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Introduction

Heat naturally flows "downhill", from higher to lower temperatures. A heat pump is a machine which causes the heat to flow in a direction opposite of its natural tendency or "uphill" in terms of temperature. Because work must be done (energy consumed) to accomplish this, the name heat "pump" is used to describe the device.

In reality, a heat pump is nothing more than a refrigeration unit. Any refrigeration device (window air conditioner, refrigerator, freezer, etc.) moves heat from a space (to keep it cool) and discharges that heat at higher temperatures. The only difference between a heat pump and a refrigeration unit is the desired effect--cooling for the refrigeration unit and heating for the heat pump. A second distinguishing factor of many heat pumps is that they are reversible and can provide either heating or cooling to the space.

One of the most important characteristics of heat pumps, particularly in the context of home heating/cooling, is that the efficiency of the unit and the energy required to operate it are directly related to the temperatures between which is operates. In heat pump terminology, the difference between the temperature where the heat is absorbed (the "source") and the temperature where the heat is delivered (the "sink") is called the "lift." The larger the lift, the greater the power input required by the heat pump. This is important because it forms the basis for the efficiency advantage of the geothermal heat pumps over air-source heat pumps. An air-source heat pump, must remove heat from cold outside air in the winter and deliver heat to hot outside air in the summer. In contrast, the GHP retrieves heat from relatively warm soil (or groundwater) in the winter and delivers heat to the same relatively cool soil (or groundwater) in the summer.

As a result, geothermal heat pump, regardless of the season is always pumping the heat over a shorter temperature distance than the air-source heat pump. This leads to higher efficiency and lower energy use.

Vapor Compression Cycle

All heat pumps use a vapor compression cycle to transport heat from one location to another. In heating mode, the cycle starts as the cold liquid refrigerant within the heat pump passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (fluid circulated through the earth connection). The refrigerant evaporates into a gas as heat is absorbed. The gaseous refrigerant then passes through a compressor where it is pressurized, raising its temperature to over 180 degrees F. The hot gas then circulates through a refrigerant-to-air heat exchanger where the heat is removed and sent through the air ducts. When the refrigerant loses the heat, it changes back to a liquid. The liquid refrigerant cools as it passes through an expansion valve, and the process begins again.

Although heat pumps are complex internally, they are marvels of compact design for reliability. Some include features such as additional heat exchangers for water heating, and microprocessor-based automatic controls and protection devices.

Click here for an illustrative diagram.

Efficiency

GeoSource heat pumps are much more efficient than air source heat pumps because earth temperatures are much more uniform through the year than air temperatures. Not only are earth temperatures more constant, but also the range of temperatures in ground water is rather small in the United States, varying from upper 40s to upper 70s nationwide.

To further improve efficiency, many manufacturers use variable-speed, electronically-controlled motors on the duct system fans. Depending on unit size, manufacturers may opt for reciprocating, inertia, rotary, or scroll compressors -- all of which are hermetically sealed and mounted in the indoor cabinet. Some advanced heat pumps feature two-speed or variable speed operation while others feature dual compressors to vary output capacity and match loads.

Energy In Versus Heat Out, or COP

Common Efficiency Factors for Furnaces:

Sizing

Numerous studies have shown that residential heat pumps are often sized too large. For maximum efficiency, heating and cooling loads should be determined according to Air Conditioning Contractors Association (ACCA) Manual J or an approved equivalent. The actual unit size should be within 15 percent of Manual J recommendations. Heat pumps that are too large waste energy and do not provide proper humidity control. Where heating needs are much greater than cooling needs, advanced heat pumps (see above) can improve Summer comfort.

Performance

Performance of GeoSource heat pumps can be determined from American Refrigeration Institute (ARI) ratings of certified products. Rated heat pumps are always marked "ARI certified." ARI provides peak performance information as an Energy Efficiency Ratio (EER) for cooling and a Coefficient of Performance (COP) for heating.

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