1、AN-04-5-2 How Much Energy Could Residential Furnace Air Handlers Save? Harvey M. Sachs, Ph.D. Member ASHRAE ABSTRACT Available information indicates that improving the air- handler motors and fan systems in residential furnaces and heat pumps promises substantial, cost-efective eficiency gains (hund
2、reds of kWh per year). Unfortunately, the manda- tory test methods used give on ineficient units use several times more. The savingspotential is comparable to the electricity use of a 2001-compliant refrigerator (about 500 kWh/yr). INTRODUCTION The air handler of residential split systems includes t
3、he cabinet (below the furnace in an upflow furnace), the fan, and the fan motor. Improvements could result from changes in aerodynamics, the fan, motor, and/or motor controls. Such measures show potential for substantial, cost-effective effi- ciency gains. During the heating cycle, the difference be
4、tween an advanced fardmotor system and an ordinary one is approx- imately the total energy consumption of a 2001 -compliant refrigerator: in the range of 500 kilowatt-hours per year (kWh/ yr). In the air-conditioning mode in “average” climates, we estimate that the better air-handler motors reduce d
5、emand by about 250 watts (including the benefits of reduced heat rejec- tion by the fan motor) and save 200 kWh in a typical cooling climate (Sachs and Smith 2003). In this paper, many numbers are “engineering estimates.” In some cases, actual data are proprietary and unpublished. In others, uncerta
6、inties arise from gaps between field measurements and predictions that would be made from rating methods. Thus, our conclusions should be taken as indicators of opportunities and additional Sandy Smith, Ph.D. Table 1. An Indication of Electricity Savings Potential for the Most Efficient Condensing F
7、urnaces* Based on 114 furnaces listed in ACEEE (ZOOI), which only considered AFUE. The “high” use group is inferred to use PSC motors, and the “low” use group to be equipped with ECPM motors. research needs rather than definitive results for regulatory purposes. Gas furnace electricity consumption v
8、aries greatly, even within size classes. To illustrate this, one estimator of electri- cal use by furnaces is Eae, the annual electrical consumption computed according to the annual fuel utilization efficiency (AFUE, ASHRAE 2000) test protocol. Table 1 shows the aver- age value of Eae for extremely
9、efficient condensing fumaces (AFUE almost all 94% or better), taken from ACEEE (2001 I). Routes to Improved Performance Most residential units use permanent split capacitor (PSC) motors. Typically, multiple taps give fixed speed . Pp. 24-27, table titled “Most Eficient Gas Furnaces. Harvey M. Sachs
10、is director of the Buildings Program and Sandy Smith is senior research associate at American Council for an Energy-E%- cient Economy (ACEEE), Washington, D.C. 02004 ASHRAE. 43 1 Table 2. Efficiency Estimates for 1/2 Horsepower PSC and ECPM Motors* Technology Based on discussions with industry autho
11、rities. See also Nadel et al. (2002, “Efficiency (Oh) up to 82%” from . options. The electronically commutated DC permanent magnet motor (ECPM, ECM? ICM, DCPM, and other terms) has 5% to 10% of the “premium” market.3 The ECPM motor costs substantially more today, but it offers many benefits, includi
12、ng continuously variable speeds-and much higher efficiency. Table 2 compares efficiency for % horsepower versions of the two motor types. The efficiencies noted are electrical energy conversion efficiencies, or “wire-to-shaft” rather than “wire-to-air” effi- ciencies. The efficiency of fractional ho
13、rsepower motors (4 hp) is not governed by national EPAct standards (NEMA 1997), and the relevant standard (NEMA 2002) does not include non-induction single-phase fractional motors, such as permanent magnet motors. Thus, estimates in this paper are based on reviews of manufacturers literature4 and di
14、scussions with industry experts. Conventionally, residential systems use high-speed fan operation in air-conditioning mode and a lower speed for heat- ing. This is required because the contrast between the supply air temperature and the desired room temperature is much smaller in cooling than heatin
15、g: roughly 20F (1 1 OC) in cool- ing and 5OoF-7O0F (28OC-39”C) in heating with gas or oil fuel. Thus, getting the same effect requires moving more air mass in cooling, which is accomplished through higher fan speeds. Ironically, because the air-handler fan is located in the condi- tioned airstream,
16、reducing electricity used in heating will slightly increase gas consumption. On the other hand, the decreased heat rejection by an efficient motor in the cooling cycle decreases compressor work and electricity used for cool- ing, improving efficiency. ECMm is a trademark of one manufacturer. Estimat
17、e based on roughly 300,000 ECMyr (10-year average ACHR 20031) and roughly 4.5 million new furnaces (GAMA data) plus roughly 1.5 million new heat pumps (AM data) per year. Calling it 400,000 ECM for 2002 to allow for increasing market share would give about 9%. As an example, one efficiency graph in
18、a product brochure shows a remarkable 70% efficiency at 400 rpm (about 150 watts) and peak efficiency of 82% at . 12. The artificially high number ofmodels makes inferences ofstatis- tical robustness inadvisable. furnaces and because there is little difference in the two numbers across the range of
19、furnace efficiencies considered. Indeed, although total range is fairly small-90% to 96+%- roughly 85% (of common updraft models) have efficiencies no higher than 93%. The scatter plot of Eue (electricity use) versus furnace capacity suggested a “trough” or “valley” with few models. Empirically, thi
20、s seemed to separate a graph region of systems with high electrical efficiency/low Eae from a graph area with much higher electricity use. When we plot- ted the data for the best 20% (lowest kwhyr) in each size class against the most inefficient 20% (most kWWyr) within capac- ity classes, there were
21、 clear differences. To further explore this pattern, we created an informal efficiency metric, the electricity use ratio or EUR. This is the ratio of the annual electricity use, Eae, divided by the fumace capacity, in thousands of Bhuh (kBtu/h). For a furnace with Eae = 500 kWhyr and 50,000 Bhuh (50
22、 kBtdh) capacity, EUR = 500/50 = 1 O. EUR has two operational virtues: (1) EUR “normalizes” electricity use across furnace capacities, and (2) everything needed for assessing EUR is available in the stan- dard columns of the GAMA on-line database for gas furnaces. EUR can be used to select high- eff
23、iciency electrical furnaces. Table 4 tabulates these data by furnace size.13 The average saving, across all sizes, is 5 1 1 kWh/yr. For purposes of this study, we use 500 kWhyr as the average elec- tricity saving in the heating season by high-efficiency furnace air handlers. In contrast, GE uses 894
24、 kwh as the national average (GE 2001). This value is decremented by the esti- mated fraction of condensing furnaces sold in relatively warm climates (Kendall 2002b). We assumed 90% conversion effi- ciency for gas condensing furnaces. The sales-weighted aver- age may be higher, which would make ACEE
25、Es estimates of gas used conservative, reducing our savings estimate margin- ally. These savings have not been adjusted by the ratio of field to test static pressure or power found empirically (Proctor and Parker 2000) because the savings of ECPM motors under high static pressure may be less than an
26、ticipated (Pigg 2003). Statehegional heating and cooling load hour data were interpolated graphically from ARI (2003) Appendix A, with use normalized for specific states and areas. For heating loads in specific regions, we adjusted state or region HLH by the ratio of that state or regions HLH (CLH)
27、to the national aver- age of 2,080 HLH. For cooling season savings, we began by estimating the reduced demand (kW) of an ECPM motor. Since most utilities face their peak demands in summer, the reduction in power required (kW) is significant. From exam- ination of manufacturers literature, discussion
28、s with industry representatives, and professional judgment, we inferred that the average PSC furnace fan will have a power requirement about 190 W higher than an advanced motor (or other advanced air handler). The reduced power represents heat not rejected to the buildings air and, thus, relieves so
29、me compres- 13. We suspect that almost all furnaces for which EUR 6 also have ECM systems, but in combination with very high internal static pressures that require high wattages to move enough air. 434 ASHRAE Transactions: Symposia Tariffs U.S. Average New England Wisconsin Pacific NW, Mountains Ele
30、c, fkWh $0.08 $0.1 1 $0.08 $0.06 Electricity Saved, kwh Gas /therm $0.80 $0.96 $0.70 $0.90 Heating 500 548 617 685 A/C 200 131 125 125 sor load. For a COP - 3.3, this corresponds to an additional load reduction of 60 W or so, yielding a total demand reduc- tion of 250 W (0.25 kW) for an advanced fur
31、nace fan that supports a central air conditioner. Fan energy and compressor load savings in cooling season were computed by multiplying the demand reduction (kW) derived above by cooling-load hours (CLH) for particular states. We drew electricity and gas cost data from the Energy Information Adminis
32、tration (EIA) residential revenue for unit sources. For gas, we used national and state residential price data from the natural gas price series (EIA date unknown). For electricity, we used EIA (2000). Together, these data were the basis for conclusions on savings presented in Table 5. POTENTIAL AND
33、 PERFORMANCE The electricity use of furnaces is not regulated. For central air conditioners, the SEER test condition gives a 365 Wlcfm default criterion but sets no maximum on actual power use for the fan. Thus, it is useful to consider how much energy is actu- ally needed for the air-handling funct
34、ion, given ARI default I Total kWYr 700 679 742 810 $/yr $57 $75 $56 $49 Extra Gas Needed therms 19 23 23 22 external static considerations. Table 6 suggests that less than 200 W/lOOO cfm would be needed for common units if exter- nal static pressures were as low as assumed by ARI 210/240 (2003), an
35、d the intemal static pressures (with filters) are no higher than 0.5 in. W.C. (inches water column) (125 Pa). Actual unit air-handling efficiency (W/l O00 cfin) depends on design decisions by the manufacturer, particularly the internal static pressure drop across the filter, heat exchanger, and othe
36、r components of the unit. For example, if the internal static forthe 3-tonunit drops from 0.5 in. to 0.3 in., the estimated power requirement drops from 160 W/1000 cfm to 110 W/cfm. As expected, better performance has been demonstrated in laboratory prototypes that may have had both advanced motors
37、and reduced internal pressure drops. l4 Conversely, the higher than stipulated external static pressures measured in the field are correlated with average fan power of 470 W/1000 cfin (Proctor and Parker 2000). Net Saving GE Estimate 14. Report from manufacturer who requested anonymity. $15 $22 $9 $
38、20 $42 $53 $47 $29 $97 $146 $89 $9 1 ASH RAE Transactions: Symposia 435 Table 6. Air-Handler Power Requirements for 3-TOn and 5-TOn Systems* Parameter External static pressure? Internal static pressure Total static 3-Ton 5-Ton Units 0.15 (37) 0.2 (50) in. water column (Pa) 0.5 (125) 0.5 (125) in. wa
39、ter column (Pa) 0.65 (162) 0.7 (175) in. water column (Pa) Air supplied through system Conversion factor I Power needed. ho I 0.12 I 0.22 I hv delivered to air I 1200 2000 cfm 6350 6350 (cfrn) * (in. water)hp Conversion factor Power needed, W I an eficiencv I 65% I 65% I I 746 746 WhP 92 164 W deliv
40、ered to air Calculated Values Power required to fan for 1 O00 cfm 141 253 W Assume AM external static pressure requirements (AR1 210/240), stipulated internal static for filter and heat exchangers, 400 cfmton air supply, 65% fan efficiency, and 74% fan motor efficiency. This approach was suggested b
41、y Prof. S. Kavanaugh, Mechanical Engineering, University of Alabama. As in test conditions of AR1 (2003, Table 6). Motor input Motor efficiency required W/lOOO cfm RESULTS: ECONOMICS 190 340 W 74% 74% 160 170 W/lOOO ch Unit Savings Clearly, a more efficient air handler will save electricity and, thu
42、s, save money for the consumer. On the other hand, the air-handler fan and motor are located in the airstream. All of the energy delivered to the fan motor is ultimately dissipated as heat in the airstream. Thus, a more efficient motor actually means that less heating energy is delivered to the hous
43、e. All other things being equal (since AFUE does not reflect this electricity use), more gas will be burned to compensate. On the other hand, during the cooling season, the more efficient air handler will improve air-conditioner efficiency by reducing parasites. Table 5, from Sachs and Smith (2003),
44、 considers these gains and losses to project energy and dollar savings for three colder regions. It also includes a national savings aver- age. As noted above, it has been more difficult to estimate savings in regions where furnaces are sold with relatively large fans to meet larger cooling loads, s
45、o the national estimate is rougher. Nonetheless, on average, these estimates are lower than those by a major motor manufacturer (GE 2001) by a factor of 2.4, so we consider them conservative. Two other savings estimates are available. In a study of 3 1 Wisconsin houses, Pigg (2003) found that ECPM m
46、otors yielded only about 450 kWh electricity savings per year for heating and cooling operation, with the median ECPM furnace using about half of the electricity per therm for heating compared to the median non-ECPM furnace if the fan was not operated in continuous ventilation mode. However, Pigg (2
47、003) also noted that “electricity use for heating operation was higher on average than standard rating data would indicate for ECPM furnaces; this is most likely due to generally higher static pressures encountered in the field compared to rating test conditions.” When external static pressures are
48、greater than the ARI (2003) defaults, the PSC motor may use less elec- tricity, but the ECPM motors tested by Pigg will increase power to maintain the preset airflow. Looked at from another perspective, we infer that the ECPM motors saved less because they actually provided the airflow services to d
49、istant rooms that the PSC motors could not adequately serve. This implies that the ECPM motor can increase gas use if it provides more uniformly warm temperatures in the house during heating than the PSC. In a field study with two identical unoccupied houses equipped with mid-efficiency furnaces, Gusdorf et al. (2002) alternated between PSC and ECPM motors in the test house, thereby keeping all other variables constant. Their work suggests very large electricity savings (up to 74%), but the results are not directly applicable, since their test condition
