Patent Publication Number: US-11384971-B2

Title: Intelligent defrost control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/384,824, filed on Dec. 20, 2016. U.S. patent application Ser. No. 15/384,824 claims priority to and incorporates by reference the entire disclosure of U.S. Provisional Patent Application No. 62/270,235, which was filed on Dec. 21, 2015. U.S. patent application Ser. No. 15/384,824 and U.S. Provisional Patent Application No. 62/270,235 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to heat pump systems and more particularly, but not by way of limitation, to a method for controlling a defrost cycle of a heat pump system. 
     BACKGROUND 
     In a heat pump system running in a heating mode, it is common for frost to form on an exterior coil of the heat pump system. While the heat pump system is operating in the heating mode, the exterior coil can become extremely cool as the heat pump system attempts to transfer heat from exterior ambient air to a refrigerant in the exterior coil. If a temperature of the exterior coil cools to a temperature below a dew point temperature of the exterior ambient air, condensation occurs on the exterior coil. If the temperature of the exterior coil drops to a temperature below freezing or the exterior ambient air is below freezing, the condensation will turn into frost on the exterior coil. Formation of frost on the exterior coil is common in most areas where heat pump systems are used. 
     The formation of frost on the exterior coil reduces the effectiveness of the exterior coil as a heat transfer unit. The exterior coil is designed to transfer heat from the exterior ambient air to the refrigerant inside the exterior coil. To achieve this function, an exterior fan is typically used to draw exterior ambient air across the exterior coil. When frost forms on the exterior coil, an ability of the exterior fan to draw air across the exterior coil is reduced, which reduces the exterior coil&#39;s ability to absorb heat from the exterior ambient air. 
     Methods have been developed to defrost the exterior coil to remove frost that has built up on the exterior coil. One defrost method involves switching the heat pump system into a defrost mode during which the heat pump system operates as an air conditioner to transfer heat from the interior of an enclosed space, such as, for example, a house, to the exterior coil to melt any frost that has formed thereon. The heat pump system then operates as a typical air conditioner to transfer heat from the interior of the house to the exterior coil via a compressor and expansion valve system. In the defrost mode, the refrigerant in the exterior coil becomes warmer such that frost that has formed on the exterior coil melts. Meanwhile, the refrigerant in the interior coil becomes cooler. Interior air that is passed over the cooled interior coil blows out into the heated space. This is known in the industry as “cold blow.” Cold blow is typically counteracted with auxiliary heating elements. 
     When the heat pump system initiates a defrost cycle to remove frost from the exterior coil, three events typically occur: 1) the exterior fan is deactivated; 2) a reversing valve shifts from the heating mode to the defrost mode; and 3) the auxiliary heating elements are activated. The exterior fan is deactivated to stop the cooling effect on the frost formed on the exterior coil and to allow the frost to melt. The reversing valve is shifted to reverse the flow of the refrigerant within the heat pump system to provide hot refrigerant to the exterior coil to melt the frost. The auxiliary heating elements are activated to heat the interior air that is blown over the cool interior coil and into the interior of the building in order to provide warm air. 
     SUMMARY 
     A controller for initiating a defrost cycle of a heat pump system is configured to measure a temperature of an evaporator coil and to determine if the temperature of the evaporator coil is less than a freezing temperature. Responsive to a determination that the temperature of the evaporator coil is less than the freezing temperature, the controller is configured to determine if a current dew point temperature of air is greater than the temperature of the evaporator coil. Responsive to a determination that the current dew point temperature of air is greater than the temperature of the evaporator coil, the controller is configured to calculate a frost-collection rate. Responsive to a determination that the frost-collection rate is greater than a frost-collection-rate threshold, the controller is configured to initiate a defrost cycle. 
     A method of initiating a defrost cycle using a controller of a heat pump system includes measuring a temperature of an evaporator coil and determining whether the temperature of the evaporator coil is less than a freezing temperature. Responsive to a determination that the temperature of the evaporator coil is less than the freezing temperature, determining whether a current dew point temperature of air is greater than the temperature of the evaporator coil. Responsive to a determination that the current dew point temperature of air is greater than the temperature of the evaporator coil, calculating a frost-collection rate. Determining whether the frost-collection rate is greater than a frost-collection-rate threshold, and, responsive to a determination that the frost-collection rate is greater than the frost-collection-rate threshold, initiating a defrost cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of an illustrative heat pump system; 
         FIG. 2  is graph of a frost map; and 
         FIG. 3  is a flow diagram of an illustrative process for defrost control for a heat pump. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present invention will now be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
     Prior heat pump systems have incorporated defrost-cycle algorithms based on one or both of condenser-coil temperature and time since a most-recent defrost cycle. However, these algorithms are often inefficient and unreliable because they fail to consider environmental humidity and temperature conditions. For example, it is possible for heat pump systems to operate in environmental conditions where the ambient air temperature is below freezing but the exterior coil temperature is above the dew point. In such conditions, no condensation will form on the exterior coil and no defrost cycle is necessary. If the heat pump system uses a defrost algorithm that does not consider the environmental humidity and temperature conditions, an unnecessary defrost cycle may be initiated due to the exterior temperature being below freezing. Running unnecessary defrost cycles is a waste of energy and also prevents the heat pump system from operating as a heat pump to provide heat to the interior space because, during the defrost cycle, the heat pump system operates as an air conditioner to provide warm refrigerant to the evaporator coil. 
     Heat pump systems typically include an exterior coil that operates as an evaporator coil and an interior coil that operates as a condenser coil. A person having skill in the art will appreciate that when the heat pump systems operate in the defrost mode, the outdoor coil operates as a condenser coil and the indoor coil operates as evaporator coil. For the purposes of this application, the term “evaporator coil” is used to refer to the exterior coil and the term “condenser coil” is used refer the interior coil irrespective of the operating mode being described unless specifically stated otherwise. 
     During operation of the heat pump system, if the temperature of the evaporator coil drops below the dew point temperature, water may begin to condense from the ambient air that surrounds the evaporator coil onto the evaporator coil. If the evaporator coil temperature is below freezing, the condensed water freezes to form frost on the evaporator coil. For the heat pump system to operate efficiently, the heat pump system includes a defrost control to periodically initiate a defrost cycle to melt the frost that has accumulated on the evaporator coil. 
     The rate at which frost forms on the evaporator coil is referred to as a frosting rate. The frosting rate is a function of environmental temperature and humidity. In a typical embodiment, and in contrast to prior defrost-cycle algorithms, the illustrative defrosting method utilizes local environmental humidity and temperature data to determine when a defrost cycle is necessary. 
     In some embodiments, the environmental humidity and temperature data are provided to the heat pump system via, for example, a weather-data service. For example, information from the weather-data service may be obtained by a system controller of the heat pump system via an internet connection. The weather-data service may provide the environmental humidity and temperature data, for example, periodically (e.g., every hour, etc.) or on a “push” basis (e.g., the weather-data service provides updates to the heat pump system whenever the data changes). In some embodiments, the environmental humidity and temperature data may be obtained with one or more sensors, such as, for example, temperature and humidity sensors that are positioned proximal to the evaporator coil. Utilizing the environmental humidity and temperature data enables the heat pump system to more accurately determine if the defrost mode should be initiated. 
     Referring now to  FIG. 1 , a schematic diagram of an illustrative heat pump system  100  is shown. The heat pump system  100  includes an evaporator coil  102 , a reversing valve  104 , a compressor  108 , and a condenser coil  112  that are coupled together to form a circuit through which a refrigerant may flow. The heat pump system  100  also includes a controller  122  that controls the operation of the components within the heat pump system  100 . In a typical embodiment, the controller  122  comprises a computer that includes components for controlling and monitoring the heat pump system  100 . For example, the controller  122  comprises a CPU  126  and memory  128 . In a typical embodiment, the controller  122  is in communication with a thermostat  123  that allows a user to input a desired temperature for the enclosed space  101 . The controller  122  may be an integrated controller or a distributed controller that directs operation of the heat pump system  100 . In a typical embodiment, the controller  122  includes an interface to receive, for example, thermostat calls, temperature setpoints, blower control signals, environmental conditions, and operating mode status for the heat pump system  100 . For example, in a typical embodiment, the environmental conditions may include indoor temperature and relative humidity of the enclosed space  101  (shown in  FIG. 1 ). 
     The refrigerant flows through the heat pump system  100  in a continuous heating cycle. Starting from the evaporator coil  102 , an outlet  103  of the evaporator coil  102  is coupled to a suction line  106  of the compressor  108  via the reversing valve  104  to feed the refrigerant to the compressor  108 . The compressor  108  compresses the refrigerant. A discharge line  110  feeds compressed refrigerant from the compressor  108  through the reversing valve  104  to the condenser coil  112 . In the heat pump configuration, refrigerant traveling from the condenser coil  112  flows through a first bypass valve  114 , avoiding a first throttling valve  116  that is in the closed position, and is directed to the evaporator coil  102 . Just before the refrigerant enters the evaporator coil  102 , the refrigerant passes through a second throttling valve  120 , avoiding a second bypass valve  118  that is in a closed position. The second throttling valve  120  reduces a pressure of the refrigerant as it enters the evaporator coil  102  and the heating cycle begins again. The behavior of the refrigerant as it flows through the heat pump system  100  is discussed in more detail below. 
     During operation of the heat pump system  100 , low-pressure, low-temperature refrigerant is circulated through the evaporator coil  102 . The refrigerant is initially in a liquid/vapor state. In a typical embodiment, the refrigerant is, for example, R-22, R-134a, R-410A, R-744, or any other suitable type of refrigerant as dictated by design requirements. Ambient air from the environment surrounding the evaporator coil  102 , which is typically warmer than the refrigerant in the evaporator coil, is circulated around the evaporator coil  102  by an exterior fan  130 . In a typical embodiment, the refrigerant begins to boil after absorbing heat from the ambient air and changes state to a low-pressure, low-temperature, super-heated vapor refrigerant. Saturated vapor, saturated liquid, and saturated fluid refer to a thermodynamic state where a liquid and its vapor exist in approximate equilibrium with each other. Super-heated fluid and super-heated vapor refer to a thermodynamic state where a vapor is heated above a saturation temperature of the vapor. Sub-cooled fluid and sub-cooled liquid refers to a thermodynamic state where a liquid is cooled below the saturation temperature of the liquid. 
     The low-pressure, low-temperature, super-heated vapor refrigerant leaving the evaporator coil  102  is fed into the reversing valve  104  that, in the heat pump mode, directs the refrigerant into the compressor  108  via the suction line  106 . In a typical embodiment, the compressor  108  increases the pressure of the low-pressure, low-temperature, super-heated vapor refrigerant and, by operation of the ideal gas law, also increases the temperature of the low-pressure, low-temperature, super-heated vapor refrigerant to form a high-pressure, high-temperature, superheated vapor refrigerant. The high-pressure, high-temperature, superheated vapor refrigerant leaves the compressor  108  via the discharge line  110  and enters the reversing valve  104  that, in the heat pump mode, directs the refrigerant to the condenser coil  112 . 
     Air from the enclosed space  101  is circulated around the condenser coil  112  by an interior fan  132 . The air from the enclosed space  101  is typically cooler than the high-pressure, high-temperature, superheated vapor refrigerant present in the condenser coil  112 . Thus, heat is transferred from the high-pressure, high-temperature, superheated vapor refrigerant to the air from the enclosed space  101 . Removal of heat from the high-pressure, high-temperature, superheated vapor refrigerant causes the high-pressure, high-temperature, superheated vapor refrigerant to condense and change from a vapor state to a high-pressure, high-temperature, sub-cooled liquid state. The high-pressure, high-temperature, sub-cooled liquid refrigerant leaves the condenser coil  112  and passes through the first bypass valve  114 . The first throttling valve  116  is in the closed position while the heat pump system operates as a heat pump. Just before the high-pressure, high-temperature, sub-cooled liquid refrigerant enters the evaporator coil  102 , the high-pressure, high-temperature, sub-cooled liquid refrigerant passes through the second throttling valve  120 . 
     The second throttling valve  120  abruptly reduces the pressure of the high-pressure, high-temperature, sub-cooled liquid refrigerant and regulates an amount of refrigerant that travels to the evaporator coil  102 . Abrupt reduction of the pressure of the high-pressure, high-temperature, sub-cooled liquid refrigerant causes sudden, rapid, evaporation of a portion of the high-pressure, high-temperature, sub-cooled liquid refrigerant, commonly known as “flash evaporation.” The flash evaporation lowers the temperature of the resulting liquid/vapor refrigerant mixture to a temperature lower than a temperature of the ambient air. The liquid/vapor refrigerant mixture leaves the second throttling valve  120  and returns to the evaporator coil  102 , and the cycle begins again. This cycle continues as needed or until the heat pump system  100  determines that a defrost cycle needs to be run to remove frost that has built up on the evaporator coil  102 . 
     As shown in  FIG. 1 , the heat pump system  100  is operating as a heat pump to provide heat to the enclosed space  101 . However, in order to defrost the evaporator coil  102 , the heat pump system  100  is configured to operate in the defrost mode. To initiate the defrost mode, the controller  122  reverses the flow of the refrigerant through the heat pump system  100  to cause the evaporator coil  102  to act as a condenser coil and to cause the condenser coil  112  to act as an evaporator coil. Repurposing the evaporator coil to act as a condenser coil causes the temperature of the evaporator coil  102  to increase, thereby melting any frost that has accumulated on the evaporator coil  102 . To operate the heat pump system  100  in the defrost mode, the controller  122 : 1) switches the reversing valve  104  to the valve configuration illustrated as reversing valve  104   a  to reverse the flow direction of the refrigerant through the heat pump system  100 ; 2) closes the first bypass valve  114  and opens the first throttling valve  116 ; and 3) closes the second throttling valve  120  and opens the second bypass valve  118 . So configured, the heat pump system  100  provides warm refrigerant to the evaporator coil  102  to melt frost from the evaporator coil  102 . However, with the condenser coil  112  operating as an evaporator coil, the air blown over the condenser coil  112  by the interior fan  132  is cooled by the condenser coil  112 , which now has cold refrigerant passing therethrough. To counter this cooling effect, a heating element  133  is activated to warm the air. In a typical embodiment, the heating element  133  is a resistive heating element. In other embodiments, the heating element  133  may comprise other devices that permit air passing around the heating element  133  to be warmed. 
     In a typical embodiment, the controller  122  is configured to communicate with the components of the heat pump system  100  to monitor and control the components of the heat pump system  100 . Communication between the controller  122  and the components of the heat pump system  100  may be via a wired or a wireless connection. In a typical embodiment, the controller  122  is configured to control operation of one or more of the reversing valve  104 , the compressor  108 , the first bypass valve  114 , the first throttling valve  116 , the second bypass valve  118 , the second throttling valve  120 , the exterior fan  130 , the interior fan  132 , and the heating element  133 . The heating element  133  is used during the defrost cycle to heat air from the enclosed space  101  that is blown over the condenser coil  112  by the interior fan  132 . The controller  122  controls whether the reversing valve  104  is in the heat pump mode or the defrost mode. The controller  122  also controls whether or not the compressor  108  is operating. In some embodiments, the compressor  108  may be a variable or multispeed compressor. In such embodiments, the controller  122  controls the speed at which the compressor  108  operates. The controller  122  also controls whether the first bypass valve  114 , the first throttling valve  116 , the second bypass valve  118 , the second throttling valve  120 , are in the open or closed position. The controller  122  also controls the whether the exterior fan  130  and the interior fan  132  are operating. In some embodiments, one or both of the exterior fan  130  and the interior fan  132  may be variable or multispeed fans. In such embodiments, the controller  122  controls the speed at which the exterior fan  130  and the interior fan  132  operate. 
     In a typical embodiment, the controller  122  can communicate with an external data source  150  via an antenna  124 . In some embodiments, the controller  122  may use the antenna  124  to communicate with a router  154 . The router  154  may be, for example, an internet access point that is connected to the Internet. The external data source  150  provides data regarding local environmental conditions to the controller  122  and may be, for example, an internet weather-data service. In a typical embodiment, the data from the external data source  150  may include: temperature, humidity, dew point temperature, forecast information, and the like. Forecast information can include predictions about future temperature, humidity, dew point temperature, and the like. In some embodiments, the controller  122  can monitor the temperature of the evaporator coil  102  and humidity data from a first sensor  160  that positioned proximal to the evaporator coil  102 . In some embodiments, additional environmental data may be measured with a second sensor  162  positioned proximal to the evaporator coil  102 . In some embodiments, the first sensor  160  and the second sensor  162  may include multiple sensors to monitor multiple aspects of the environmental conditions, such as, for example, humidity and temperature of an area in proximity to the evaporator coil  102 . 
     In some embodiments, the controller  122  calculates the dew point temperature using temperature and relative humidity data provided by the external data source  150 . The controller  122  may use some or all of the data from the external data source  150  to determine if a defrost cycle should be initiated. Use of data from the external data source  150  to initiate a defrost cycle will be discussed in more detail below. In some embodiments, the controller  122  calculates the dew point temperature using temperature and relative humidity data provided by at least one of the first sensor  160  and the second sensor  162 . 
     In some embodiments, the controller  122  may rely upon, in part or in whole, on data obtained from one or more components of the heat pump system  100  to determine if a defrost cycle should be initiated. For example, the controller  122  may monitor the power consumption of the exterior fan  130 . During normal operation, the controller  122  controls the exterior fan  130  to maintain a certain revolutions per minute (RPM) so that a certain cubic feet per minute (CFM) of air flows around the evaporator coil  102 . In order to maintain that RPM, the exterior fan  130  consumes a certain amount of power. During operation of the heat pump system  100 , the controller  122  can monitor either or both of the RPM and the power consumed by the exterior fan  130 . When frost forms on the evaporator coil  102 , flow of air around the evaporator coil  102  is inhibited. The reduction of air flow around the evaporator coil  102  causes the RPM of the exterior fan  130  to drop. In order to maintain the desired RPM, additional power is provided to the exterior fan  130 . In response to the RPMs of the exterior fan  130  crossing an RPM threshold or the power consumption of the exterior fan  130  increasing beyond a power threshold, the controller  122  may initiate a defrost cycle. After the defrost cycle has been run, the controller  122  can confirm that the defrost cycle was successful in removing frost from the evaporator coil  102  by checking to see if the RPM or power consumption of the exterior fan  130  no longer exceeds the threshold value. 
     In some embodiments, the controller  122  can monitor a speed of the compressor  108  to determine the speed of the exterior fan  130 . During operation of the heat pump system  100 , the speed of the exterior fan  130  is related to the speed of the compressor  108 . As frost begins to form on the evaporator coil  102 , the ability for the heat pump system  100  to provide heat to the enclosed space  101  decreases. To combat the loss in heating performance, a speed of the compressor  108  is typically increased to provide additional heating capacity. As a result of increasing the compressor speed, the speed of the exterior fan  130  is also increased. Thus, the controller  122  can initiate a defrost cycle in response to a speed of the compressor  108  exceeding a threshold value. 
     Referring now to  FIG. 2 , a graph demonstrating a frost map  200  is shown. For illustrative purposes, the  FIG. 2  will be described relative to the heat pump system  100  of  FIG. 1 . The frost map plots the temperature of the evaporator coil  102  versus dew point temperature. The term frost potential refers to the difference between dew-point temperature and the temperature of the evaporator coil  102 . When the temperature of the evaporator coil  102  is greater (i.e., warmer) than the dew-point temperature or above freezing, the frost potential is negative. In other words, no frost can accumulate on the evaporator coil  102 . Therefore, no defrost cycle is needed. In contrast, when the temperature of the evaporator coil  102  is less than (i.e., colder) than the dew-point temperature and is also at or below freezing, the frost potential is positive. In other words, frost collection on the evaporator coil  102  is possible. Therefore, a defrost cycle may be necessary. 
     A freeze line  202  identifies the freezing point of water for a given environment. For the purposes of  FIG. 2 , it is assumed that the freezing point of water is 32° F. It will be appreciated by a person of ordinary skill in the art that the freezing point of water may vary slightly based on environmental conditions, such as, for example, altitude. A dew point line  204  identifies conditions for which the formation of frost may occur. As illustrated in  FIG. 2 , the temperature of the evaporator coil  102  must be at or below freezing and below the dew-point temperature in order for frost to collect on the evaporator coil  102 . If the temperature of the evaporator coil  102  is greater than freezing or at or above the dew-point temperature, frost cannot form on the evaporator coil  102 . 
     The freeze line  202  and the dew point line  204  intersect and divide the frost map  200  into Regions I-IV. In Region I, the temperature of the evaporator coil  102  is above the freeze line  202  and above the dew point line  204 . Thus, no condensation will form on the evaporator coil  102  and a defrost cycle does not need to be run. In Region H, the temperature of the evaporator coil  102  is above freeze line  202  and below the dew point line  204 . Formation of condensation on the evaporator coil  102  will occur in Region II. However, because the temperature of the evaporator coil  102  is above the freeze line  202 , no frost will form on the evaporator coil  102  and a defrost cycle does not need to be run. In Region III, the temperature of the evaporator coil  102  is below the freeze line  202  and below the dew point line  204 . In Region III, frost can begin to form on the evaporator coil  102 . When the heat pump operates in Region III, a defrost cycle will need to be run periodically to insure that too much frost does not build up on the evaporator coil  102 . In Region IV, the temperature of the evaporator coil  102  is below the freeze line  202  and above the dew point line  204 . No condensation or frost will form on the evaporator coil  102  while the heat pump operates in Region IV, thus a defrost cycle does not need to be run when the heat pump is operating within Region IV. 
     Referring now to  FIG. 3 , a flow diagram of an illustrative process  300  for defrost control for a heat pump is shown. For illustrative purposes, the process  300  will be described relative to the heat pump system  100  of  FIG. 1 . A person having skill in the art will recognize that the process  300  may be utilized by other systems for which a defrost cycle is used. The process  300  can be carried out by, for example, the controller  122 . The process  300  begins at a step  302 . At step  302 , the heat pump system  100  begins to operate and a heating timer is initiated. The heating timer tracks the amount of time the heat pump system  100  has been in operation. After the heat pump system  100  has begun operating, the process  300  proceeds to step  304 . 
     At step  304 , the controller  122  determines whether a temperature of the evaporator coil  102  is below the freeze temperature. The temperature of the evaporator coil  102  may be obtained via the first sensor  160  or may be determined by measuring the temperature of the refrigerant passing through the evaporator coil  102 . If it is determined at step  304  that the temperature of the evaporator coil  102  is above the freeze temperature, the process  300  proceeds to step  306 . However, if it is determined at step  304  that the temperature of the evaporator coil  102  is below the freeze temperature, the process  300  proceeds to step  312 . 
     At step  306 , a no-frost timer is started and the process  300  proceeds to step  308 . At step  308 , the controller  122  determines if a heating demand for the enclosed space  101  has been met. If it is determined at step  308  that the heating demand has been met, the process  300  proceeds to step  310 , where the heat pump system  100  ceases operation and the process  300  ends. However, if it is determined at step  308  that the heating demand has not been met, the process  300  returns to step  304 . 
     At step  312 , the controller  122  determines whether the temperature of the evaporator coil  102  is greater than the current dew point temperature. In a typical embodiment, information regarding the current dew point temperature is received from the external data source  150 . In some embodiments, the current dew point temperature is calculated using information from the external data source  150  or the first sensor  160  and the second sensor  162 . If it is determined that the current dew point temperature is less than the temperature of the evaporator coil  102 , no frost can form on the evaporator coil  102  and the process  300  proceeds to step  306 . However, if it is determined that the current dew point temperature is greater than the temperature of the evaporator coil  102 , frost can form on the evaporator coil  102  and the process  300  proceeds to step  314 . 
     At step  314 , a frost timer is started and the controller  122  calculates several values before proceeding on to step  316 . In a typical embodiment, the controller  122  calculates the following values: 1) a mass flow rate of air that is being blown over the evaporator coil  102  by the exterior fan  130 ; 2) an amount of moisture in the air at a present exterior temperature; 3) an amount of moisture in the air at an apparatus dew point temperature of the evaporator coil  102 ; and 4) a frost-collection rate. The mass flow rate of air can be determined based upon a speed at which the exterior fan  130  is blowing. The speed of the exterior fan  130  can be determined using a sensor associated with the exterior fan  130  or can be determined based upon the speed of the compressor  108  as discussed above. Knowing the speed of the exterior fan  130  allows the CFM of air that the exterior fan  130  moves over the evaporator coil  102  to be calculated. In a typical embodiment, the CFM of the exterior fan  130  is a performance property of the exterior fan  130  that is known. The mass of the air being blown over the evaporator coil  102  can then be calculated by multiplying the CFM by the density of air. The density of air is determined based upon the present exterior air conditions. In particular, the density of air is a function of the ambient temperature, the relative humidity, and the altitude. 
     In a typical embodiment, the amount of moisture in the air at the present exterior temperature is a constant for a particular exterior temperature and relative humidity. In a typical embodiment, a table of values of the grains of moisture per pound of air based on various outdoor temperatures and relative humidities can be stored in the memory  128  of the controller  122 . In a typical embodiment, the controller  122  obtains the present exterior temperature and relative humidity from the external data source  150 . In some embodiments, the controller  122  obtains the present exterior temperature from the first sensor  160 . Once the controller  122  has obtained the present exterior temperature and the relative humidity, the controller  122  may reference the table of values of grains of moisture per pound of air to determine the amount of moisture in the air at the present conditions. 
     In a typical embodiment, an amount of moisture in the air at an apparatus dew point temperature of the evaporator coil  102  is a constant for a particular temperature. In a typical embodiment, the table of values of grains of moisture per pound of air at various temperatures and relative humidities can be referenced by the controller  122  to determine the amount of moisture in the air at the present apparatus dew point temperature of the evaporator coil  102 . In a typical embodiment, the controller  122  obtains the present apparatus dew point temperature of the evaporator coil  102  from the second sensor  162  and the relative humidity from the external data source  150 . In some embodiments, the controller  122  may obtain the present apparatus dew point temperature of the evaporator coil  102  by measuring a temperature of refrigerant within the evaporator coil  102 . Once the controller  122  has obtained the present exterior temperature and the relative humidity, the controller  122  may reference the table of values of grains of moisture per pound of air to determine the amount of moisture in the air at the present apparatus dew point temperature of the evaporator coil  102 . 
     For the purposes of calculating the amount of moisture in the air at the present apparatus dew point temperature it is assumed that the air flowing over the evaporator coil  102  is cooled to a temperature equal to the temperature of the evaporator coil  102 . As the air flowing over the evaporator coil  102  is cooled, an ability of the air flowing over the evaporator coil  102  to retain moisture is reduced. As a result of this reduction, moisture settles out of the air and onto the evaporator coil  102 . 
     In a typical embodiment, the frost-collection rate describes a theoretical maximum rate at which frost can begin to form on the evaporator coil  102  given the current environmental conditions in which the heat pump system  100  is operating. The frost-collection rate is calculated by subtracting the amount of moisture in the air at an apparatus dew point temperature from the amount of moisture in the air at the present exterior temperature and multiplying the result by the mass flow rate of air. In some embodiments, the controller  122  adjusts the frost-collection rate with a correction factor. It is acknowledged that the air flowing over the evaporator coil  102  is not cooled to the same temperature as the evaporator coil  102  due to various inefficiencies relating to a transfer of heat between the evaporator coil  102  and the air flowing over the evaporator coil  102 . In order to account for this difference, a correction factor may be used to more closely reflect an actual amount of moisture that settles on the evaporator coil  102 . For example, the calculated frost-collection rate may be multiplied by the correction factor to more accurately reflect an actual amount of moisture that settles on the evaporator coil  102 . After the calculations of step  314  have been determined, the process  300  proceeds to step  316 . 
     At step  316 , the controller  122  determines if the frost-collection rate is greater than a frost-collection-rate threshold. The frost-collection-rate threshold is a value that can be set as desired. Higher values for the frost-collection-rate threshold allow the heat pump system  100  to continue to operate for longer periods of time before a defrost cycle is initiated. However, as frost that accumulates on the evaporator coil  102 , an ability of the heat pump system  100  to heat the enclosed space  101  decreases. Lower values for the frost-collection-rate threshold helps prevent large amounts of frost from forming on the evaporator coil  102  because defrost cycles will occur more often. However, running defrost cycles more often requires that the heating element  133  be used more often, which negates efficiencies and cost savings regarding the providing of heat to the enclosed space  101  compared to heating the enclosed space  101  in the heat pump mode. If it is determined at step  316  that the frost-collection rate is greater than the frost-collection-rate threshold, the process  300  proceeds to step  320 . However, if it is determined that the frost-collection rate is less than the frost-collection-rate threshold, the process  300  proceeds to step  318 . 
     At step  318 , the controller  122  calculates the weight of frost that has formed on the evaporator coil  102  and compares the weight of that the frost that has formed to a frost-weight threshold. The frost-weight threshold is a value that can be set as desired. Higher values for the frost-weight threshold allow the heat pump system  100  to continue to operate for longer periods of time before a defrost cycle is initiated. However, as frost that accumulates on the evaporator coil  102 , an ability of the heat pump system  100  to heat the enclosed space  101  decreases. Lower values for the frost-weight threshold helps prevent large amounts of frost from forming on the evaporator coil  102  because defrost cycles will occur more often. However, running defrost cycles more often requires that the heating element  133  be used more often, which negates efficiencies and cost savings regarding the providing of heat to the enclosed space  101  compared to heating the enclosed space  101  in the heat pump mode. If it is determined at step  318  that the frost weight is greater than the frost-weight threshold, the process  300  proceeds to step  320 . However, if it is determined that the frost weight is less than the frost-weight threshold, the process  300  returns to step  316 . 
     At step  320 , the controller  122  initiates a defrost cycle. As discussed above, in order to defrost the evaporator coil  102 , the controller  122 : changes the reversing valve  104  to the configuration of reversing valve  104   a ; closes the first bypass valve  114 ; opens the first throttling valve  116 ; opens the second bypass valve  118 ; closes the second throttling valve  120 ; and activates the heating element  133 . After the defrost cycle has begun, the process  300  proceeds to step  322 . 
     At step  322 , the controller  122  determines if the temperature of the evaporator coil  102  is greater than a thawing-temperature threshold. The thawing-temperature threshold is a value that can be set as desired. In a typical embodiment, the thawing-temperature threshold is set at value well above the freeze temperature. For example, the thawing-temperature threshold may be set at 60° F. In other embodiments, the thawing-temperature threshold may be set to other temperatures as desired. In general, higher thawing-temperature threshold values cause the defrost cycle to run for longer periods of time and lower thawing-temperature threshold values cause the defrost cycle to run for shorter periods of time. If it is determined at step  322  that the temperature of the evaporator coil  102  is less than the thawing-temperature threshold, the process  300  returns to step  320 . However, if it is determined at step  322  that the temperature of the evaporator coil  102  is greater than the thawing-temperature threshold, the process  300  proceeds to step  324 . At step  324 , the defrost cycle ends. After step  324 , the process  300  returns to step  302 . 
     The process  300  described above may be modified to satisfy various design parameters. For example, steps may be removed, added, or changed. In some embodiments, the process  300  may evaluate weather-forecast data. For example, the controller  122  may receive weather-forecast data from the external data source  150  that informs the controller  122  about future weather conditions. Information regarding future weather conditions may be relevant to the decision regarding whether a defrost cycle should be initiated. For example, once the process  300  reaches step  314 , the controller  122  could include an additional step that is carried out before the step  314  that considers the weather-forecast data. If the weather-forecast data includes a forecast that the ambient temperature will rise above freezing in the near future, the controller  122  can decide not to initiate the defrost cycle and instead proceed to back to step  302 . Initiating a defrost cycle when the forecast indicates that the ambient temperature will be above freezing in the near future is unnecessary because frost that has formed on the evaporator coil  102  will begin to melt due to ambient temperature being above freezing. 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As will be recognized, the processes described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. The scope of protection is defined by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.