Date: 2021-11-01 14:18:50
In a highly unusual development, a law was passed in 2020 to change the metric used to rate the efficiency of large-diameter ceiling fans made for sale in the United States and Canada.
By the AMCA INTERNATIONAL NORTH AMERICA REGION AIR MOVEMENT ADVOCACY COMMITTEE
Editor’s note: This article is adapted from the AMCA International white paper “Introducing Ceiling Fan Energy Index (CFEI) and Changes to the U.S. Regulation for Large-Diameter Ceiling Fans.” For the full white paper and related resources, go to the AMCA International large-diameter-ceiling-fan microsite at www.amca.org/ldcf.
On May 27, 2021, the U.S. Department of Energy (DOE) published in the Federal Register a final rule codifying the change of the regulatory metric for large-diameter ceiling fans (LDCF) from cubic feet per minute per Watt (cfm/W) to ceiling fan energy index (CFEI).
The change is a result of the Ceiling Fan Improvement Act, part of the Energy Act of 2020 provision in the omnibus bill signed into law on Dec. 27, 2020. The Ceiling Fan Improvement Act resolves constraints of the cfm/W metric on high-performing LDCF while making passage of low-performing LDCF from a simple reduction in speed more difficult.
This article describes cfm/W and explains why a change in metric was needed.
Part 1: The Problematic cfm/W Metric
The DOE defines LDCF as a ceiling fan with a blade span greater than 7 ft (2.1 m). Blade span is fan diameter plus the extent to which fan diameter is enlarged by “wing tips” extending from the blades (Figure 1).
A different test procedure is used for ceiling fans with diameters less than or equal to 7 ft (2.1 m).
Photograph courtesy of Big Ass Fans
ANSI/AMCA Standard 230, Laboratory Methods of Testing Air Circulating Fans for Rating and Certification, describes how to perform laboratory measurements for LDCF and calculate cfm/W for a given speed, specifying that, for variable-flow LDCF, tests be conducted at five speeds: 20 percent, 40 percent, 60 percent, 80 percent, and 100 percent. An average cfm/W is not calculated as part of the ANSI/AMCA Standard 230 procedure.
ANSI/AMCA Standard 230 describes how to uniformly measure or calculate and report:
- Thrust (pound-force or newton)
- Airflow rate (cfm or cubic meters per second [m3/s])
- Power (W)
- Efficacy (cfm/W or m3/s/W at one speed)
- Efficiency (air power/electrical input power)
The initial DOE test procedure requires the same five speeds, but additionally requires that power consumption be measured while a fan is idle. In calculations of cfm/W, the five operating speeds are equally weighted by time (2.4 hr, for a total of 12 operating hours per day) and averaged. Standby-power time is estimated to be
12 hr per day.
FIGURE 1. “Wing tips” need to be included in measurements of blade span. Photographs courtesy of 4Front Engineered Solutions
cfm/W vs. CFEI. Table 1 compares cfm/W and CFEI.
TABLE 1. Comparison of cfm/W and CFEI.
Because of the relationship between power and airflow defined in the fan laws, cfm/W can be gamed more easily than can CFEI. Figure 2 shows airflow vs. power for five different 24-ft- (7.3 m) diameter fans. For a given airflow, the lower a curve on the chart, the more efficient (less power) the fan. Despite differences in true operating efficiency at any common duty point, each of the five fans has a rating of 234 cfm/W (0.110 [m3/s]/W). In contrast, CFEI ratings at high speed are dramatically different. The most efficient fan (lowest power for a given airflow) is Fan 1, which has a CFEI rating of 1.72. The least efficient fan (highest power for a given airflow) is Fan 5, which has a CFEI rating of 0.63. While both Fan 1 and Fan 5 would comply with the cfm/W minimum-efficiency requirement, Fan 1 would significantly exceed the DOE minimum-efficiency requirement of 1.00 at high speed, while Fan 5 would be non-compliant, as its CFEI is less than 1.00. Table 2 summarizes the CFEI and cfm/W ratings for each of the fans in Figure 2. As illustrated by this example, CFEI provides a better representation of how efficiently a LDCF performs.
FIGURE 2. Input power vs. airflow at five test speeds for five 24-ft fans with average cfm/W and CFEI at high speed shown in legend.
TABLE 2. Relative component efficiencies required to achieve 234 cfm/W at various airflows, 24-ft-diameter fan.
As previously discussed, the relationship between power and airflow dictated by the fan laws provides an inequitable efficiency requirement for LDCF. Fan 3 is representative of a current high-efficiency LDCF product. Fan 1 represents a high-airflow (for the given diameter) LDCF with the same cfm/W as Fan 3. Note that, despite increasing the airfoil efficiency by 10 percent (no small task) and the drive efficiency to 99 percent, the motor would have to increase its efficiency by 18 percent for Fan 1 to comply with the cfm/W requirements. This essentially makes Fan 1 impossible to manufacture. On the other hand, Fan 5 represents a low-airflow (for the given diameter) LDCF with the same cfm/W as Fan 3. Note that, even though both fans have the same cfm/W rating, Fan 5 has a 10-percent-less-efficient airfoil, a 35-percent-less-efficient motor, and a 4-percent-less-efficient drive. This gives Fan 5 a free pass on efficiency compliance that leaves a lot of potential energy savings on the table. It should be noted that Fan 5 would have to be made roughly 27 percent more efficient to comply with CFEI requirements at high speed.
CFEI was developed to make inefficient fans less likely to comply through the use of slower speeds, such as those used to game the cfm/W metric, and to remove the unintentional barrier to compliance for high-performing high-utility fans.
For discussion of CFEI and how it came to replace cfm/W, how to comply with CFEI requirements in the U.S. Code of Federal Regulations, and how CFEI is reflected in AMCA International standards, testing, and certification, see the full “Introducing Ceiling Fan Energy Index (CFEI) and Changes to the U.S. Regulation for Large-Diameter Ceiling Fans” white paper on the AMCA International large-diameter-ceiling-fan microsite at www.amca.org/ldcf.