Abstract:
A system for monitoring the service life of an HVAC air filter is disclosed and includes an airflow sensor that is positioned in an HVAC duct in relatively close proximity to the air filter. The airflow sensor output is sent to a processor that is pre-programmed with a filter evaluation algorithm. Each time the HVAC blower is activated begins a new duty cycle during which airflow signals are generated are sampled by the processor/algorithm. Selected sampled values are averaged to calculate a peak airflow velocity, V peak , for each duty cycle. The peak airflow velocity, V peak , is then compared to a base reference, V reference , to determine whether the air filter requires service/replacement. The value of the base reference, V reference , can be established during an initializing procedure and thereafter updated using the peak airflow velocity, V peak .

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to devices and methods for efficiently filtering air in heating, ventilation and air conditioning (HVAC) systems. More particularly, the present invention pertains to air filter monitors for HVAC systems. The present invention is particularly, but not exclusively, useful for monitoring an HVAC air filter to determine whether the air filter needs servicing or replacement. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nearly all commercial and residential buildings have an HVAC system that includes an air handler to condition and circulate air within the building. Moreover, all of these systems include at least one air filter to filter the circulating air. Generally, the HVAC systems include tubular structures (ducts) to deliver and remove air from the building. Air filters are often placed in a duct upstream of the system&#39;s blower (return duct) to remove dust and other particles from the building before the air is recirculated. 
         [0003]    Many types of air filters are commercially available including cloth filters, single use, disposable, fibrous media filters, washable metal screen filters, etc., and all or these filters have one thing in common. When they get dirty, they lower the overall efficiency of the system. For example, dirty filters can cause the blowers to work harder and use more energy than normal. In addition, dirty filters can cause HVAC components to undesirably heat to temperatures where they become inefficient. 
         [0004]    Most HVAC systems are thermostatically controlled. In many of these systems, the blower runs intermittently, and generally only when needed. The consequence of this is that the conditions within the ducts and near the filter can vary considerably. In particular, pressures and flow velocities within the system can vary. Factors causing these conditions to vary include the temperature and moisture content of the air in the building and, in some cases, the outdoor air. In addition, these factors can include the overall dirt and particle levels in the building. Also, at any given time, the conditions in the system ducts are dependent on the length of time that the blower, heater, etc. have been operating and the previous cyclical operation of these components. 
         [0005]    As indicated above, a clogged filter can decrease system efficiency and waste energy. Crude methods for determining a filter&#39;s condition include holding the filter in front of a light source and visually determining how much light passes through the filter. This technique can be grossly unreliable. Rather than a visual inspection, another technique involves simply replacing a filter, without inspection, according to a periodic replacement schedule, e.g. monthly or yearly. Unfortunately, both of these techniques are inefficient, and can result in either 1) an otherwise usable filter being discarded, or, 2) the inefficient use of a clogged filter that should have been replaced earlier. 
         [0006]    As disclosed herein, the airflow velocity near a filter can provide an indication of filter cleanliness. However, in some cases, due to the varying conditions that can be present in the ducts as described above, simple airflow measurement techniques can provide inaccurate results. For example, a reference airflow may be determined under an initial set of duct conditions. Later, a filter measurement may be made and compared to the reference to gauge filter cleanliness. However, if the two measurements are made under substantially different duct conditions, a relative clean filter may appear to be dirty, or vice versa. 
         [0007]    With the above in mind, it is an object of the present invention to provide a system and method for accurately monitoring an HVAC filter to determine whether an air filter needs servicing. It is another object of the present invention to provide a system and method for accurately monitoring an HVAC filter to estimate a period of time before an air filter needs to be replaced. Another object of the present invention is to provide systems and methods for monitoring air flow efficiency that are relatively easy to manufacture, simple to use and is comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0008]    A system for monitoring the service life of an HVAC air filter includes an airflow sensor that is positioned in an HVAC duct in relatively close proximity to the air filter. For the system, the airflow sensor outputs signals that are indicative of the airflow velocity of air flowing through the duct. These airflow signals are then sent to a processor. In accordance with the invention, the processor is pre-programmed with a filter evaluation algorithm. The processor inputs the airflow signals into the filter evaluation algorithm and runs the algorithm to determine whether the air filter requires replacement. For the system, an indicator can be operationally connected to the processor to generate a user perceptible output such as an audio alarm or a visual display when the filter requires servicing or replacement. 
         [0009]    As indicated above, the processor/algorithm performs operations on the airflow signals to determine whether the air filter requires service/replacement. In one implementation, airflow through the duct occurs periodically. Each time the HVAC blower is activated begins a new duty cycle that is typically longer than about three minutes. During a duty cycle, the airflow signals that are generated are sampled by the processor/algorithm. This sampling can include one or more reading cycles within each duty cycle. In addition, for each reading cycle, a specific sampling plan may be conducted. For example, for each reading cycle, the processor/algorithm may sample the airflow signals at approximately four second intervals for a period of about eighty seconds. Typically, the first reading period is conducted within three minutes from the beginning of a new duty cycle. The number of reading periods per duty cycle and the temporal spacing between reading periods can also be included in the sampling plan. 
         [0010]    The result of the sampling plan described above is a number of digitized airflow velocity values (i.e. magnitudes) that can be manipulated by the processor/algorithm to determine whether the air filter requires service/replacement. More specifically, for each duty cycle, this manipulation can include the step of determining a maximum airflow velocity value, V max , for each reading period in the duty cycle. The algorithmic manipulation can further include the step of averaging the maximum airflow velocity values, V max , to determine a peak value V peak , for each duty cycle. Once the peak value V peak , is calculated for a duty cycle, the processor/algorithm can compare the peak value V peak , to a base reference, V reference , to determine whether the air filter requires service/replacement. For example, the processor/algorithm may provide an alarm output indicating a dirty filter when the peak value V peak , is less than a preselected percentage, P of said base value V reference  (i.e. V peak &lt;P×V reference ). Typically, suitable values of P are in the range of about 70 to 90 percent. 
         [0011]    For the present invention, the value of the base reference, V reference  can be established during an initializing step when the filter is new or the base reference, V reference , can be established during normal HVAC system operation. In either case, the base reference, V reference  can be held constant over the life of the filter or can be updated by the processor/algorithm. In one embodiment of the algorithm, the base reference, V reference  is updated by comparing the current base reference, V reference  with the most recently calculated peak value V peak , and updating the base reference, V reference  with the peak value V peak , when the peak value V peak , exceeds the base reference, V reference . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0013]      FIG. 1  is a schematic view of a portion of a building environment showing a system for monitoring the service life of an HVAC air filter operationally positioned in an HVAC unit; 
           [0014]      FIG. 2  a schematic view showing the components of a system for monitoring the service life of an HVAC air filter; 
           [0015]      FIG. 3A  shows a perspective view of a one-piece system for monitoring the service life of an HVAC air filter having a fan-style airflow sensor, circuitry portion having a processor for running a preprogrammed algorithm and a battery section, shown folded in an operational configuration; 
           [0016]      FIG. 3B  shows a top plan view of a one-piece system shown in  FIG. 3A  folded into a compact configuration for storage or transport; 
           [0017]      FIG. 4  is a flowchart illustrating the algorithmic steps for determining whether an air filter requires service/replacement in accordance with one aspect of the present invention; 
           [0018]      FIG. 5  shows a plot of the analog signal output from an airflow sensor on a graph of voltage versus time and illustrates a plan for sampling the output; and 
           [0019]      FIG. 6  shows a plot of voltage versus time in which each dot represents a calculated V peak  value for a duty cycle and illustrates a method for updating a reference voltage that is used to gauge whether a filter should be serviced or replaced. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring initial to  FIG. 1 , a portion of a building environment  10  is shown having an HVAC unit and a system (generally designated  12 ) for monitoring the service life of an HVAC air filter  14 . As shown, the system  12  includes an airflow sensor  16  that is positioned in an HVAC duct  18  at a distance “d” along the duct  18  from the air filter  14 . For the arrangement shown in  FIG. 1 , the HVAC unit includes and air handler  20  having a blower  22  which may be operationally coupled with an optional heating, air conditioning, humidifier and/or dehumidifying subsystem(s)  24 . As shown, air is forced to circulate into and through duct  26  by blower  22 . Air in duct  26  is then introduced (arrow  28 ) into a space or room in the building environment  10  through vent  30 . Also shown, air returns (arrow  32 ) from a space or room flowing into return duct  18  through vent  34 . From vent  34 , air flows through duct  18 , past sensor  16 , and through filter  14  to the air handler  20 . 
         [0021]    Cross-referencing  FIGS. 1 and 2 , it can be seen that the system  12  includes a processor that is pre-programmed with a filter evaluation algorithm (processor/algorithm  36 ) and an input/output device  38 . As shown, the sensor  16  is electronically connected with the processor/algorithm  36  via link  40  which can be, for example, a wire or wires, a wireless connection, a bus or the two can be connected over a network such as an internet connection. The processor/algorithm  36  and sensor  16  may be integrated (sharing one or more components), co-located or the processor/algorithm  36  may be remotely located from the sensor  16 . Also, shown, the input/output device  38  is electronically connected with the processor/algorithm  36  via link  42  which can be, for example, a wire or wires, a wireless connection, a bus or the two can be connected over a network such as an internet connection. The processor/algorithm  36  and input/output device  38  may be integrated (sharing one or more components), co-located or the processor/algorithm  36  may be remotely located from the input/output device  38 . Thus, for the present invention, the entire system  12  may be one integral unit located within a duct  18 , or, a sensor may be position in the duct  18  having a link (wireless or wire) to outside the duct  18 , with the other system  12  components located in close proximity, remotely or a combination thereof. 
         [0022]    For the system  12 , the airflow sensor  16  outputs signals that are indicative of the airflow velocity of air flowing through the duct  18 . Suitable airflow sensor include, but are not limited to, fan-type sensors having a blade which rotates in an airflow and coils/magnets which generate an electrical output that is proportional (linearly or non-linearly proportional) to the blades RPM. Alternatively, flaps may be used which pivot to an extent that is proportional (linearly or non-linearly proportional) to an airflow velocity. Flow meters based on Bernoulli&#39;s principle such as single static air pressure sensor may be used. Typically, these sensors output an electrical signal have a voltage or amplitude that is proportional (linearly or non-linearly proportional) to an airflow velocity. Any other type of airflow sensor known to those skilled in the pertinent art which outputs an electrical signal having at least one signal parameter such as voltage or amplitude that is proportional (linearly or non-linearly proportional) to an airflow velocity can be used in the system  12 . 
         [0023]    The airflow sensor  16  is typically mounted in a return duct  18  upstream of the air filter  14  at a location in the cross-section of the duct  18  where laminar flow is most likely to occur. In some cases, as shown in  FIG. 1 , the airflow sensor  16  is positioned in duct  18  at a distance “d” along the duct  18  from the air filter  14 . Typically, this distance “d” is approximately six to eight inches. In some cases, a service access (not shown) is provided about 1 foot from the filter. In these cases, the airflow sensor  16  can be conveniently positioned between the service access and filter. Several techniques can be used to mount the airflow sensor  16  in the duct  18  including, but not limited to, sheet metal screws, two sided tape or a magnetic mount. 
         [0024]    Also for the system  12 , the processor/algorithm  36  can include a processor such as a microcomputer (programmable or programmed), personal computer, logic circuit or a combination thereof, with memory, or any other device known to those skilled in the pertinent art capable of processing instructions and implementing the algorithms described herein. The algorithms described may be programmed into hardware, firmware, software or a combination thereof. The processor/algorithm  36  may be pre-programmed with the algorithm prior to delivery of the system  12  to the user and/or may be programmed with an algorithm that is updatable or accepts/requires user input (see below). Typically, the algorithm is programmed into an application level software program which is translated into machine language and processed by a microprocessor or personal computer. 
         [0025]    Also for the system  12 , the input/output device  38  can include one or more output devices including speakers for audio output and/or displays for displaying visual information. For example, the speakers can be provided for producing an audible alarm such as a siren, buzz and/or screech, or may produce spoken status reports such as “battery low”; “filter change needed”, etc. The display may be as simple as a light (e.g. LED), a panel of lights or a multi-pixel display. The LED&#39;s may indicate state such as initialization, low airflow, low battery, etc. An onboard or detachable LCD may be used to display information such as “battery low”; “airflow drop {appropriate percent}” filter life left {appropriate life time}, etc. The output may be a touchscreen or computer monitor. Some or all of the system  12  may be connected to a network such as a LAN, the internet, etc. In this case, the output may include email notifications or an update to a website. For the system  12 , the input/output device  38  can include one or more input devices which can include, for example, input buttons such as a single button to check battery life, a multi-button panel, a five point, round and center panel allowing menu navigation, for example, if an LCD is present, etc. Other known forms of input devices such as touchscreens, keyboards, a mouse, a bluetooth device such as a cellphone, infra-red remote control, etc. can be used as an input to the system  12 . 
         [0026]      FIGS. 3A and 3B  show a one-piece system  12  having a fan-style airflow sensor  16 , circuitry portion  44  having a processor for running a preprogrammed algorithm and a battery section  46 . As shown, the system  12  includes an LED lamp  47  for indicating whether filter replacement/service is required. Screw mount holes  48   a,b  are provided to screw the unit to a duct wall. Folding arms  50   a,b  allow the fan portion to pivot between a stowed configuration ( FIG. 3B ) and an operational configuration ( FIG. 3A ). 
         [0027]      FIG. 4  is a flowchart illustrating algorithmic steps for determining whether an air filter requires service/replacement. As shown, the process can begin by inputting a sampling plan (Box  51 ) for sampling the signal output from an airflow sensor, an initial base reference V reference , and a percentage factor, P. The sampling plan, base reference V reference , and/or percentage factor, P can be pre-programmed into the processor/algorithm  36  or, in some cases, the one or more of these items can be created and/or modified by the user, for example, using an I/O device  38  described above. When a new filter is used, an initialization process may be used to ensure the new filter is performing correctly. This process can include calculating a maximum airflow velocity value, V max-empty , for a reading period without a filter installed in the HVAC unit. The new filter can then be installed and a maximum airflow velocity value, V max-new , for a reading period can be measured. The measured maximum airflow velocity value, V max-new , can then be compared with the value, V max-empty  to determine whether the new filter is performing correctly within initial specifications for the filter. For example, a new filter may be determined to be out of specification if the value, V max-new , is less than a preselected percentage, P new  of the value, V max-empty  (i.e. V max-new &lt;P new ×V max-empty ). For example, a suitable value of P new  may be in the range of about 85 to 95 percent. 
         [0028]      FIG. 5  illustrates a plan for sampling an analog signal  52  from an airflow sensor  16  (see  FIG. 1 ). The analog signal  52  shown in  FIG. 5  represents the signal output of an airflow sensor  16  for an illustrative duty cycle. As shown, the dots  54  in  FIG. 5  represent sampling events and corresponding voltage values. The processor/algorithm  36  samples the analog signal  52  at specific times, t, according to a pre-programmed sampling plan. For this purpose, the processor/algorithm  36  can include a clock and logic for sampling the analog signal  52  according to the pre-programmed sampling plan. As shown, the sampling plan can include sampling within a number of reading cycles  56   a - c  within the duty cycle. In addition, for each reading cycle  56   a - c,  a specific sampling plan may be conducted.  FIG. 5  illustrates three reading cycles  56   a - c,  in each of which the analog signal  52  is sampled at approximately four second intervals for a period of about eighty seconds (twenty samples per reading cycle). Typically, the first reading cycle  56   a - c  is conducted within three minutes from the beginning of a new duty cycle. The number of reading periods per duty cycle and the temporal spacing between reading periods can also be included in the sampling plan. 
         [0029]    The result of the sampling step (Box  58 ) shown in  FIG. 4  are a set of digitized airflow velocity values corresponding to each sampling event. Box  60  of  FIG. 4  shows that the next step is to determine the maximum airflow velocity value, V max , for each reading period. This corresponds to dots  54   a,    54   b  and  54   c  in  FIG. 5 . For example, a simple compare and replace algorithm which compares each new value with the previous maximum value and includes a counter to stop at the end of a reading cycle can be used. 
         [0030]    Box  62  of  FIG. 4  shows that the next step is to average the maximum airflow velocity values, V max , to determine a peak value V peak , for each duty cycle. Typically, this can be implemented as a call to an averaging subroutine. Next, as shown in Box  64 , the peak value V peak , is compared to a base reference, V reference . As shown, when the peak value V peak , is less than a preselected percentage, P of the base reference V reference  (i.e. V peak &lt;P×V reference ), the system  12  outputs an alarm, warning light, etc. (Box  66 ). On the other hand, when V reference &lt;V peak , the base reference can be update (Box  68 ) and the updated V reference  used for the next duty cycle. Lastly, when V reference &gt;V peak &gt;P×V reference , the system waits for the next duty cycle (Box  70 ) and repeats Boxes  58 - 70 , as applicable with the current base reference V reference  and new airflow sensor analog signal. Typically, a value of P in the range of about 70 to 90 percent is used. 
         [0031]      FIG. 6  further illustrates the decision Box  64  of  FIG. 4 . More specifically, each dot  72  in  FIG. 6  represents a calculated V peak  value for a duty cycle. Dots  72  for the last nine duty cycles at the end of a filter&#39;s life are shown. Also, an initial base reference V reference  is illustrated by dotted line  74  and an updated base reference V reference  is illustrated by dotted line  76 . As shown, the earliest two V peak  values are slightly below the initial base reference V reference  (dotted line  74 ). For these two, the algorithm does not activate the alarm (Box  66 ) or update the Base reference (Box  68 ). However, the third dot  72   a  represents a V peak  value above the initial base reference V reference  (dotted line  74 ) so the base reference V reference  is updated (Box  68 ) and an updated Base Reference (dotted line  74 ) is used for future duty cycles. The next five V peak  values are slightly below the updated base reference, V reference  (dotted line  76 ). For these five, the algorithm does not activate the alarm (Box  66 ) or update the Base reference (Box  68 ). The last V peak  value (dot  72   b ) is less than P*V reference  so the algorithm activates the alarm (Box  66 ). 
         [0032]    In an alternate embodiment, the algorithmic output of Box  64  can be used to drive an automated filter changing and/or filter cleaning apparatus. For example, U.S. Pat. No. 6,152,998 granted on Nov. 28, 2000, and titled AUTOMATIC FILTER CARTRIDGE to James Eric Taylor, discloses an automatic filter cartridge having a supply roller and takeup roller. As disclosed, a motor can be used to rotate the take-up roller and replace a dirty portion of a filter roll with a clean portion. 
         [0033]    The algorithm shown in  FIG. 4  can be modified or augmented to generate and output other process parameters including a total cumulative run time for the air filter and an estimated time for a replacement of the air filter. For example, in the calculation of an estimated time for a replacement of the air filter, an empirical or theoretically derived relationship between the quantity (V reference  minus V peak ) and the estimated time for a replacement of the air filter can be used. 
         [0034]    While the particular System for Monitoring Air Flow Efficiency as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.