1、AN-04-5- 1 Energy-Saving Opportun it es in Residential Air-Handler Efficiency Mark A. Kendall ABSTRACT This paper demonstrates the technical considerations that are important to understanding the energy-saving potential of encouraging the use ofbrushless direct current (BDC) motors in residential ai
2、r handlers. Energy savings estimates are provided, and the regulations that are already in place are explained. Some misperceptions about the testing and opera- tion of residential furnaces and air conditioners are also covered. Usingone set ofparameters that assume typical cyclic operation, the con
3、sumer payback of purchasing BDC circu- lating fan motors rather than standardpermanent split capac- itor motors is 26 years. This drops to just under 4 years if the fan is operated continuously. INTRODUCTION An air handler is the component in a central heating or cooling system that is responsible f
4、or circulating conditioned air through the conditioned space. The air handler is usually part of the central furnace or indoor unit on a heat pump. In cases where there is no furnace, but there is a central air condi- tioner, the air handler is a distinct piece of equipment. The standard air handler
5、 utilizes a centrifugal blower and a permanent split capacitor (PSC) motor. The PSC motor oper- ates on alternating current. Speed control, which is required for fans that operate at one speed during the cooling season and at another during the heating season, is accomplished by applying voltage to
6、different sets of motor windings. This introduces slip, which slows the motor down, but at a consid- erable efficiency penalty. A PSC motor operating at part speed draws almost as much power as one operating at full speed. A competing motor technology found in many high-end air handlers is the brush
7、less direct-current (BDC) motor. The BDC motors construction and its speed-control technique are inherently more efficient than the PSC motor, but there is a substantial price premium. Some advocates of energy efficiency, utilities, legisla- tures, and regulators have considered requiring or promoti
8、ng more efficient air handlers based on BDC technology. Many of these promoters presume that the air handler is not regulated, or is under-regulated, by the federal government, and that additional energy savings can result from the promotion of efficient air handlers as distinct from the air conditi
9、oning or heating systems they serve. There are several points that must be considered when deciding whether to apply additional regulation or incentive to promote more efficient air handlers. There are also some common misperceptions about the way in which furnaces and air conditioners are tested an
10、d how air-handler electricity consumption is measured. These factors lead to lower net energy savings and higher incremental equipment prices than might first be apparent. ENERGY SAVINGS DURING THE HEATING SEASON In 1997 there were 56.6 million residences in the United States equipped with a central
11、 warm air furnace. Ofthose, 35.1 million were installed in combination with a central air condi- tioner. In the remaining 2 1.5 million homes, only a central furnace is present. Homes with a central air conditioner but no central furnace number 12.1 million. Other homes with air handlers include tho
12、se heated and cooled by electric heat pumps, which number 10.5 million (EIA 1997). Since the air handler and the furnace are one and the same in over 7 1% of all households with air handlers, improving the electrical efficiency of the air handler results in electricity Mark A. Kendall is the vice pr
13、esident, Technical Affairs, Gas Appliance Manufacturers Association, Arlington, Va. 02004 ASHRAE. 425 Table 1. Typical Furnace Electrical Consumption Characteristics AFUE 80% When Equipped with a When Equipped with Electricity Use Metric Stages PSC Fan Motor a BDC Fan Motor Savings E, kWh (GJ) 1 730
14、 (2.6) 420 (1.5) 42% 2 590 (2.1) 180 (0.6) 67% BE, W 1 450 250 47% 2 - LOW 320 60 79% 2 - HIGH 450 160 58% 90% EA, kwh (GJ) 1 660 (2.4) 470 (1.7) 38% 2 620 (2.2) 250 (0.9) 63% savings during the heating season. However, while electricity savings are substantial, they are offset by increased fuel con
15、sumption. BE, W The Efficiency Advantage of BDC Motors A survey of seven residential furnace manufacturers in 2002 quantified the electricity savings that can be achieved in typical single- and two-stage residential furnaces with BDC motors. The results are presented in Table 1. Respondents provided
16、 average annual electricity consumption, EAE, and blower power consumption, BE, as measured by the federal test procedure for their “most sold” furnace models. The survey requested efficiency certification test data for nominal 75 kBtu/h (22 kW) input furnaces capable of accom- modating a 3-ton (1 1
17、 kW) cooling system. Savings shown are the average of savings compared to products from the same manufacturer rather than across manufacturers. Since there are no single-stage BDC furnaces available today, the results represent an estimate based on comparative BDC-PSC perfor- mance data. These data
18、reveal that the common single-stage furnace, if it were equipped with a BDC motor, would save 3 10 kWh/ yr (0.9 GJ/yr), or 42%, compared to an identical PSC- equipped model. For condensing funiaces, the savings would be less at 190 kWh/yr (0.7 GJ/yr), or 38%. Using average national retail electricit
19、y prices of $0.0841 per kWh ($23.3/ GJ) (DOE 2003), this corresponds to an annual electricity cost savings of $26 for noncondensing funiaces and $16 for condensing furnaces. The power consumption of a typical furnace circulating fan motor in heating mode is 400 W. At 30% efficiency, the motor suppli
20、es 280 W of supplementary electric resistance heat. A BDC motor operating in low speed at 70% efficiency would supply the equivalent of a 40 W resistance heater-a savings of 240 W. If all 56.6 million central furnaces operated simultaneously, their circulating fan motors would generate 15.8 GW in su
21、pplementary resistance heat. Contrast that with 1 550 330 44% 2 - LOW 400 120 77% 2 - HIGH 580 320 44% the 13 million electric resistance furnaces in the US. (EIA 1997). If all operated simultaneously, assuming 1 O kW each, they would produce 130 GW of resistance heat, nearly 10 times the power cons
22、umption of all central furnace fan motors. When combined with two-stage gas controls and two- speed inducer motors that allow the fumace to operate at two different capacities to better match the heating load, electricity savings are even more substantial. The data also show that electricity savings
23、 almost double those for single-stage equip- ment can be achieved by using BDC motors in combination with two-stage controls. So, the typical two-stage noncon- densing furnace equipped with a BDC, which is available today, would save 4 1 O kWhiyr (1.5 GJ/yr), or 67%, versus the single-stage version.
24、 Electricity Savings Are Offset by Increased Fuel Use While these electricity savings are impressive, consumers do not receive the full benefit in terms of energy savings or util- ity cost savings. Increasing the efficiency of the fan motor reduces the amount of heat it generates. For fumaces in the
25、 heating season, the heat from the motor supplements the heat from the burner and helps to satisfy the heat load in the home. The motor effectively behaves like an electric resistance heater. With less heat from the motor, the furnace must run longer to match the heating load. The extra fuel consume
26、d offsets the electricity savings on a site energy basis. Average annual fuel consumption, EF, is stipulated in the federal test procedure for residential furnaces. For two furnaces with identical input rate and efficiency, the calcula- tion procedure yields a higher fuel consumption for furnaces wi
27、th more efficient air handlers. This is due to the increase in burner operating hours (BOHss) that results from the decrease in heat output from the furnace air handler. Gusdorf et al. (2002) confirmed this effect in a study of two identical homes-one with a variable-speed BDC fan motor and one with
28、 a conventional fan motor. With both fans in continuous operation, they found that the variable-speed fan 426 ASHRAE Transactions: Symposia reduced the homes total heating season electricity use by 26%. More striking, the homes natural gas consumption increased by 14%, which was roughly equivalent t
29、o the elec- tricity savings on a site energy basis. These results would be somewhat less dramatic in a typi- cal U.S. home with intermittent fan use and a lower heating load. Also, federal test procedures in the U.S. assume cyclic fan operation. Regardless, increasing the efficiency of the furnace f
30、an is equivalent to shifting heating demand from elec- tricity to gas, oil, or propane during the heating season when most jurisdictions are trying to conserve heating fuel. So the effect of regulating or otherwise promoting air- handler efficiency above that which is already encouraged by SEER will
31、 save no site energy. The only means for accom- plishing that are to (1) reduce heat loss from the flue, as measured by AFUE, (2) reduce heat loss from the ducts to unconditioned spaces, or (3) reduce the heat load in the home by improving the building shell. While they cannot save site energy, more
32、 efficient air handlers may provide benefits associated with burning heating fuel on site rather than heating with electricity produced at fossil-fuel-fired power plants. On an energy basis, electricity is a more expensive fuel than natural gas by a factor of three. Therefore, consumers can expect a
33、n overall decrease in heat- ing costs by shifting from electric resistance heat to gas heat, and the nation can expect a reduction in power plant emissions and primary energy use. Also, in areas that rely on gas-fired power boilers or turbines as marginal electricity-generating units during the heat
34、ing season, more efficient air handlers will also result in net decrease in fuel consumption in that area. ELECTRICITY SAVINGS DURING THE COOLING SEASON The air handler also operates during the cooling season. However, unlike in the heating season, the electrical efficiency of the air handler is alr
35、eady measured and regulated as part of the total cooling system. There are some misconceptions, however, about the adequacy of the testing and regulation that exists. Air-Handler Efficiency is Regulated through SEER and HSPF Heat pumps and packaged cooling-only equipment have integral air handlers.
36、The air-handler electricity consumption is measured directly as part of the federal energy efficiency test. The federal test procedure and regulations for cooling and heating efficiency are rated in SEER and HSPF, respectively. For split cooling-only systems that depend on furnaces for their air han
37、dling, a common misperception is that, since air-handler electricity consumption is not regulated in the heating season, there are unregulated or under-incentivized efficiency gains to be achieved in the cooling season. Although it is true that annual fuel utilization efficiency (AFUE), the fumace f
38、uel efficiency metric, does not account for electrical efficiency, regulations and incentives to promote SEER also strongly influence furnace design. When an air conditioner is sold with a frnace, that combination receives a SEER rating regulated by the federal government. The electricity use of the
39、 furnace air handler is included in the SEER calculation. One option manufacturers have for exceeding the federal minimum SEER is to improve the electrical efficiency of the furnace air-handler fan. Other options, which may cost less for the same SEER benefit, include improving the efficiency of the
40、 compressor or heat exchanger or adding a thermostatic expansion valve. Since new furnaces are matched with new air conditioners in over half of all applications, furnaces today must be capable of achieving the 1 O SEER federal minimum with any common condenser and coil. In effect, the efficiency of
41、 every furnace air handler is regulated by the fact that they likely will be sold with air conditioners with which they have already been regulated as part of the cooling system. Adding a separate furnace fan regulation or incentive, since it will not result in an increase in SEER, does not provide
42、electricity savings. However, it does restrict the equipment designers ability to optimize the system to meet the SEER target using the lowest cost combination of design options. There are three other misperceptions that lead to the belief that the SEER test does not properly account for air-handler
43、 efficiency: SEER does not capture additional cooling load that results from air-handier inefficiency. The default fan power consumption in the SEER test is too low. The static pressure requirement in the SEER test is too low. Each of these claims is examined below. Additional Cooling Load. One argu
44、ment for favoring air-handler efficiency over condenser efficiency is that heat produced by the air handler represents sensible cooling load during the cooling season that is not considered in the SEER test. Without that SEER incentive, some argue, designers are not being properly rewarded for impro
45、ving air-handler effi- ciency compared to condenser or coil improvements that have the same energy conservation benefits. In fact, heat produced in the air handler is reflected in the SEER test, both through measurement of electrical input to the circulating fan and through reduced cooling capacity.
46、 In other words, in the addi- tional sensible heat from the motor is properly considered in the SEER test through its effect on system capacity. There are two basic air-handier configurations: blow- through and draw-through. In draw-through designs, the fan motor lies downstream from the evaporator
47、coil. Draw- through designs, because they provide for better utilization of coil surface and therefore better system efficiency, are typi- cally used in packaged equipment, indoor fancoils, and heat pumps. In a blow-through design (typical of units installed with indoor furnaces), the fan motor lies
48、 upstream from the ASH RAE Transactions: Symposia 427 evaporator coil. In both cases, the fan motor is in the airstream and the heat from the motor is released into the conditioned air. In both the blow-through and draw-through designs, the heat produced by the fan motor acts to lower SEER. By reduc
49、- ing the temperature difference between the return air and supply air, heat from the fan reduces calculated cooling capac- ity. This effect is independent of the position of the motor in relation to the coil. Default Fan Power. Another argument used to favor air- handler efficiency over condenser efficiency is that the SEER test allows manufacturers to elect a default blower power consumption value of 365 W/Mcfm/min (0.5 W/m3/s). Improvements in blower wattage that take place above the limit, since they are not credited toward SEER, would reduce the incentive for designers
