Heating, ventilation, and air conditioning systems (HVAC systems) are used in residential and/or commercial areas for heating and/or cooling to create comfortable temperatures inside those areas. These temperature controlled areas may be referred to as comfort zones. Comfort zones may comprise different zone conditions (i.e., temperature, humidity, etc.) and the locations in which the HVAC systems are installed or otherwise associated with for the purpose of performing heat exchange (sometimes referred to as an ambient zone) may also have different conditions. Both the zone conditions and the conditions of the location affect operation of the HVAC systems and, where the conditions are different, may result in otherwise substantially similar HVAC systems operating at different efficiencies. Some HVAC systems are heat pump systems. Heat pump systems are generally capable of cooling a comfort zone by operating in a cooling mode for transferring heat from a comfort zone to an ambient zone using a refrigeration cycle (i.e., Rankine cycle). When the temperature of an ambient zone in which a portion of an HVAC system is installed or otherwise associated with is colder than the temperature of a comfort zone with which the HVAC system is associated, the heat pump systems are also generally capable of reversing the direction of refrigerant flow (i.e., a reverse-Rankine cycle) through the components of the HVAC system so that heat is transferred from the ambient zone to the comfort zone (a heating mode), thereby heating the comfort zone.
One example of rating the heating energy efficiency of an HVAC system is the use of the Heating Season Performance Factor (HSPF) rating. To obtain a HSPF rating, the HVAC system is tested under prescribed conditions (i.e., certification conditions) to determine the efficiency at which it generates an energy output based on an energy input. The prescribed conditions generally involve very strict control over the zone conditions and the ambient conditions of the location of the installation of the HVAC system being tested. A higher HSPF rating is indicative of a more energy efficient HVAC system. The higher HSPF rating indicates that the HVAC system may be operated at a lower energy cost than an HVAC system having a lower HSPF rating.
In some cases where moisture is present in the cold ambient zone, the moisture condenses on the HVAC system (e.g., the components of the HVAC system). Accordingly, when the ambient temperature is below a freezing point, frost and/or ice may accumulate on the HVAC system. This accumulation of frost and/or ice is detrimental to the ability of the HVAC system to perform at its optimum energy efficiency. In order for the HVAC system to perform efficiently, the frost and/or ice on the HVAC system should be removed (e.g., defrosted). Accordingly, the HVAC systems that provide refrigerant-based heating are often configured to perform a defrost function whereby the components of the HVAC system that are at least partially covered in frost and/or ice are heated to melt the frost and/or ice performing the defrosting is achieved by reversing the direction of refrigerant flow from the direction of flow used in the heating mode. Specifically, the refrigerant flow is such that heat is transferred from the comfort zone to the ambient zone during the defrosting of the HVAC system components. The heat pump systems generally use a reversing valve for rerouting the direction of refrigerant flow between the compressor and the heat exchangers associated with the comfort zone and the ambient zone. This act of defrosting consumes energy that could be used to heat the comfort zone, and therefore, the benefit of defrosting must be carefully weighed against the alternative of simply allowing the HVAC system to operate at a less energy efficient state with the frost and/or ice buildup intact.