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Dehumidifying Heat Pipes and New Designs for Heat Exchangers: Dehumidifying heat pipes improve the ability of heat pumps and air-conditioners to remove excess moisture from the air in humid climates. They were invented for the US space program to dehumidify the cabin air inside a space vehicle, and to save electrical energy.

Dehumidifying heat pipes are a system of sealed tubes with liquid refrigerant inside. They transfer heat from one area to another without using any electrical energy. When hot, humid air passes through one end of the pipes, the pipes heat up, evaporating the refrigerant inside. This absorbs some of the air's heat. The air stream then flows through the heat pump's (or air-conditioner's) evaporator coil, where it is further dehumidified and cooled. The vaporized refrigerant moves to the other side of the heat pipes, where the cooler air passing through it condenses it, giving off the heat it had absorbed earlier, and raising the temperature of the air stream to the required level. Grooves on the interior of the pipes act as a wick to return the liquid to the opposite end of the pipes. Once there, it repeats the evaporation cycle.

Most models of heat pumps and air-conditioners can be retrofitted with dehumidifying heat pipes. They can also be built into new heat pump or air-conditioner heat exchanger coils. Tests have found that these devices increase the dehumidifying capacity of a cooling system by up to 91%, while slightly increasing electricity consumption, because the deeper coils create slightly greater air resistance for the blower fan. Heat pipes save electricity in another way as well. They allow the cooling system to be reduced in size.

There are also new types of heat exchanger coils for air-conditioners and heat pumps. One is the PF (plate-fin) coil. Test indicated that they improve the performance of a heat pump by a significant percentage. Another type of heat exchanger is the O-coil. This design permits a better airflow across the coils, and uses fewer welds, which minimizes the chances of refrigerant leaks. The O-coil also takes up less space, shrinking the unit's physical size or "footprint."

Gas-Fired Heat Pumps: This type of heat pump uses natural gas to run a small engine, which in turn drives the heat pump's compressor. To date, only the one company has the license to manufacturer this type of heat pump. Overall efficiencies are said to be in the 126% range. This is just slightly lower than the efficiency of an electric air-source heat pump. This technology also recovers much of the waste heat from the engine to either heat the house or to make domestic hot water.

The main advantage of this system is not so much energy savings as it is economics. In some parts of the nation natural gas costs less per Btu of heat than electricity does. However, this does not include the annual "tune-up" that's necessary to keep the system operating correctly.

Ductless Heat Pumps: Ductless heat pumps (also known as mini-split systems) function the same as any other heat pump, except that they do not require distribution ducts, where some heating or cooling energy is lost. Such systems offer energy savings of as much as 50% over a ducted air distribution system. Ductless systems consist of an outside unit containing the usual heat pump components, and an indoor unit that can be located on any nearby wall. The usual heat pump (or air-conditioner) refrigerant tubing connects the two sections. These are ideal for smaller houses, remote rooms in large houses, or houses that have an open floor plan. These systems can also have several indoor units connected to one outdoor unit. This gives the occupants of the house the ability to "zone" or control heating and cooling to different parts of the house.

Heat Pumps with Gas Back-up Heat (Dual Fuel): Even though combustion furnaces that use more than one type of fuel are not new, they are less common than dual fuel with heat pumps. This is due to a lack of awareness of these appliances, and the small number of manufacturers producing them. In theory, any heat pump can be equipped with a gas (LP or natural gas) burner that supplements the heat pump when the outside temperature falls below about 35° F (1.6° C). Gas back-up heat helps solve the problem of the heat pump delivering relatively cool air during cold weather. It also lowers the electrical consumption of the equipment, because the electric resistance coils (the major source of high electric bills in cold weather) are not used.

Since there are few heat pump manufacturers that incorporate both types of heat supply in one box, these configurations are often two smaller, side-by-side, standard systems sharing the same ductwork. The combustion fuel half of the system could be propane, natural gas, or oil, or even coal and wood.

In comparison with a combustion fuel-fired furnace or standard heat pump alone, this type of system is also economical. Actual energy savings depend on the relative costs of the combustion fuel relative to electricity.

Geothermal Heat Pumps: Geothermal heat pumps (sometimes referred to as earth-coupled, ground-source, or water-source heat pumps) have been in use since the late 1940s. Geothermal heat pumps use the constant temperature of the earth as the exchange medium instead of the outside air temperature. This allows the system to reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days.

As with any heat pump, geothermal and water-source heat pumps are able to heat, cool, and, if so equipped, supply the house with hot water. Some models of geothermal systems are available with two-speed compressors and variable fans for more comfort and energy savings. They are quieter, last longer, need little maintenance and do not depend on the temperature of the outside air relative air-source heat pumps.

At least one manufacturer has combined an air-source heat pump with a geothermal heat pump. This is called a "Dual-Source" heat pump. These appliances combine the best of both systems. Dual-source heat pumps have higher efficiency ratings than air-source units, but are not as efficient as geothermal units. The main advantage of dual-source systems is that they cost much less to install than a single geothermal unit, and work almost as well.

Even though the installation price of a geothermal system can be several times that of an air-source system of the same heating and cooling capacity, the additional costs are returned to you in energy savings in 5 to 10 years. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. There are approximately 40,000 geothermal and water source heat pumps installed in the United States each year.

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