AIR-SOURCE HEAT PUMP EFFICIENCY

         AIR-SOURCE HEAT PUMP EFFICIENCY

The heating efficiency of a heat pump is measured by its Heating Season Performance Factor (HSPF), which is the ratio of heat provided in Btu per hour to watts of energy used. This factor considers the losses when the equipment starts up and stops, as well as the energy lost during the defrost cycle.

New heat pumps manufactured after 2005 are required to have an HSPF of at least 7.7. Typical values for the HSPF are 7.7 for minimum efficiency, 8.0 for medium efficiency, and 8.2 for high efficiency.

Variable speed heat pumps have HSPF ratings as high as 9.0, and geothermal heat pumps have HSPFs over 10.0. The HSPF averages the performance of heating equipment for a typical winter in the United States, so the actual efficiency will vary in different climates.

To modify the HSPF for a specific climate, a modeling study was conducted and an equation was developed that modifies the HSPF, based on the local design winter temperature. In colder climates, the HSPF declines and in warmer climates, it increases. In Climate Zone 4, the predicted HSPF is approximately 15% less than the reported HSPF.

GEOTHERMAL HEAT PUMPS

Unlike an air-source heat pump, which has an outside air heat exchanger, a geothermal heat pump relies on fluid-filled pipes, buried beneath the earth, as a source of heating in winter and cooling in summer, Figures 7-3, 7-4. In each season, the temperature of the earth is closer to the desired temperature of the home, so less energy is needed to maintain comfort. Eliminating the outside equipment means higher efficiency, less maintenance, greater equipment life, no noise, and no inconvenience of having to mow around that outdoor unit. This is offset by the higher installation cost.

There are several types of closed loop designs for piping:

·         In deep well systems, a piping loop extends several hundred feet underground.

·         Shallow loops are placed in long trenches, typically about 6 feet deep and several hundred feet long. Coiling the piping into a "slinky" reduces the length requirements.

·         For homes located on large private lakes, loops can be installed at the bottom of the lake, which usually decreases the installation costs and may improve performance.

Proper installation of the geothermal loops is essential for high performance and the longevity of the system. Choose only qualified professionals, who have several years experience installing geothermal heat pumps similar to that designed for your home.

Geothermal heat pumps provide longer service than air-source units do. The inside equipment should last as long as any other traditional heating or cooling system. The buried piping usually has a 25-year warranty. Most experts believe that the piping will last even longer because it is made of a durable   plastic with heat-sealed connections, and the circulating fluid has an anticorrosive additive.

Geothermal heat pumps cost $1,300 to $2,300 more per ton than conventional air-source heat pumps. The actual cost varies according to the difficulty of installing the ground loops as well as the size and features of the equipment. Because of their high installation cost, these units may not be economical for homes with low heating and cooling needs. However, their lower operating costs, reduced maintenance requirements, and greater comfort may make them attractive to many homeowners.

GEOTHERMAL HEAT PUMP EFFICIENCY

The heating efficiency of a geothermal heat pump is measured by the Coefficient of Performance (COP), which measures the number of units of heating or cooling produced by a unit of electricity. The COP is a more direct measure of efficiency than the HSPF and is used for geothermal heat pumps because the water temperature is more constant. Manufacturers of geothermal units provide COPs for different supply water temperatures. If a unit were installed with a COP of 3.0, the system would be operating at about 300% efficiency.

FURNACE EQUIPMENT

Furnaces burn fuels such as natural gas, propane, and fuel oil to produce heat and provide warm, comfortable indoor air during cold weather. Furnaces come in a variety of efficiencies. The comparative economics between heat pumps and furnaces depend on the type of fuel burned, its price, the home’s design, and the outdoor climate. Recent energy price increases have improved the economics of more efficient equipment. However, due to the long-term price uncertainty of different forms of energy, it is difficult to compare furnaces with various fuel types and heat pumps.

FURNACE OPERATION

Furnaces require oxygen for combustion and extra air to vent exhaust gases. Most furnaces are non-direct vent units—they use the surrounding air for combustion. Others, known as direct vent or uncoupled furnaces, bring combustion air into the burner area via sealed inlets that extend to outside air.

Direct vent furnaces can be installed within the conditioned area of a home since they do not rely on inside air for safe operation. Non-direct vent furnaces must receive adequate outside air for combustion and exhaust venting. The primary concern with non-direct vent units is that a malfunctioning heater may allow flue gases, which could contain poisonous carbon monoxide, into the area around the furnace. If there are leaks in the return system, or air leaks between the furnace area and living space, carbon monoxide could enter habitable areas and cause severe health problems.

Most new furnaces have forced draft exhaust systems, meaning a blower propels exhaust gases out the flue to the outdoors. Atmospheric furnaces, which have no forced draft fan, are not as common due to federal efficiency requirements. However, some furnace manufacturers have been able to meet the efficiency requirements with atmospheric units. Atmospheric furnaces should be isolated from the conditioned space. Those units located in well ventilated crawl spaces and attics usually have plenty of combustion air and encounter no problem venting exhaust gases to the outside.

However, units located in closets or mechanical rooms inside the home, or in relatively tight crawl spaces and basements, may have problems. Furnace mechanical rooms must be well sealed from the other rooms of the home (see Figure 7-5). The walls, both interior and exterior, should be insulated. Two outside-air ducts sized for the specific furnace should be installed from outside into the room, one opening near the floor and another near the ceiling, or as otherwise specified in your locality’s gas code.

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