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ON-DEMAND EXTERNAL HEAT EXCHANGER OPTION It’s also possible to use an external heat exchanger to preheat domestic water using heat from a buffer tank. This option is illustrated in Figure 7-25. Whenever there is a draw for domestic hot water that exceeds some minimum threshold (typically 0.6 to 0.7 gpm in residential systems), a flow switch closes its contacts. This energizes a relay, or one zone in a multi-zone relay center, to power up a circulator that creates flow from the top of the buffer tank through the primary side of a stainless steel brazed plate heat exchanger. Cold domestic water passes in counterflow through the other side of this heat exchanger and absorbs heat. The temperature of the domestic water leaving the heat exchanger depends on the temperature at the top of the buffer tank and the size of the heat exchanger. A suggested guideline is to size the heat exchanger for a 5oF approach temperature difference based on the average anticipated water temperature at the top of the tank and the design flow rate of cold domestic water entering the heat exchanger. In Figure 7-25a, the preheated water leaving the heat exchanger passes through a tankless electric water heater, which supplies any necessary boost in temperature. Domestic hot water leaving the tankless heater passes through an ASSE 1017-rated mixing valve before flowing to hot water fixtures. It’s also possible to use a tank-type water heater in place of the tankless heater, as shown in Figure 7-25b. In systems supplying heating and cooling, the circulator associated with the domestic water heat exchanger would be disabled during cooling mode operation. This would transfer the full domestic water heating load to the tankless water heater. CHILLED WATER COOLING DETAILS One of the foremost benefits of air-to-water heat pumps is their ability to provide both heating and cooling. The latter is provided as a stream of chilled water typically ranging in temperature from 45 to 60oF. There are several potential ways to use chilled water to reduce the temperature of interior air, as well as lower its moisture content. Some of those details are discussed below. CHILLED WATER TERMINAL UNITS The most common way of using chilled water for building cooling is to route it through copper tubing in a water-to-air heat exchanger, often called a “coil,” while room air passes by aluminum fins attached to the copper tubing. The air movement across the coil is created by a fan or a blower. This combination of hardware has several different names depending on its size and mounting location. They include: • Fan-coil • High wall cassette • Air handler A fan-coil is intended to be mounted within a finished interior space. In most cases, it is located at the base of a wall just above the floor. Some fan-coils are surface mounted, others are mounted into a recessed cavity in the wall. Figure 7-26 shows an example of a modern surface- mounted fan-coil. Figure 7-26 Courtesy of Myson Any fan-coil used for chilled water cooling must include a condensate drip pan. This pan catches water droplets falling from the coil and routes them to a piping connection. A plastic drain tube then carries the condensate to a suitable drain. Most fan-coils must be supplied by line voltage to operate the fan or blower. Some can be turned on and off by a single contact closure, while others are operated by a handheld remote. Most have integral controls that allow the fan speed to be set as desired. A high wall cassette is a special type of fan-coil designed to be mounted a few inches below ceiling level. An example is shown in Figure 7-27. 69PDF Image | Heat Pump Systems 2020
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