The present invention relates to an improved heat pump dehumidification system, consisting of at least one of an air source heat pump, a water source heat pump, and a direct expansion heat pump, incorporating a unique combination of an additional interior heat exchange means (typically an air handler) for use when the system has satisfied the thermostat setting in the cooling mode of operation, but when humidity levels remain excessively high, as well as an optimum design sizing for a heat pump system's interior air heat exchange means while operating in one of a dehumidification mode, a cooling mode, and a heating mode. 
Air source heat pump systems typically circulate a refrigerant, such as R-22 or the like, as a means to extract heat from the exterior air in the heating mode of operation, and as a means to reject heat into the exterior air in the cooling mode of operation.  An electric fan typically enhances the circulation of air over a first array of exterior finned heat exchange tubing exposed to the exterior air.  Having gained heat from, or rejected heat into, the exterior air, the heated or cooled refrigerant is then circulated, by means of a refrigerant compressor, through the refrigerant transport tubing into a second array of finned heat exchange tubing, with airflow augmented by means an electric fan, located within the interior space, with a second heat exchange step comprising a transfer of heat to or from the refrigerant to heat or cool interior air space, depending on the direction of the flow of refrigerant in the heating mode or in the cooling mode.  The operation of an air source heat pump is well understood by those skilled in the art. 
Rather than using exterior air as an exterior heat exchange means, ground source/water source heat pump systems typically utilize fluid-filled closed loops of tubing buried in the ground, or submerged in a body of water, so as to either absorb heat from, or to reject heat into, the naturally occurring geothermal mass and/or water surrounding the buried or submerged tubing.  Water-source heating/cooling systems typically circulate, via a water pump, water, or water with anti-freeze, in plastic underground geothermal tubing so as to transfer heat to or from the ground, with a second heat exchange step utilizing a refrigerant, such as R-22 or the like, to transfer heat to or from the water, and with a third heat exchange step utilizing an array of interior finned refrigerant transport tubing, with airflow augmented by an electric fan, to transfer heat to or from the refrigerant to heat or cool interior air space.  The operation of a water source heat pump is well understood by those skilled in the art. 
Direct eXpansion (herein referred to as “DX”) ground source systems, where the refrigerant transport lines are placed directly in the sub-surface ground and/or water, typically circulate a refrigerant fluid, such as R-22, in sub-surface refrigerant lines, typically comprised of copper tubing, to transfer heat to or from the sub-surface elements, and only require a second heat exchange step to transfer heat to or from the interior air space by means of the interior air's exposure to an array of finned heat transfer tubing, with the interior's air flow augmented by an electric fan. 
Consequently, DX systems are generally more efficient than water-source systems because of less heat exchange steps and because no water pump energy expenditure is required.  Further, since copper is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the copper tubing of a DX system generally has a greater temperature differential with the surrounding ground than the water circulating within the plastic tubing of a water-source system, generally, less excavation and drilling is required, and installation costs are generally lower with a DX system than with a water-source system. 
Also, since DX systems do not require a defrost cycle in the heating mode, and since the sub-surface geothermal heat exchange temperatures are far more stable than those of widely fluctuating exterior air in ever-changing atmospheric conditions, DX systems are generally more efficient than air source systems. 
While most in-ground/in-water heat exchange designs are feasible, various improvements have been developed intended to enhance overall system operational efficiencies in DX heat pumps.  Various such design improvements are taught in U.S. Pat.  No.  5,623,986 to Wiggs; in U.S. Pat.  No.  5,816,314 to Wiggs, et al.;  in U.S. Pat.  No.  5,946,928 to Wiggs; in U.S. Pat.  No.  6,615,601 B1 to Wiggs; in Wiggs' U.S. patent application Ser.  No.  10/073,513; in Wiggs' U.S. patent application Ser.  No.  10/127,517; in Wiggs' U.S. patent application Ser.  No.  10/251,190; in Wiggs' U.S. patent application Ser.  No.  10/335,514; in Wiggs' U.S. patent application Ser.  No.  10/616,701, in Wiggs' U.S. patent application Ser.  No.  10/757,265, and in Wiggs's U.S. patent application Ser.  No.  10/804,698, the disclosures of which are incorporated herein by reference. 
Since DX heat pump applications are generally the most efficient types of heat pumps, and since the interior air heat exchange means is basically the same for all heat pump systems, only the best heat pump design will be demonstrated herein, although the subject invention can be utilized in an identical manner for all heat pump systems, whether air source, water source or DX. 
Virtually all heat pump systems utilize a compressor, an interior heat exchange means, an exterior heat exchange means, thermal expansion devices, an accumulator, a refrigerant fluid (such as R-22, R-410A, or the like), and operatively connected refrigerant transport tubing, as is well understood by those skilled in the art.  Also, most all heat pump systems utilize an interior air handler, comprised of an array of finned refrigerant transport tubing with airflow augment by an electric fan, as the interior heat exchange means, as is well understood by those skilled in the art.  However, occasionally, the interior heat exchange means may be comprised of a refrigerant to water heat exchange means, with the water circulated within the interior space, which is commonly referred to as a hydronic type interior heat exchange means, as is well understood by those skilled in the art. 
As explained, virtually all heat pump systems utilize an array of finned refrigerant transport tubing, with the interior airflow passing over same augmented by means of an electric fan, which is commonly referred to as an air handler.  As the interior air passes over the finned tubing, the air absorbs heat from the hot refrigerant in the heating mode, and rejects heat into the cold refrigerant in the cooling mode.  The air handler may be comprised of one or multiple sets of arrays of finned refrigerant transport tubing, and the air handler's electric fan may be designed to operate at one or at multiple speeds.  All of this is well understood by those skilled in the trade. 
While virtually all heat pump systems operate in a reverse-cycle mode, and may be switched from a heating mode to a cooling mode, and vice versa, by simply changing the setting on the system's thermostat, as is well understood by those skilled in the art, the present invention is principally concerning the operation of heat pump systems in the cooling mode. 
When operating in the cooling mode, cold refrigerant is circulated through the interior air handler, with the warmer interior air being blown over the cold finned refrigerant transport tubing by means of an electric fan.  Heat is absorbed by the cold refrigerant, as heat always travels to cold, and is thereby removed from the interior air.  The removal of heat is commonly referred to as the sensible load work performed by the system.  However, in the cooling mode of operation, a second consequence of the heat pump's operation occurs.  Namely, naturally occurring moisture in the interior air is also removed.  This removal of moisture, which is commonly referred to as the latent load work performed by the system, results because the cold refrigerant is below the dewpoint.  Consequently, when operating in the cooling mode, all heat pump systems' interior heat exchange means are equipped with a moisture condensate drain to remove the condensed moisture from the interior space.  Typically, the condensate drain consists of a PVC tube, or the like, which typically simply carries the water to the exterior of the house via gravity.  If the condensate water must travel uphill to be removed, such as from a basement area, a small condensate water pump, electrically operated, is situated at a low point within the condensate drain line to pump the water out of the structure.  All of the above is well understood by those skilled in the art, and consequently, the condensate drain is not shown herein. 
In many areas, excessive moisture can create health concerns, such as fostering molds and dust mites, as well as decreasing comfort levels.  While heat pump systems do remove moisture from the interior air when operating in the cooling mode, as explained above, heat pump systems do not remove moisture from the air when the thermostat is satisfied and the system is inoperative, as heat pump systems are virtually always solely designed to provide thermostatic sensible load comfort levels without regard to interior latent load humidity levels.  The removal of excessive interior humidity has simply been an historical advantageous by-product of the heat pump system while it is operating in the cooling mode. 
In areas where high humidity levels can cause discomfort and/or associated health concerns from mold accumulation and the like, the removal of high levels of humidity is desirous.  It is reported that data released by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (“ASHRAE”) suggests that maintaining relative humidity levels between 30% and 60% limits the harmful effects of many unwanted bacteria, viruses, fungi, mites, allergic rhinitis and asthma, and other respiratory related conditions.  It is reported that The Journal of Allergy and Clionical Immunology has recommended that interior relative humidity levels be maintained below 51% to inhibit dust mites and to improve healthfulness. 
While all refrigerant based heat pump cooling equipment generally removes humidity, the cooler the refrigerant the more humidity that is removed.  However, historically, as explained, humidity is only removed when the heat pump's cooling system is operating.  When the system's thermostat setting is reached, typically at about 70 degrees F., the cooling system typically shuts off until the interior air warms enough to register at the thermostat and to thereby re-engage the cooling system.  When the cooling system is shut off, the system's compressor and interior air handler both are typically shut off, thereby stopping both the sensible cooling of the interior air and the associated latent load removal of interior air humidity, as there is no air flow over the cold refrigerant within the finned heat exchange tubing of the interior air handler. 
In order to continuously remove humidity with conventionally designed systems, one must continuously operate the system in the cooling mode.  Such continuous operation typically results in excessive cooling, to the point of being uncomfortably cold.  While one could continuously operate a small cooling system in an effort to continuously remove humidity, and engage a larger cooling system only when the small unit could not remove the interior heat load, during cooler nighttime periods even the small cooling system could still make the interior space uncomfortably cold.  Further, smaller systems may not have the ability to remove large amounts of humidity present when the primary larger cooling system is shut off. 
Historically, excessive humidity levels are addressed by the utilization of a dehumidifier, which dehumidifier is a system totally separate and independent of a heat pump system, as is well understood by those skilled in the trade.  However, traditional dehumidifiers are not particularly efficient to operate, require additional space, do not have the typically higher design load capacities of heat pump systems, and often require the owner to manually dispose of trays of accumulated water. 
Since excessively high humidity levels can be both uncomfortable as well as a health concern, particularly with the requisite introduction of certain quantities of fresh air into schoolrooms and the like, a means to utilize existing heat pump systems to expressly remove excessive humidity, exclusive of the thermostat level, and without the need to operate a totally separate and independent dehumidification system is desirable. 
Thus, it is an objective of this subject invention to disclose a means to utilize existing heat pump systems for dehumidification purposes, exclusive of the need to obtain and operate a separate and independent dehumidification system, and exclusive of the need to solely rely upon heat pump system operation in the cooling mode controlled by a thermostat.