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The economical heat pump, which uses ambient heat to both heat and cool a house, is an increasingly popular alternative to more conventional heating and cooling systems—sometimes replacing them, sometimes serving as an auxiliary system. Similar in function to an air conditioner except that its role can be re versed, the heat pump in one mode transfers heat into the house, and in another mode extracts it. Furthermore, a single thermostat setting senses which mode is required and reverses the heat pump automatically. The key ingredient in a heat pump is the refrigerant in its coils, usually a sub stance called Freon, which vaporizes into a gas at a boiling point far lower than the 212° F. that water requires. When the refrigerant boils, changing from a liquid to a gas, it absorbs heat from its surroundings; it can do so even when those surroundings are relatively cool. As the refrigerant changes back into a liquid from a gas, it gives up its heat to the atmosphere. This process of transformation from liquid to gas and back again is controlled by an expansion valve and an electric compressor. Heat pumps are most efficient in the cooling mode. As a source of heat their efficiency varies according to the out door temperature. When the temperature dips below about 400, heat pumps must draw more heavily on supplemental electrical resistance coils built into the unit. In cold climates, heat pumps can be effective as an alternative source of heat, installed in tandem with an existing fuel- fired heating system and hooked to a thermostat that switches from one sys tem to the other whenever the outdoor temperature is high enough for the heat pump to function economically. Heat pumps can be cheaper to operate than other heating systems because, by tapping into the free heat in outdoor air, they give back more energy—in the form of heat—than the equivalent amount of electrical energy they consume. Thus they are more efficient and—under optimal conditions—less expensive to operate than gas or oil furnaces or electrical resistance systems; the amount of energy, expressed in BTUs, that is gobbled up by these conventional systems always exceeds the BTUs of heat they produce. Heat pumps are steadily gaining popularity, but they are not for everybody, because not everybody has the optimal conditions that guarantee a good pay-back (-- 8) for the initial cost of the pump. The long-term saving that a heat pump can offer will depend on climate, house construction, insulation, and the relative cost of other sources of heat in a particular area. In rating heat pumps for both heating and cooling capacity, manufacturers use two terms that the potential purchaser should be familiar with. Both compare the ratio of energy input to energy out put. For the heating mode, the ratio is called the coefficient of performance, or COP, calculated by dividing BTU input into BTU output; the COP of a typical heat pump can range from 2.8 at 60° out door temperature to 1 at 10°. For the cooling mode, the ratio is called the energy efficiency ratio, or EER, a figure that is determined by dividing the unit’s watt age into the BTUs removed; typically, the EER can range from 6 to 10. The higher the EER and the COP ratios, the more efficient the unit. Also to be considered is the source of heat energy tapped by the heat pump — air, water or even the sun. Those that use air, called air-to-air heat pumps, are avail able in one-, two- and three-piece systems. One-piece systems, called single- package units, are similar to room air conditioners. These units sit in a window or are installed outdoors, usually on the roof or at the side of the house, and are connected directly to the house ductwork. Two- and three-piece systems, called split systems, have an outdoor section connected to one or two indoor sections by pipes running through the house wall. The indoor section can be tied into an existing forced-air heating system to share the same ducts and blower, in much the same way as central air conditioning is connected. Heat pumps that tap the energy of water, called water-to-air systems, are connected to a well or to a pond that does not freeze over. Water-to-air heat pumps are one-piece systems, drawing water from the well or a pond and expelling the used water into a discharge well or into a lawn sprinkler system. Because their source of heat remains at a fairly constant temperature—about 50°—through out the year, water-to-air systems are often 25 percent more efficient than air- to-air systems, although they can be more expensive to install. One variation of a heat pump, more modest in scale than the kind used for house heating and cooling, can heat the domestic water supply. Actually a cabinet-sized air-to-water system, this small heat pump absorbs heat from the indoor air and transfers it to the water tank. The unit maintains the water at a temperature of 120° to 140° while lowering the temperature of the surrounding air only 1° or 2°. In addition, this type of unit serves as a dehumidifier. A heat-pump water heater monitors water temperature in the tank by means of a sensor, and the unit is automatically activated whenever hot water is with drawn and replaced by cold water. In most cases, no backup heat source is needed, but the existing heat source—oil, gas or electricity—remains in place and can be turned on if the heat pump is closed down for servicing. A typical heat-pump water heater is sold as a kit containing the heat-pump cabinet, fittings for the inlet and outlet shutoff valves and, in some models, a new pressure-relief valve with a longer sensing tube to replace the tank’s existing short-tube pressure-relief valve. Hoses for the pump’s water connections and for the drain line that carries off condensation are included, but in some localities these flexible hoses will have to be re placed with rigid piping; check your local plumbing codes. The unit plugs into a 20- ampere, 115-volt circuit like that required for central air conditioners and heavy appliances. If there is no such circuit in the vicinity of the unit, one will have to be installed at the main electrical panel. [106 ] The Magical Mechanics of a Heat Pump An air-to-air system. In the heating mode, cold refrigerant liquid, vaporized to a mist under low pressure, passes through the outdoor coils, absorbs heat from the air and changes from a mist to a gas. The gas is pulled through the reversing valve into the compressor, which makes it denser and hotter. The hot gas is then pumped to the indoor coils, giving up its heat to the air circulating around the coils. The heated air, blown past supplementary heating coils out of sight near the blower, is distributed through the house ducts. The refrigerant emerges from the indoor coils as a liquid. It passes through a low-pressure expansion valve, where it changes back into mist, drops in temperature, and enters the outdoor coils to repeat the cycle. In the cooling mode, the direction of the flow of refrigerant is changed by the reversing valve: The refrigerant in mist form first flows through the indoor coils to pick up heat from the air. Following a series of changes exactly opposite those of the heating mode, the refrigerant then expels heat outside, aided by an exhaust fan. In three-piece systems, the compressor is located indoors in a separate cabinet and is connected by refrigerant pipes to the outdoor and the indoor coils. In one- and two-piece systems, the compressor is located outdoors. A water-to-air system. In the heating mode, water from the supply well is pumped into pipes that circle past the refrigerant coils of the first of two heat exchangers, which are similar in function to the coils of an air-to-air system. The refrigerant in the coils of this first heat exchanger absorbs heat from the water and changes from a mist to a gas. The water, having given up its heat, is discharged into a well or a sprinkler system. The compressor makes the gas denser and hotter and then pumps it to the second heat exchanger, where the gas releases heat to air that will be distributed through the house. Emerging from the second heat exchanger as a liquid, the refrigerant is changed to a mist by an expansion device and returns to the first heat exchanger to repeat the cycle. In the cooling mode, the path is reversed. After absorbing heat from the indoor air, the refrigerant passes heat to the water circling past the coils of the first heat exchanger’s pipes. The water is then discharged. [107 ] A Pump for Heating Water An air-to-water heat pump. The components of this system are similar to those of the water-to- air heat pump. Cool water from the bottom of the tank is pumped into the tube- within-a-tube heat exchanger—an inner tube of water surrounded by an outer tube of refrigerant. There, the refrigerant, heated to a gas by the compressor, loses its heat to the water, condenses to a liquid, and passes through an expansion valve to become a mist that is lower in temperature than the surrounding air. As it absorbs heat from the air, the refrigerant passes through evaporator coils and returns to the condenser as a liquid. The cycle continues until all the water in the tank is heated to the tempera ture set by the thermostat. Meanwhile, water in the air condenses on the outside of the cool evaporator coils and drips into a pan, from which a tube leads it away to a drain. Joining the Pump to the Heater 1 Draining the water tank. Shutoff the heat source to the water tank: On an electric tank, turn off the power at the main electrical panel; on an oil-fired tank, turn off the burner switch; close the gas valve on the supply line of a gas-fired tank. Connect a garden hose to the drain valve at the bottom of the tank, and run the hose out doors or to a floor-level drain or sump pump. Turn off the cold-water supply valve at the top of the tank and open all the hot-water faucets in the house. Open the drain valve, and drain the tank to the level of the drain valve. Disconnect the garden hose; remove the drain valve. [108 ] 3 Installing the tank outlet valve. Unscrew the overflow pipe from the existing pressure- relief valve, located on the side or top of the tank, and set the pipe aside. Remove the pressure-relief valve, and replace it with the 2-inch nipple of the new outlet fitting --. Thread the T fitting onto the nipple, then thread the new pressure-relief valve onto the T end. Thread the shutoff valve onto the center tap of the T fitting, and insert the overflow pipe in the new relief valve. 4 Connecting the hoses. Remove the cap plugs from the pump inlet and outlet. Connect one hose to the pump outlet and to the shut off valve at the top of the tank. Connect the other hose to the bottom shutoff valve on the tank and to the pump inlet. Connect the clear plastic tubing to the condensation drain on the back of the pump, and run the tubing to an indoor floor drain, to a sump pump or to an outdoor location. Turn the two shutoff valves to the fully closed position; then turn on the tank’s cold- water supply valve. Allow the tank to fill until water flows in a steady stream from the open hot-water faucets throughout the house. Turn off the faucets and open the tank shutoff valves. Plug the pump into a 20-amp, 115- volt circuit and turn it on. 2 Installing the tank inlet valve. Thread a 4- inch nipple -- into the drain-valve opening, and screw the T fitting to the other end of the nipple. Screw the original drain valve onto the T end, and thread the shutoff valve on the center tap of the tee. To ensure water tight joints, first coat the threads with pipe- joint compound or plastic joint tape, and don’t overtighten the threaded sections. [109 ] Prev | Next: Hot Water in a Hurry |