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2-pipe buffers should only be used when the heat source is turned on and off based on buffer tank temperature. The reasoning is as follows: if the heat pump flow rate and load flow rate are similar, there will be very little flow through the tank. This could cause the heat pump to shut off based on satisfying the space heating thermostat, without adding much heat to the tank. In this scenario, the tank is not well “engaged” in the heat flow process. However, when the heat pump is controlled directly from tank temperature, the heat pump should continue to run even after the space heating thermostat is satisfied, storing heat that’s immediately ready to flow to the next zone requesting it. The 3-pipe buffer tank configuration is an excellent compromise between the strengths and limitations of the 4-pipe and 2-pipe configurations. It provides a way to pass heat directly from the heat pump to the load when both are operating. It also forces flow returning from the load through the lower portion of the tank, and thus, ensures that the tank’s thermal mass is engaged. This “direct-to-load” configuration has been modeled using TRNSYS simulation software and found to increase the seasonal COP of the heat pump by keeping the lower portion of the tank slightly cooler than is possible with other piping configurations. Buffer tanks connected to heat pumps tend to have minimal temperature stratification. This happens because most heat pumps have recommended flow rates of 3 gpm per ton (12,000 Btu/hr) of capacity. A typical 4-ton air- to-water heat pump operating at these conditions would “turn over” an 80-gallon buffer in less than 7 minutes. Those flow rates, especially if introduced vertically into the tank, create lots of internal mixing. Figure 7-13 shows an example of an air-to-water heat pump supplying a highly zoned distribution system through a buffer tank with a 3-pipe configuration. The split system air-to-water heat pump supplies a combination of low-temperature panel radiators and radiant floor panel circuits. The heat output of each radiator and radiant panel circuit is regulated by a non- electric thermostatic valve. A variable-speed pressure- regulated circulator automatically adjusts speed based on the status of these valves. The heat pump is turned on an off to maintain the water temperature at the middle of the buffer tank between 100 and 110oF. In addition to buffering the heat pump against short cycling, the tank provides hydraulic separation between the heat pump’s internal circulator (P1) and the variable-speed distribution circulator (P2). DIRT SEPARATION The refrigerant-to-water heat exchangers used in modern air-to-water heat pumps often have narrow flow passages. It is essential that the fluid passing through these heat exchangers is clean. A low-velocity zone dirt separator, as shown in Figure 7-13, is one solution. If the system has one or more ECM-based circulators, a magnetic dirt separator is recommended. AUXILIARY HEATING PROVISIONS Although it’s possible to size an air-to-water heat pump such that it can supply design heating load, or even for that load plus a safety factor, such sizing is not always economically justified. The Boston house example described in section 5 demonstrated that a low-ambient air-to-water heat pump having a balance point capacity of about 65% of the design load could supply about 96% of the total seasonal heating energy. The nominal 4-ton heat pump would have to be increased to a nominal 6-ton unit to meet the design load. Under typical partial load conditions, the larger heat pump would be more subject to short cycling. The suggested solution was to install an electric boiler that could provide supplemental heat during the few hours in a typical year when it was needed, while still maintaining the heating system as “all electric.” Figure 7-14 shows an electric boiler piped in parallel with the air-to-water heat pump. Using an electric boiler for auxiliary heat has the advantage of allowing a wider selection of possible buffer tanks and boiler heating capacity. A wider selection of buffer tanks is possible because of the limited number of buffer tanks currently available with internal electric heating elements. The boiler could be sized to provide supplemental heating in combination with the heat pump, or it could be sized to provide 100% backup heating, even at design loads should the heat pump be down for service. Large electric boilers operating on typical residential 240 VAC single phase power can require high amperage. The minimum electric service panel rating should be 200 amps. Electric boilers also allow easy servicing since they are not integrated into a tank or other system component. One prerequisite of using an electric boiler is that a separate circulator is required. The system shown in Figure 7-14 has this circulator. It also has a check valve, purging valve and pressure-relief valve in each heat source circuit. These components allow the buffer tank to be supplied by either heat source or both at the same time. They also allow either heat source to be completely isolated from the remainder of the system without having to shut down heat delivery. 59PDF Image | Heat Pump Systems 2020
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