in accordance with an embodiment of the present invention; 
           [0037]      FIG. 4  is a schematic diagram depicting an implementation of the FDD system in accordance with an embodiment of the present invention; 
           [0038]      FIG. 5  is a flow chart describing the training and monitoring phases of the FDD system in accordance with an embodiment of the present invention; and 
           [0039]      FIG. 6  is an exemplary performance report generated by the remote server in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    A high-level system diagram of the intelligent FDD system for HVAC equipment, in accordance with one embodiment of the present invention, is shown in  FIG. 1 . HVAC equipment  102  for heating, ventilating and air conditioning is located in a substantially enclosed building, for example, a house  100 . The HVAC equipment  102  is a typical system which may include a control board  104 , a heat exchanger  106 , a evaporator coil  108 , a blower  110 , a filter  112 , and a thermostat  114 . During operation of an exemplary HVAC equipment, a user sets the desired parameters, for example but not limited to, humidity or temperature on the thermostat  114 , which informs the control board  104  to start the blower  110 . The blower  110  draws in return air through the filter  112 . This air is then heated by the heat exchanger  106  or cooled by the evaporator coil  108  and re-supplied to the house  100  through the supply air path. The exhaust produced by the HVAC equipment  102  is expelled outside as flue gas. 
         [0041]    In one embodiment of the present invention, various sensors are installed within the HVAC equipment to sample the performance of the system. In the exemplary embodiment as shown in  FIG. 1 , temperature  120  and humidity  122  sensors, are installed in the return air path. Temperature  124 , carbon monoxide  126  and air flow  128  sensors are installed in the supply air path  132 , and temperature sensor  130  is installed in the flue gas exit path. In other embodiments, additional sensors may be installed to measure other environmental conditions, for example but not limited to, external temperature, external humidity. Such external conditions may also be provided by the thermostat  114  or the local weather stations (not shown). These sensors allow comprehensive sampling of the performance of the HVAC equipment, which is then analyzed by the FDD system  138 . The FDD system  138  is able to control the HVAC equipment through the control board  104 . It is also able to communicate with a remote system, such as a remote server  140 , a computer  142  or a wireless terminal  144 , for example but not limited to, a smart phone through the Internet  146 . In one embodiment, communication is established wirelessly by a wireless adapter  150  that is plugged into the homeowner&#39;s local internet access device  152 . The remote server  140  allows further processing of the information communicated by the FDD system  138 . Furthermore, the FDD system  138  may be controlled remotely, for example but not limited to, by a remote server  140 , a computer  142  or a smart phone  144 . While the FDD system  138  is designed to autonomously maximize the efficiency of the HVAC equipment  102 , its control of the HVAC equipment  102  may be manually overridden. However, the FDD system  138  may still continue to monitor and report the performance of the HVAC equipment  102 . In the illustrated embodiment in  FIG. 1 , the FDD system  138  is separate from the HVAC control board  104 , however, it should be apparent to a person skilled in the art that the FDD system  138  and the HVAC control may reside on the same board, or the HVAC control board  104  may be implemented as a HVAC control module. 
       Fault Detection and Diagnosis (FDD) System 
       [0042]    Referring to  FIGS. 1 and 2 , a block level diagram of the FDD system  300  in accordance with an embodiment of the present invention is shown. The sensors  302  in communication with the FDD system  300  receive raw data {right arrow over (x)}, {right arrow over (y)}, {right arrow over (x)}=(x 1 (t), x 2 (t), . . . , x m (t)): vector that stores for external driving conditions, e.g., internal room temperature and humidity, {right arrow over (y)}=(y 1 (t), y 2 (t), . . . , y p (t)): vector that stores sampled internal state variable from n sensors installed within a HVAC system, e.g., supply and return air temperature, return humidity and pressure, and {right arrow over (y)}=(x 1 (t), x 2 (t), . . . , x m (t); y 1 (t), y 2 (t), . . . , y p (t))=(v 1 (t), v 2 (t), . . . , v m+p (t)): a feature vector that includes a concatenation of {right arrow over (x)} and {right arrow over (y)} vectors. As described in the above, the sensors may be installed throughout the HVAC equipment, and optionally throughout the home to measure conditions external to the HVAC equipment as mentioned above. For example, the home may be installed with a plurality of sensors to measure flue gas temperature (e.g. sensor  130 ), return air temperature (e.g. sensor  120 ), return air humidity levels (e.g. sensor  122 ), supply air temperature (e.g. sensor  124 ), supply air carbon monoxide levels (e.g. sensor  126 ) or supply air flow (e.g. sensor  128 ). There may also be sensors that measure the external driving conditions of the HVAC equipment. For example, internal room temperature and humidity level of the house may be measured and taken into consideration. Measurement of the external driving conditions may be done by separate sensors installed throughout the home (outside and inside) or by existing hardware, such as the thermostat  114 . Alternatively or additionally, conditions external to the HVAC equipment may be provided by external sources such as the local weather station. 
         [0043]    The raw data {right arrow over (x)}, {right arrow over (y)}, {right arrow over (v)}, from the sensors  302  are then received by the filtering and converting module  304  for signal conditioning and conversion into digital data. In the filtering and converting module  304 , raw data is converted into digital data, for example, by using an analog to digital converter (ADC). The output {right arrow over (v)} from the filtering and converting module  304  becomes the input data into the classifier  308 . Optionally, where the FDD system  300  is for use in a transient state operation, a windowing module  306  may be included. The windowing module  306  is required because the measured data from the sensors  302  is a sequence of ordered samples in a transient environment. During the transient operation, for example, when the furnace fires-up, the sampled signals may vary in time. Therefore, a sequence of measurements from one or more sensors over a window of time may need to be observed. Thus, the windowing module  306  is used to extract a window of those ordered samples A[1: L] for use in the classifier  308 . In other words, the windowing module  306  assembles a sensor&#39;s symbols into a chronological sequence of consecutive symbols, where the number of symbols per sequence is defined by the window length L. On the other hand, at some time after fire up, the stead state operation of the furnace may be observed, once measured data stop varying over time. During the steady state operation, one sample is collected at a time, looking at all sensor measurements. 
         [0044]    In one exemplary embodiment of steady-state operation, the input {right arrow over (v)} to the classifier  308  is the direct output from the filtering and converting module  304 . In another exemplary embodiment of transient-state operation, the input to the classifier  308  is further processed by the windowing module, to produce fixed-size sequences according to some length L. A[1: L]=v(1), v(2), . . . , v(L): a sequence of chronologically ordered values sampled from a state variable i in {right arrow over (v)}. This module is only relevant for monitoring transient state of input signals over time. The classifier  308  matches the input data to a model of either normal HVAC behaviour, or of some fault condition, and produces a classifier score that is either an indication of the overall performance of the HVAC equipment or the occurrence of some fault condition. For FDD system  300  in the steady-state operation, the classifier  308  may be based on various one-class classifiers or density estimation algorithms, for example but not limited to, the Gaussian Mixture Model (GMM) or the one-class Support Vector Machine (SVM). For FDD system  300  in the transient-state operation, the sequence classifier  308  may be based on, e.g., the Hidden Markov Model (HMM) classifier. By using the classifier  308 , which is described in detail below, degradation in performance and fault detection may be detected with high accuracy since the classifier  308  is trained in situ, i.e. while the HVAC equipment is running, using real data samples measured from the particular HVAC equipment installation and the surrounding environment. This allows the FDD system  300  to adapt to the particular intricacies of the HVAC FDD monitoring equipment as installed and not on an ideal or industry standard data. 
         [0045]    The output of the classifier  308  is a classifier score S(A[1: L]) or S({right arrow over (v)}) that is used by the decision module  310  to assess the operating conditions and fault conditions of the HVAC system. This is achieved by comparing the classifier score to a decision threshold set during the training phase of FDD system  300 . Similarly to the classifier  308 , the decision threshold may be selected based on the in situ data samples measured from the HVAC equipment and the surrounding environment. When the performance of the HVAC system falls below a certain threshold or when a fault condition is detected, the FDD system  300  provides a notification to, for example but not limited to, the remote server  320  for verification and further instructions. In one embodiment, notification to the remote server  320  is accomplished through a wireless interface  316 . Also, data samples from the conditions that triggered the fault may be stored in memory  314 . For example, the raw measured data from the sensors  302 , the input data to the classifier  308 , the classifier parameters, the classifier score, and the decision threshold that triggered the fault may all be stored in memory  314 . The classifier score and input data may also be used by a performance degradation estimator  312 , which provides relationship between classifier score and the performance of the HVAC system, and to estimate the overall degradation in efficiency of the HVAC equipment based on the classifier score and the input data. For example, the efficiency may be estimated based on the physical properties of HVAC equipment using measured data samples. The result of the estimated performance degradation may be communicated to the remote server  320 , which may be taken into consideration in determining the next step of the monitoring process. 
       Classification and Decision Modules 
       [0046]    Referring to  FIG. 3 , classifier  308 , decision module  310 , performance degradation estimator  312 , data storage  314  and remote server  320  as shown in  FIG. 2  are described. 
         [0047]    The classification system  308  may comprise N one-class classifiers  3080 ,  3081 , . . .  308 N. Classifier  3080  is defined by a plurality of classifier parameters, which are estimated during the training phase using real data samples measured in situ from the particular HVAC equipment installation and the surrounding environment. Then, during the monitoring phase, using the plurality of classifier parameters estimated during the training phase, classifier  3080  is used to compare input samples from sensors to the normal system behaviour of the HVAC equipment. 
         [0048]    The classifier  308  may also have classifiers  3080 ,  3081 , . . .  308 N. to monitor for a specific fault condition of the HVAC equipment. The classifiers  3080 ,  3081 , . . .  308 N are trained using data collected according to fault conditions specifically diagnosed during the operation of the HVAC equipment as discussed below. 
         [0049]    One-class classification refers to a special type of pattern recognition problem. Let C 0  be a certain class of interest. For input vector {right arrow over (v)} or input sequence A[1: L], a one-class classifier outputs a score S({right arrow over (v)}) or S(A[1: L]) that indicates the confidence or likelihood that the input produced was sampled from C 0 . If the input does not belong to C 0 , it is called an anomaly (or a novelty). Generally, the objective of training a one-class classifier is to estimate the parameters that correspond to an efficient mapping between the input sample space, say {right arrow over (Z)}, and class C 0 . That mapping defines the decision region R 0  for class C 0  such that R 0 ={{right arrow over (Z)}: f({right arrow over (Z)})≧T}, where T is a decision threshold. If f({right arrow over (Z)})≧T, {right arrow over (Z)} is classified as C 0  (familiar or normal behaviour); otherwise, {right arrow over (Z)} is classified as C 1  (unfamiliar or abnormal behaviour). 
         [0050]    The input to the classifiers  3080 ,  3081 , . . .  308 N depends on whether the FDD system  300  is used to analyze steady-state or transient state operation. For steady-state operations, the input to a classifier i among  3080 ,  3081 , . . .  308 N is a vector {right arrow over (v)} i (t), where t represents time at which the HVAC equipment, the external environment or both were sampled. For transient state operations, the input to a classifier i among  3080 ,  3081 , . . .  308 N is a sequence A i [1: L] output from the windowing module  306 . A i [1 L] is a sequence of ordered samples captured by the windowing module according to a window of length L, from the HVAC equipment or the external environment or both. The classifier  3080  is trained to operate on normal conditions, while classifiers  3081 , . . .  308 N are trained and operate independently based on specific fault conditions. 
         [0051]    With the inputs, classifiers  3080 ,  3081 , . . . ,  308 N produce classifier scores S 0 , S 1 , . . . S N  that are input into the decision module  310 . The decision module  310  compares respective scores to decision thresholds T 0 , T 1 , . . . T M . In the exemplary embodiment shown in  FIG. 3 , there is one decision threshold,  3100 ,  3101 , . . .  310 N for each classifier  3080 ,  3081 , . . .  308 N. For classifier  3080 , the score S 0  indicates the likelihood that an input sample (measures from the current behaviour of the HVAC equipment) corresponds to the behaviour of the HVAC equipment when it was initially tuned in situ in the installed environment. If the classifier score S 0  falls below the minimum threshold T 0 , a flag is raised, which is then communicated to the remote server  320 . The administrator at the remote server  320  may perform further analysis before notifying the homeowner of the anomaly in the HVAC equipment. The flag, along with accompanying data such as inputs {right arrow over (v)} 0 (t) or A 0 [1: L], classifier parameters of the classifier  3080 , classifier score S 0 , and decision thresholds T 0  may also be stored in memory  314 . If the FDD system  300  includes a performance degradation estimator  312 , for example, for estimating efficiency of the system, the output of the performance degradation estimator  312  may also be communicated to the remote server  320  and stored in data storage  314 . 
         [0052]    The classifier  308  may include additional classifiers  3081 , . . .  308 N, and the FDD system  300  may also monitor fault conditions. With the inputs vector {right arrow over (v)} 1 (t), a sequence A 1 [1: L], the classifiers  3081 , . . .  308 N produce scores S 1  . . . S N  which are compared against decision thresholds T 1  . . . T N  to determine whether specific fault conditions arose. If one of the score S 1  . . . S N  exceed or is equal to the maximum allowed threshold T 1  . . . T N , a fault flag is raised, which is again communicated to the remote server  320 . Data samples relevant to the fault flag may also be stored in memory  314 . 
         [0053]    While the above was described using one-class classifiers, two-class or multi-class classifiers are also reasonably contemplated within this invention. However, the training phase requires that the classification parameter be re-estimated with data samples from both normal and abnormal behaviour. 
         [0054]    A schematic diagram of an embodiment of a FDD system  400 , which implements an embodiment of the FDD system  300  discussed above, is shown in  FIG. 4 . The FDD system  400  samples the parameters of the HVAC equipment. In this particular embodiment, the FDD system  400  accepts input from the supply air sensor  402  including thermistor, carbon monoxide sensor and air flow sensor. In order to measure the temperature of the flue gas, thermistor  406  is also installed in the flue gas path. In this exemplary embodiment, because the FDD system  400  is installed in the return air path (as shown in  FIG. 1 ), the FDD system  400  has internal thermistor  408  and humidity sensor  410  to measure the return air characteristics. While not shown in  FIG. 4 , the FDD system  400  may also accept measurements from sensors or receive communications from external systems, for example, a weather network server, about the external conditions to the HVAC equipment. 
         [0055]    The sensors  402 - 410  are connected to the signal conditioner  412 , the output of which is coupled to the analog to digital converter (ADC)  414 . Once the readings from the sensors are converted into digital format by the ADC  414 , the information is used by the microcontroller  416  for processing. The microcontroller  416  takes the plurality of classifier parameters calculated during the training phase and produces a classifier score that is communicated to the wireless transceiver  418 . A wireless transceiver  418  may be used by the FDD system  400  to communicate to a remote server. The remote server confirms the performance of the HVAC equipment using the information provided by the FDD system  400 . The remote server may then communicate further information or instructions to the FDD system  400 . The wireless transceiver  418  may also be used to allow the technician or homeowner to monitor and correct system performance by communicating with the FDD system  400 . Once the communication with the remote server is complete, and the next step determined, it is used by the digital I/O  420  to control the HVAC equipment through the HVAC control relay  424 , which is coupled to the HVAC control board  430 . In one embodiment, the thermostat  440  is also connected to the FDD system  400 . The FDD system  400  may be remotely commanded to give control of the HVAC equipment to the thermostat. Furthermore, the homeowner may use the FDD system bypass switch  422  to disable the control of the HVAC equipment by the FDD system  400 . The bypass switch  422  may be used if the FDD system  400  is malfunctioning. In a further embodiment, a humidifier  450  may be coupled to the HVAC control relay  424 . This allows the FDD system  400  to also control humidity in the home. 
       Operation of the FDD System 
       [0056]    Referring to  FIGS. 3 ,  4  and  5 , the operation of the FDD system in accordance with an embodiment of the present invention will be explained in detail. Prior to operating the system, the homeowner or the technician installs the sensors  402 , and  406  and FDD system  400 . Other sensors to measure environmental conditions such as room temperature and outside temperature may also be installed. The thermostat  440  and HVAC control board  430  are also connected to the FDD system  400 . 
         [0057]    Before starting the training phase for the classifier  3080  modelling the normal behaviour of the HVAC equipment in situ, the technician or homeowner ensures that the HVAC equipment is operating properly, for that purpose, a communication to the remote server may need to be established. With the connection in place, the technician or homeowner commences the training phase, for example via the Internet to the FDD system  400  or by pressing the reset button  426 . 
         [0058]    During the training phase, the classifier  3080  modelling the behaviour of the HVAC equipment in situ is initiated and trained. When the FDD system  400  is activated, step  502  may be employed to collect normal data samples corresponding to the normal steady state or transient state behaviour of the HVAC equipment, as installed in situ in the operation environment. This initial ‘normal’ validation data may serve for cross validation of classifier  3080  performance in modelling the normal behaviour of the HVAC equipment in situ. 
         [0059]    The training phase consists in repeatedly sampling sensor measurements from the HVAC equipment and the environment where the HVAC equipment is installed  504 . For each sample, the classifier&#39;s parameters are re-estimated 506, and the classification scores are calculated using the validation data  508 . As discussed above, in the exemplary training phase, the validation data is initially collected at step  502 . Finally, the scores computed over validation samples allow assessing the overall performance of classifier  3080 . As an example, performance may be computed as the log-likelihood of an HMM classifier over all validation samples. The iterative sampling  504 , re-estimating of parameters  506  and calculating of scores  508  make up what is referred to as “on-line” learning of classification parameters. This term refers to the training of the FDD system  400  using real data samples as they are measured from the HVAC equipment as installed, serviced or modified in situ. On-line learning is repeated until some performance criterion for the on-line learning is met  510 . For example, as an criterion, classifier performance may be monitored to determine if it is constant over a pre-defined number, N, of successive samples. Alternatively or additionally, the on-line learning may be repeated until a maximum number of samples, N max , are collected. Finally, learning may also end by using some stopping classifier-specific criteria that does not depend on validation data. 
         [0060]    If the training phase is performed shortly after installation of the HVAC equipment, the sampled data should closely reflect the initial data collected at step  502 . However, if the training phase is initiated many years since the installation, for example but not limited to after major service, modification or replacement to the HVAC equipment, the sampled data at step  504  may differ significantly from initial data measured at step  502 . Despite the difference, the HVAC equipment may still be running optimally with the altered components. In such a case, the technician may re-initiate step  502  to recollect the validation data, which is representative of the normal behaviour of the HVAC equipment. Thus, by training the classifier using initial data collecting 502 and sampling  504 , the classifier is able to evolve and change with the life of the HVAC equipment. 
         [0061]    With the classifier parameters refined during the on-line learning process, the decision threshold T is set at step  512 . The decision threshold T may be set using, for example, statistical property of the classifier, confidence limits, and a cost analysis for different types of errors. Alternatively, the decision threshold T may be set using a cost-sensitive Receiver Operational Characteristic (ROC) analysis. The classifier parameters and decision threshold are stored in memory. 
         [0062]    After training classifier  3080  in situ, the FDD system  400  may enter the monitoring phase. During this phase, the FDD system  400  samples the HVAC equipment and the external conditions  514 , and calculates the classifier score  516 . Unlike calculation step  508  of the training phase, in calculation step  516 , the classifier scores is not computed over several validation samples as in step  502 . Instead, during the monitoring phase, the classifier score is computed by presenting a new data sample measured during operations (see Step  514 ) to classifier  3080 . The score is then compared to the threshold T (step  518 ) determined during the training phase. Steps  514 ,  516 , and  518  are repeated at a predetermined interval during the monitoring phase. Referring also to  FIG. 3 , when classifier score falls below the decision threshold in the case of efficiency monitoring  3100 ,  518 , a flag is raised (step  520 ), which is communicated to the remote server for further instructions (step  522 ). Information related to the flag may be stored in memory for future analysis and training. Additionally, for each classifier score computed at step  516 , the FDD system  400  may also estimate the degradation in performance of the HVAC equipment. 
         [0063]    A flag is an indication that the HVAC equipment has deviated from the normal behaviour. However, the specific reason for the deviation is not yet known until the HVAC equipment is diagnosed. The diagnosis may be carried out remotely by a technician or, if necessary, the technician may diagnose the problem on site. Once the cause of the deviation in the performance is determined, the FDD system  400  may be trained to detect the specific fault condition. Referring to  FIG. 3 , the classification system  308  is composed of N classifiers  3080 ,  3081 , . . . ,  308 N. Classifier  3080  models the normal behaviour of the HVAC equipment, while classifiers  3081 , . . . ,  308 N model a respective specific fault condition of the HVAC equipment. If the performance of the HVAC equipment had never deviated from normal behaviour, the classification system  308  may only have classifier  3080  that models the normal behaviour of the HVAC equipment. Subsequently, for each flag raised, the FDD system stores data samples and other pertinent information, and uses this information to train other fault condition classifiers  3081 , . . . ,  308 N. After each flag is diagnosed, for example, flag was raised because of a malfunctioning compressor, the FDD system is trained for new fault condition classifiers. At this point, the HVAC equipment is presumably still running abnormally as the problem has been diagnosed but not yet fixed. Data samples collected at Step  521  provide ‘abnormal’ data for train and validate classifiers  3081 , . . . ,  308 N. Thereafter, the classifier parameters are estimated using the stored information, that is the information stored when the flag was raised, at step  521 . The steps  504 - 508  are repeated until the exit condition is met at step  510 . The threshold is then set at step  512 . Over time, a new classifier is trained to model each specific abnormal behaviour. With each of the classifiers  3081 , . . . ,  308 N, the FDD system can monitor for a respective specific fault condition that was identified and diagnosed by the technician. Referring also to  FIG. 3 , the classifier score is compared  518  to one of the thresholds T 1 , . . . , T N  for each of the classifiers  3081 , . . . ,  308 N. In the case of fault monitoring, a flag is raised  520  when the score exceeded a threshold. For example, the technician may determine that the condition generating the flag was related to a leaking refrigerant. After the diagnosis, the technician trains a new classifier to detect for this specific fault condition by using the data samples and conditions that generated the flag. In future monitoring of the HVAC equipment, the new classifier should be able to detect the specific fault condition and self-diagnose the problem. 
         [0064]    As more classifiers  3081 , . . . ,  308 N are trained to detect specific fault conditions, the FDD system is able to pro-actively diagnose more problems in the HVAC equipment. This information may be used in a variety of ways. For example, the technician may use the information to remotely control the HVAC equipment to fix problems without travelling to the site. In other instances, the technician may personally contact the homeowner with possible fixes to improve the performance of the HVAC equipment or to schedule an appointment for on-site maintenance. In another embodiment of the invention, the remote server may generate an e-mail containing detailed information and recommendation for the owner. Such an e-mail may include possible fixes to the fault detected by the remote server and other recommendations to improve the general health of the HVAC equipment. Furthermore, the remote server may send monthly e-mails about the performance of the HVAC equipment. As an illustration,  FIG. 6  shows information about performance for the month of February that may be included in the monthly e-mail. Since February is a cold month, the e-mail contains information related to the heating performance of the HVAC equipment. As it has detected a decline in performance, several suggestions are provided, including recommended parts for purchase. Related to comfort is the humidity level. Because the remote server detected a low level of humidity, it has suggested the addition of a humidifier. Various other useful information may be incorporated for the homeowner. 
         [0065]    In this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. 
         [0066]    It will be further understood that the terms “comprises” or “comprising”, or both when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0067]    One or more embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.