A heat pump is a year-round air-conditioning system that provides warm air during the winter and cool air during the summer. It is basically a compressor-cycle air-conditioning system (similar to the one described previously) that can operate in reverse. During the reverse operation, the condenser functions as an evaporator, and the evaporator functions as the condenser. The overall refrigerant cycle, however, remains the same. (See page 230 and FIG. 17-1 for the operational flow details.)
When the system is operating, the condenser (which is located in the house) is cooled by the air that is circulated around the house. As this air passes over the condenser, it absorbs heat that is used for heating the house. For the cycle to operate properly, the evaporator (which is located on the outside) must absorb heat from the outside air. Even when the outside temperature drops to as low as 20° F. the evaporator can absorb heat because the refrigerant within the evaporator is at a lower temperature. However, as the temperature of the outside air drops, the ability of the evaporator to absorb heat decreases, decreasing the effectiveness of the heat pump. Even though a heat pump may be operational at temperatures as low as 20° F, the Btu output is sufficiently reduced so that auxiliary heaters are usually required.
A heat pump should be sized for the air-conditioning load and not the heating load. Otherwise, the air conditioner will be oversized. Except for a small section of the Deep South, the heating load on a house will always be greater than the cooling load. A heat pump will produce approximately 20 percent more Btus per hour for heating than it does for cooling. Consequently, in most parts of the country, when a house is heated by a heat pump, auxiliary heat will also be needed. For example, in the New York area, a typical eight-room house would require a furnace that could produce about 100,000 Btus per hour for winter heating and also a 31⁄2 ton (42,000 Btu/hour) air-conditioning unit for summer cooling. If a properly sized heat pump was used for heating, it would produce only 50,400 Btu/hour. The remaining 49,600 Btu/hour would have to be provided by auxiliary heaters, which are usually electrical resistance heaters.
The auxiliary heaters in heat pumps are automatically activated when the pump cannot supply sufficient heat to keep up with the heat loss of the structure during the winter. In northern communities where there is a considerable amount of moisture and low temperatures during the winter, there is a tendency toward an ice buildup on the metal fins of the outdoor evaporator coil. An excessive ice buildup could cut off air circulation across the coil and result in a loss of heating capacity. With some heat pumps, this icing condition is automatically controlled by a defrost cycle that reverses the flow of the refrigerant for a short time. The hot refrigerant heats the outdoor coil and melts the ice. During the defrost cycle, the auxiliary heaters are usually energized to offset the cycle’s cooling effect on the indoor circulating air.
A heat pump can operate in either the heating or air-conditioning mode. Most manufacturers suggest that the unit be operated in the air-conditioning mode when the outdoor air temperature is above 65° F (unlike a regular air conditioner, where the recommended temperature is 60° F) and in the heating mode when the temperature is below 65° F. Operating a heat pump in the wrong mode can result in damage to the compressor.
The components and problems of heat pumps are basically similar to those of air-conditioning systems. Consequently, the overall inspection procedure outlined earlier in this chapter for air conditioners should be used when inspecting heat pumps. However, you should not check both modes of operation. As long as the unit is functioning properly in the mode tested, it is an indication that the major and most costly components (compressor, fans, and coils) are operational. The system should then also function in the opposite mode. If it doesn’t, a faulty reversing valve is usually the cause.
One of the benefits of air conditioning is the dehumidification of the circulating air. This benefit is not without cost. Cooling and dehumidifying the air is more costly than cooling alone. In the southwestern part of the United States, the outdoor air is relatively dry, and dehumidification is not necessary. In this area, cooling can be achieved by means of an evaporative cooler. Because of the low humidity, water readily evaporates. In the evaporation process, the water absorbs heat from its surroundings and lowers the temperature.
The typical evaporative cooler consists of a sheet-metal and plastic casing containing a fan, pads, filter, and a water source. The pads, which hold the water, can be wetted by a spray, by a trickling stream, or by passing through a reservoir on a rotating drum. In some units, the wetted pads also function as air filters. As the air passes over or through the pads, it is cooled by the evaporating water and then distributed throughout the house.
When inspecting an evaporative cooler, turn the unit on and listen for any unusual sounds or vibrations in the blower compartment. Also look for signs of water leaks and check the pads for deposits and crusting. For efficient operation, the pads may require cleaning or replacement.