Heat Pump Innovations
Since the introduction of air-source heat pumps to the marketplace in the 1950s, nearly one-third of all homes built today in the United States have at least one heat pump. The convenience of a single appliance that delivers both heating and cooling is appealing especially to those building new homes or replacing older Heating, Ventilation, and Air-conditioning (HVAC) systems. Some electric utility companies also offer rebates for higher-efficiency heat pumps [a Seasonal Energy-Efficiency Rating (SEER) of 12 or higher]. Even with these advantages, however, many heat pump owners still complain about noisy operation and inadequate heating performance. The sections below describe some of the recent improvements in heat pump technology that address these complaints.
Reverse Cycle Chillers: One of the more notable innovations is called a Reverse Cycle Chiller (RCC). It offers the advantages of allowing the homeowner to choose from a wide variety of heating and cooling distribution systems from hydronic radiant floor to multiple zones for forced air. It also offers the potential for lower winter electric bills and hotter air out of the supply vents for greater comfort. An RCC is especially economical for all electric homes or in areas where natural gas is not available. Depending on other fuel rates, it may even be the least expensive heating option over all of the remaining heating fuel choices.
The system consists of a standard 12 SEER, single speed, air source heat pump (sized to the heating load rather than the usual smaller summer cooling load); a large, heavily insulated tank of water that the heat pump heats or cools, depending on the season of the year; a fan coil with ducts or a radiant floor to distribute the stored energy to the house; and some controls to regulate the water flow.
Even if you choose a standard forced air distribution system, the RCC still does away with one of the biggest complaints about air source heat pumps, which is the periodic blowing of cool air during their defrost cycle and during the initial start of the heating cycle as the distribution ducts warm up. The RCC system solves these problems because as it defrosts, it uses the stored heat in the water tank and not the room air directly. Since the temperature of the water in the storage tank is fairly hot (comparable to a gas furnace's output temperature) the cold ductwork, and you, are quickly warmed.
The RCC system also allows the heat pump to operate at peak efficiency most of the time rather than only at above freezing outdoor temperatures like an ordinary heat pump would. This provides greater comfort and economy without the need for electric resistance auxiliary heating coils. In fact, most installations don't even have electric resistance coils at all, since the RCC by is completely capable of heating the building by itself.
For example, in one Michigan installation, the RCC system supplied 115° F (46° C) water to the air handler and a radiant floor system even though the outdoor temperature was negative 15° F (-26° C.) If the RCC is used as a Thermal Energy Storage (TES) system (see the section below for more on a TES) it becomes even more cost effective for areas where the electric utility offers time of use rates.
Another significant energy saving benefit is that the RCC can be equipped with a refrigeration heat reclaimer (RHR). This is similar to the common desuperheater coil found on the high-end heat pumps and air-conditioners (discussed below). The main difference is that the RHR not only makes hot water during the cooling season, but also does it during the heating season by using the excess capacity of the outdoor unit during the milder winter weather to make essentially free domestic hot water. In the summer it makes free hot water by reclaiming the waste heat from the house as long as the system is also cooling the building.
The combined RCC and RHR system costs about 25% more than a standard heat pump of similar size. The simple payback on the additional cost in areas where natural gas is not available is in about 2 to 3 years.
Two Speed Compressors, Variable Speed Fans, Desuperheaters, and Scroll Compressors: Most heat pumps and air-conditioners use single-speed compressors. Single-speed compressors runs at full capacity, no matter what the actual needs of the house are. This wastes energy and money. Some models of heat pumps can be equipped with a two-speed compressor. A two-speed compressor runs at the capacity that is closest to the appropriate capacity to meet the need for heating or cooling at that particular moment. This saves large amounts of electrical energy, and reduces compressor wear.
Also, unlike a gas or oil fired furnace, heat pumps (except for the RCC above) do not deliver very hot air during very cold weather. Instead, the heat pump delivers heated air at around 100° F (37.7° C). Such modest temperatures feel cool to many people who's skin temperature is around 96° F (36° C.) This is despite the fact that the heated air is more than enough to keep the house itself at a comfortable temperature.
In addition to this problem, in order to make up for the lower heating temperature, heat pumps must also push a larger volume of warm air through the ducts. Since the blower must provide this volume of warm air intermittently, it must also have a large capacity. This often makes the blower noisy, as well as wasteful, because it consumes large amounts of electricity to push the air.
Some models of heat pumps can be equipped with variable-speed or dual-speed indoor fans (blowers) outdoor fans, or both. The variable-speed controls for these fans attempt to keep the air moving at a comfortable velocity, minimizing cool drafts and maximizing electrical savings.
The two-speed compressor with a variable speed blower also operates on low capacity most of the time and only goes to its higher capacity as necessary. Low-capacity operation has many advantages. It reduces compressor on-off cycling, which is a major cause of wasted energy and compressor wear as well as temperature fluctuations in rooms. It also minimizes the initial blast of cold air often associated with more primitive models of heat pumps.
Since the system cycles less often, there is also less start-up noise. With low-capacity operation, the heat pump's heat exchanger coils operate at a temperature closer to the ambient air temperature. This reduces the differential between the outside and inside temperatures, making the unit operate more efficiently. This is especially advantageous in the winter. This feature keeps the outdoor coil from frosting, and in turn means that it needs less energy to defrost the outdoor coil.
Unlike a single-speed heat pump, a two-speed heat pump also works well for zone control systems. Zone control systems use automatic dampers to allow the heat pump to keep different rooms at different temperatures. This is a feature often found in houses larger than 4,000 square feet (372 square meters.)
Many high-efficiency heat pumps can be equipped with another energy-saving feature, known as a "desuperheater." In the heat pump's cooling mode, the unit recycles some of the waste heat from the house to generate domestic hot water. A desuperheater-equipped heat pump can heat water 2 to 3 times more efficiently than an ordinary electric water heater.
Another advance in heat pump technology is the scroll compressor. Scroll compressors differ considerably from the reciprocating compressor, which uses a piston inside a cylinder to compress the refrigerant. The scroll compressor consists of two spiral-shaped scrolls. One remains stationary, while the other orbits around it, compressing the refrigerant by forcing it into increasingly smaller areas.
A scroll compressor has a number of advantages over a piston compressor. It is simpler in design and has only one moving part. Scroll compressors are also more tolerant of compressor "flooding," and of contaminants entering the system, both of which are major causes of piston compressor failure. These advantages give scroll compressors a longer operating life. The rotary motion of the scroll compressor is also quieter. According to some reports, heat pumps with scroll compressors provide 10° to 15° F (5.6° to 8.3° C) warmer air when in the heating mode, compared to existing heat pumps with piston compressors.