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the U.S. Department of Defense to issue a ban on the use of heat pumps in military facilities due to the severity of maintenance problems. Fossil fuel continued to be the dominant energy source for heating buildings. The OPEC oil embargo, which began in 1973, reinvigorated efforts to develop reliable electrically powered heat pumps that could lessen dependence on petroleum-based heating fuels. Manufacturers of air-conditioning systems worked on methods of reversing the direction of heat flow, and thus allow low-temperature heat in outside air to be raised to temperatures sufficient for heating buildings. One of the earliest attempts at creating a vapor- compression machine that could heat as well as cool simply reversed the direction of the entire air conditioner within an opening in an exterior wall. Another used multiple dampers to change airflow directions. Manufacturers eventually discovered that the refrigerant flows used in vapor-compression air conditioners could be reversed using a combination of four hand-operated valves. In time, this approach was replaced by two valves operated by electrical solenoids. Further development led to a single 4-port, electrically operated “reversing valve.” This type of valve, which is discussed in more detail later in this issue, is now used in a wide variety of heat pumps that provide heating and cooling. Figure 1-3 Reversing valves made it practical to heat and cool homes using air-to-air heat pumps. Sales of residential air-to-air heat pumps grew rapidly during the 1970s. The primary markets were southern states with relatively mild winter temperatures and a definite need for summer cooling. Air- to-air heat pumps became heavily promoted by southern electric utilities, as well as by manufacturers such as Carrier, Westinghouse and General Electric. During the 1970s, the reliability of air-to-air heat pumps continually improved through revised compressor design, better lubrication details and techniques to reduce liquid “slugging” of compressors. By 1975, the U.S. Department of Defense lifted their previous ban on heat pumps in military facilities. A surge of interest in heat pumps during 1976 lead to an annual sales growth rate of 96%. Manufacturers were having difficulty keeping up with demand. By 1978, it was estimated that air-to-air heat pumps were installed in over 1.4 million U.S. homes. PERFORMANCE LIMITATIONS Early generation air-to-air heat pumps could not operate well at the low outdoor temperatures experienced in the Northern U.S. and Canada. Many were limited to minimum outdoor temperatures in the range of 15-20oF. If the outdoor temperature dropped below this limit, the heat pump would operate at grossly insufficient output or simply turn off. The heating load would then be assumed by electric resistance “strip heaters,” which are heating elements mounted in the supply air plenum on the heat pump’s interior unit, as shown in Figure 1-4. Strip heat was usually activated by the second stage of a 2-stage thermostat as room air temperature dropped slightly below the desired setting. Although reliable as a supplemental heat source, strip heat, like all electric- resistance heating, is expensive to operate. Some early- generation air-to-air heat pumps were also installed along with gas-fired furnaces that would assume the full heating load if the heat pump could not operate due to low outdoor temperature or some other condition. The inability to operate at the low outdoor air temperatures experienced in many northern states, and much of Canada, created a stigma that air-source heat pumps were only suitable for heating in mild climates. This limitation was one of the largest factors leading to the emergence of geothermal heat pumps during the 1980s. Because water returning from earth loops, wells or large open bodies of water was always above 32oF, even when outdoor air temperatures were extremely cold, geothermal heat pumps could provide predictable heating performance in northern climates. In milder climates, Courtesy of AD Cooper 7PDF Image | Heat Pump Systems 2020
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