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The heating distribution system is very simple. A single variable-speed pressure-regulated circulator operates continuously during the heating season. It’s speed automatically increases and decreases to maintain a constant differential pressure as the thermostatic valves on the panel radiators open, close or modulate. The thermostatic valve at each radiator allows it to operate as a separate zone, maintaining the desired comfort level in each space. The split system heat pump allows the hydronic system to operate without need of antifreeze. A magnetic dirt separator protects the heat pump’s condenser. A combined air/dirt/magnetic particle separator protects the permanent magnet motor in the ECM circulator for iron oxide. It also provides high-efficiency air separation for the system. A spring check valve prevents reverse thermosiphoning between the buffer tank and heat pump. This reduces extraneous heat loss through the piping when the heat pump is off. The heat pump can be isolated from the balance of the system for service if necessary. This system also has very simple control requirements. A single switch can be used to “enable” the system at the start of the heating season. The heat pump and its associated circulator turn on and off as necessary to maintain the target water temperature in the buffer tank. The distribution circulator (P2) operates continuously but is always tracking the differential pressure present based on the status of the thermostatic radiator valves. With current ECM circulator technology and proper component sizing, this distribution system could precisely deliver over 1,500 Btu/hr per watt of electrical energy supplied to circulator (P2). This is much higher distribution efficiency than what could be attained with forced-air delivery. SYSTEM #3 The system shown in Figure 8-3 supplies multiple zones of space heating with a mixture of heat emitters. It also provides domestic hot water. A split system air-to-water heat pump is the primary heat source. An electric boiler, piped in parallel with the heat pump’s condenser, provides a second stage of heat input if needed. It also provides a backup to the heat pump should it be down for service. Each heat source is equipped with a pressure relief valve and can be fully isolated if necessary. Depending on local codes, the electric boiler may require a low water cutoff and a manual reset high limit controller. Both heat sources supply a “tank-in-tank” buffer. The inner tank is constructed of stainless steel and holds 40 gallons of domestic water. It is surrounded by an outer tank that receives heat from the heat pump or the electric boiler. Heat flows from the system water in the outer tank to domestic water in the inner tank whenever the former is at a higher temperature than the latter. Domestic cold water enters the inner tank whenever there is a draw from a hot water fixture. The temperature of the domestic water leaving the inner tank depends on the temperature maintained in the tank shell. A thermostatically controlled tankless electric water heater boosts the domestic water to the desired supply temperature. The buffer tank has a 3-pipe configuration. Heated water from the heat pump or electric boiler can flow directly to one or more of the space heating zones when either heat source is on at the same time as one or more of the zones. Any difference in flow rates between the heat source(s) and zones passes through the buffer tank. All return flow passes into the lower portion of the buffer tank, and thus keeps its thermal mass well engaged. A spring-loaded check valve is installed in the piping leaving the heat pump and the electric boiler. These valves prevent reverse thermosiphon flow from the heated tank through either heat source when they are off. The water temperature in the tank is controlled by a 2-stage outdoor reset controller. At design load conditions, the target water temperature in the tank is 120oF. The target temperature decreases as the outdoor temperature increases. The minimum target temperature is 100oF. Maintaining the tank temperature in this range allows the heat pump to operate with relatively good COPs. It also allows the heat pump to provide the majority of the “temperature lift” required for domestic hot water. If the heat pump is not able to maintain the necessary target temperature, the controller operates the electric boiler for supplemental heat input. There is a 5-minute interstage time delay to allow time for the heat pump to stabilize its operation before turning on the electric boiler. Space heating is supplied through several types of heat emitters. The home’s main floor uses a combination of panel radiators, a towel warmer, and two areas of tube & plate underfloor heating. All of these emitters have been sized for design load output at 120oF, and thus they can all be supplied as parallel circuits. The towel warmer is combined in series with a short tubing circuit that provides a small area of floor heating in the master bathroom. The three panel radiators each have integral thermostatic 75PDF Image | Heat Pump Systems 2020
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