Abstract:
Known psychometric characteristics of air are employed to achieve accurate indoor relative humidity control to prevent condensation inside a building envelope without complex mathematical computational requirements. An HVAC system control includes a simple control algorithm employed to calculate an effective delta (ΔT) based upon a single adjustment factor (A*) and environmental inputs. The effective delta (ΔT) is then used to determine a maximum allowable indoor relative humidity. The system control is then operable to selectively activate/deactivate a device to adjust an actual indoor relative humidity to a value less than the maximum allowable indoor relative humidity to prevent condensation inside the building envelope.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The application claims priority to U.S. Provisional Application No. 60/537,527 which was filed on Jan. 20, 2004, the disclosure of which is incorporated in its entirety herein by reference.  
         [0002]     This application relates to an indoor central heating, ventilation, and air conditioning (HVAC) system wherein various units report environmental characteristics to a central control for evaluation in relation to a user input. The central control controls an indoor relative humidity to prevent condensation inside a “building envelope.” A building envelope is defined to include all building exterior walls, i.e., walls having a side exposed to the outside elements and the roof.  
         [0003]     Relative humidity is defined as the ratio of the actual amount of moisture in the air to the maximum moisture capacity at a given air temperature. It is known that as temperature increases, the capacity of the air to hold moisture in the form of water vapor also increases. Conversely, as temperature decreases, the capacity of the air to hold moisture decreases and any excess moisture condenses as water on surfaces in contact with the air.  
         [0004]     Therefore, during the winter months, the cold outdoor air has a relatively low moisture content, however, the air inside building structures is typically heated. Depending on the construction quality of a particular building, some of the cold dry outside air infiltrates into the warm indoor space and is subsequently heated to the indoor temperature. This phenomenon effectively reduces the indoor relative humidity and the indoor air becomes very dry.  
         [0005]     To address this winter dryness, humidifiers are often employed as part of the central heating system. Humidifiers introduce moisture into the heated air, increasing indoor relative humidity. Humidifiers are typically controlled by devices known as humidistats. Humidistats sense an actual indoor relative humidity and allow a homeowner to set a desired indoor relative humidity level. When the indoor relative humidity falls below the desired level, the humidistat activates the humidifier to add moisture to the air. Once the desired indoor humidity is achieved, the humidistat deactivates the humidifier.  
         [0006]     Buildings typically have thermally insulated walls and attics to minimize heat loss and reduce cold air infiltration. However, portions of the building envelope, such as windows, may be less insulated than others, and their interior surfaces may get colder. If the outdoor temperature is low enough and the indoor humidity high enough, moisture may condense on these less insulated interior surfaces, which is undesirable. Conversely, some buildings in colder climates are built to be extremely “tight” allowing minimal outdoor air infiltration levels. Without the natural drying due to outside air infiltration, internal moisture generated by the occupants and their activities allows the indoor relative humidity to reach high levels resulting in condensation even in the winter months.  
         [0007]     To address the concern of high indoor relative humidity, devices known as ventilators are often employed. Once the indoor relative humidity exceeds the desired level, the ventilator is activated to bring a controlled amount of outside dry air into the building envelope to decrease the indoor relative humidity. Ventilators typically are controlled by a second humidistat, separate from and in addition to the humidistat that controls the humidifier.  
         [0008]     In general, the colder it is outside, the lower the indoor relative humidity has to be to avoid condensation. Therefore, occupants typically notice condensation when the weather turns cold and respond by lowering the humidistat setting. However, as weather patterns change, frequent manual adjustment is often required. To date there has been no way for the occupant to know exactly how much to adjust the humidity setting. This continual trial and error process results in the indoor relative humidity level either being too high or too low in comparison with the ideal indoor humidity level.  
         [0009]     Therefore, controlling indoor relative humidity to a fixed relative humidity level, as with a simple humidistat, is undesirable.  
         [0010]     While systems have been proposed to perform detailed calculations of a maximum relative humidity level, the known proposed are quite complex. As such, it is desirable to have an HVAC system that simply, but accurately, determines the maximum allowable indoor relative humidity to prevent condensation inside a building envelope based upon indoor and outdoor temperatures.  
       SUMMARY OF THE INVENTION  
       [0011]     This invention uses known data regarding the psychometric characteristics of air to achieve accurate indoor relative humidity control to prevent condensation without complex mathematical computational requirements.  
         [0012]     An HVAC system control employs a simple control algorithm to calculate an effective delta (ΔT) based upon a single adjustment factor and environmental inputs such as indoor temperature, outdoor temperature and/or indoor relative humidity. The effective delta (ΔT) is then used to determine a maximum allowable indoor relative humidity to prevent condensation inside a building envelope.  
         [0013]     In one disclosed embodiment of this invention, the user input is a user selectable heating humidity level entered by the building owner/occupant. The occupant selects a heating humidity level from a predetermined range of 1-9 with a default value somewhere in the middle, say 5. The selected heating humidity level is subsequently employed to determine the single adjustment factor (A*). In this embodiment, the central control employs a conversion table stored in memory to convert the user selected heating humidity level to the single adjustment factor (A*). The single adjustment factor (A*) is then employed to calculate the maximum allowable indoor relative humidity based upon the user selected heating humidity level.  
         [0014]     The occupant typically sets the heating humidity level to a level just below the one that allows condensation to occur. This is accomplished through an iterative process. The occupant selectively increases the heating humidity level until condensation occurs within the building envelope. The occupant then selectively decreases the heating humidity to the level just below the level at which condensation occurred. Once the occupant has selected the indoor relative humidity level required to prevent condensation, the central control is operable to maintain the actual indoor relative humidity based upon the user selected indoor relative humidity level, continuously adjusting the actual indoor relative humidity to accommodate changing environmental conditions while preventing condensation.  
         [0015]     In another disclosed embodiment of this invention, the user input is entered by the HVAC system installer upon installation. The user input is representative of a building structural characteristic and is typically indicative of a thermal insulation level of the building envelope. The user input may be set based on past experience of the installer with respect to previous homes of similar quality. In this embodiment, the central control employs a conversion table to subsequently convert the structural characteristic into the aforementioned single adjustment factor (A*). The single adjustment factor (A*) is then employed to calculate the maximum allowable indoor relative humidity based upon the thermal insulation level of the building. Once set by the installer, the HVAC system is operable to maintain the actual indoor relative humidity level, continually adjusting to accommodate changing environmental conditions to prevent condensation.  
         [0016]     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic view of a building HVAC system.  
         [0018]      FIG. 2  is a detailed schematic view of a control for an HVAC system.  
         [0019]      FIG. 3  is a graphical representation of a relationship between an allowable relative humidity percentage and a difference between two different temperatures.  
         [0020]      FIG. 4  is an example Conversion Table.  
         [0021]      FIG. 5  is an example Allowable Humidity Table. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     A schematic view of a building HVAC system  10  is illustrated in  FIG. 1 . An indoor control unit  12  includes central control  14  which is operable to receive a user input  16  from a user interface  18  and at least one environmental input  20 . The user input  16  is a heating humidity level  22  which is selected from a predetermined range. As shown, the level is adjusted by pressing up/down arrows  24  on the user interface  18 . Of course other input devices can be utilized. An outdoor unit  26  is operable to transmit the environmental input  20  to the central control  14 .  
         [0023]     The central control  14  then calculates a desired indoor relative humidity based upon the user input  16  and the environmental input  20  and adjusts an actual indoor relative humidity to a value proximate the calculated desired indoor relative humidity by selectively activating/deactivating at least one indoor device  28 . As is known, the indoor device  28  could be a humidifier  30 , and/or a ventilator  32 , or other humidity control devices.  
         [0024]     A detailed schematic view of the central control  14  is illustrated in  FIG. 2 . Central control  14  is operable to receive a user input  16 , and at least one environmental input. A user interface  18  is operable to receive the user input  16  to set a desired temperature  19  and humidity level  22 , and transmit the user input  16  to the central control  14 . The environmental input includes an outdoor temperature T 1 , and an indoor temperature T 2 . The central control  14  also includes at least one reference table stored in a memory.  
         [0025]     Known tables have been published that relate an air temperature, t, to a humidity ratio at saturation, W s . The humidity ratio at saturation, W s , represents the maximum moisture holding capacity of the air at the temperature, t. One example table titled: Thermodynamic Properties of Moist Air, Standard Atmospheric Pressure, 14,696 p.s.i. (29.921 in. Hg.) can be found in the A.S.H.R.A.E. Fundamentals Handbook, published in 1997 (A.S.H.R.A.E. Table).  
         [0026]     In order to provide a simple, but accurate, method to calculate either t from W S  or W S  from t, the following observation has been made. The ratio of W s  at two different temperatures, t 1  and t 2 , is largely dependent on the difference between t 1  and t 2 , and not on the individual temperatures themselves. This ratio can be conveniently expressed as an allowable humidity percentage (% RH). For example, assume t 2  is greater than t 1  and the corresponding values of W S  are W S1  and W S2  As graphically illustrated in  FIG. 3 , the ratio of W S1  and W S2  (% RH) can be closely approximated, based upon the A.S.H.R.A.E. table, as a function of the difference between t 1  and t 2  (Delta T). Further,  FIG. 3  also shows that for any value of Delta T, the ratio of W S1  and W S2  is virtually the same whether t 2  is 60 degrees F. or 73 degrees F.  
         [0027]     Typically, t 2  represents an indoor temperature and t 1  represents an outdoor temperature. Therefore, for example, in a heating season, i.e. when the outdoor temperature is lower than the indoor temperature, t 2  is typically controlled between 60 degrees F. and 72 degrees F. while t 1  can typically vary from −15 degrees F. to 55 degrees F.  
         [0028]     In one theoretical situation, where a building envelope has no thermal insulation, the temperature of the building indoor surfaces will be equal to the outdoor temperature, t 1 . In this theoretical situation, condensation will occur on the building interior surfaces if an indoor moisture content (humidity ratio) exceeds W S1 , which is the saturation level for t 1 . Thus, to avoid condensation on the building indoor surfaces, the maximum allowable indoor moisture content is W S1 . In addition, it should be understood that at the indoor temperature t2, the moisture holding capacity of the indoor air is W S2 . Per the definition of relative humidity, the ratio of W S1  and W S2  is the indoor relative humidity at which condensation occurs. Therefore, the ratio of W S1  and W S2  is the allowable indoor relative humidity to avoid condensation.  
         [0029]     However, because all building envelopes have at least some level of thermal insulation, the above is simply a limiting case. In actual building envelopes, an effective Delta T is less than the actual difference between indoor temperature and outdoor temperature because the building envelope acts as an insulating barrier that reduces the effect of outdoor temperature on an indoor space. The effective Delta T (ΔT) is calculated based upon an equation: 
 
Δ T=A *( t   2   −t   1 ) 
 
 where A* is an adjustment factor and a lower adjustment factor indicates a better insulated home. 
 
         [0030]     In one embodiment, the user input  16  is a user selectable heating humidity level which is selected from a predetermined range and adjusted by pressing up/down arrows  24  on the user interface  18 . In this embodiment, the heating humidity level is typically initially entered by the homeowner and adjusted to the level just below the one that allows condensation to occur. This is accomplished through an iterative process. The occupant selectively increases the heating humidity level until condensation occurs within the building envelope. The occupant then selectively decreases the heating humidity to the level just below the level at which condensation occurred. Once set, the homeowner is not required to make any further adjustments, as the central control  14  is operable to compensate for indoor and outdoor temperature variations, controlling a maximum allowable indoor humidity to prevent condensation. Of course, the iterative process could be performed by the system installer, rather than the occupant. In this embodiment, the central control  14  employs a Conversion Table (CT), illustrated in  FIG. 3 , to convert the user input  16  into an adjustment factor A*. After conversion, the central control  14  then calculates an effective delta ΔT based upon the formula: 
 
Δ T=A *( t   2   −t   1 ) 
 
         [0031]     After calculating the effective delta ΔT, the central control  14  employs an Allowable Humidity Table (AHT), illustrated in  FIG. 4 , to determine a maximum allowable indoor relative humidity. Of course, other ways of determining a reference value to compare to such a table come within the scope of this invention. Any method of utilizing a user input and an environmental input to determine a value reference to be compared to a table comes within the scope of this invention.  
         [0032]     After determining the maximum allowable indoor relative humidity, the central control  14  is operable to selectively activate/deactivate indoor device  28  to adjust an actual indoor relative humidity to a value less than the calculated maximum allowable indoor relative humidity to prevent condensation. Whether to activate or deactivate the indoor device  28  is determined by comparing the actual indoor relative humidity to the calculated maximum allowable indoor relative humidity.  
         [0033]     If the indoor device  28  is a humidifier  30  and, upon comparison, the central control  14  determines that the actual indoor relative humidity is less than the calculated maximum allowable indoor relative humidity, the central control  14  activates the humidifier  30 . By activating the humidifier  30 , warm wet air is generated and introduced into the building envelope, effectively increasing the actual indoor relative humidity. Conversely, if upon comparison, the central control  14  determines that the actual indoor relative humidity is greater than the calculated maximum allowable indoor relative humidity, the central control  14  deactivates the humidifier  30  allowing the actual indoor relative humidity to decrease.  
         [0034]     Further, if the indoor device  28  is a ventilator  32  and, upon comparison, the central control  14  determines that the actual indoor relative humidity is greater than the calculated maximum allowable indoor relative humidity, the central control  14  activates the ventilator  32 . By activating the ventilator  32 , cool dry outside air is brought into the building envelope, effectively decreasing the actual indoor relative humidity. Conversely if, upon comparison, the central control unit  14  determines that the actual indoor relative humidity is less than the calculated maximum allowable indoor relative humidity, the central control  14  deactivates the ventilator  32  allowing the actual indoor relative humidity to increase.  
         [0035]     Finally, if the indoor device  28  includes both a humidifier  30  and a ventilator  32 , the central control  14  is operable to determine the actual indoor relative humidity and compare the actual indoor relative humidity to the calculated maximum allowable indoor relative humidity. Based upon this comparison, the central control  14  is then operable to selectively activate/deactivate either one or both of the humidifier  30  and/or the ventilator  32  to regulate the actual indoor relative humidity to a value less than the maximum allowable indoor relative humidity, preventing condensation.  
         [0036]     In another embodiment, the user input  16  is entered by the HVAC system installer. In this embodiment, the user input  16  is representative of a building structural characteristic typically indicative of the thermal insulation level of the building envelope. In this embodiment, the building structural characteristic corresponds to a heating humidity level and is typically entered by the installer of the HVAC based upon his knowledge of the thermal insulation level of the building and his past experience with buildings of similar quality. Once set by the HVAC system installer, the building owner is typically not required to make further adjustments, as the central control  14  is operable to compensate for indoor and outdoor temperature variations, controlling the maximum allowable indoor humidity based upon the thermal insulation level of the building envelope to prevent condensation.  
         [0037]     By associating a determined reference value with stored maximum allowable indoor relative humidity values, the present invention is able to provide accurate humidity control in a relatively simple system.  
         [0038]     Although two preferred embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.