What you need to know about ASHRAE 90.1-2022, FEG, and FEI - Part I

ASHRAE 90.1 revisions over time have reduced HVAC energy consumption in nonresidential buildings by 50% since the standard's introduction in 1975.  Like software updates that incorporate improvements and bug fixes, so it is with standards and code revisions. Hence this is the reason to update your project specifications to the latest standard revisions of ASHRAE 90.1.

The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) updates 90.1 every three years; the previous issue was in 2019. But what happens if a significant revision needs to be made prior to a third-year revision? ASHRAE issues an addendum. The 2022 edition of Standard 90.1 incorporates over 80 addenda to the 2019 edition. An example of the benefit of addenda and revisions to 90.1-2019 is the replacement of the FEG (Fan Efficiency Grade) metric with the FEI (Fan Energy Index) metric. The following explains why this metric change was so important to saving fan power consumption.

Fan Efficiency Grade (FEG) was developed by the Air Movement and Control Association International, Inc. (AMCA) in support of a request from ASHRAE Standard 90.1 that could be used to establish minimum acceptable fan efficiency. FEGs, as defined in AMCA 205, are designed to be a simple system to indicate the aerodynamic quality of the fan and are based on the fan’s peak total efficiency.

The total efficiency (TE) is calculated using the traditional airflow, total pressure, and input power as measured per AMCA Standard 210. It does not take into effect the efficiency of the drive (belt drive) or the motor. Fan efficiency is defined as the air power divided by the fan input power. Both Static and Total Efficiency can be calculated from fan performance data and can be plotted along with the fan curve (Figure 1).

Figure 1: Fan curve plotting both Static and Total Efficiency

The peak total efficiency occurs at the top of the “bell” shaped efficiency curve. This peak efficiency is used to determine the FEG value. Note that the peak efficiency occurs at just one point on the curve and all other points on the curve have a lower efficiency.

It is important to understand, as the efficiency curves illustrate, that each fan has a large range of efficiencies depending on the airflow and pressure of the operating point.

For example, a fan with a peak efficiency of 70% can easily be selected to operate at a point of only 50% efficiency. Another aspect of AMCA FEGs is that their value depends on the fan size. Note that smaller fans are inherently less efficient than larger fans due to running clearances between parts cannot be held as tightly in proportion to other dimensions as on larger fans.

The AMCA FEG curves have been established such that fans of a given model that are geometrically similar will each have the same, or nearly the same, grade. Once the Peak TE is known, the FEG value can be determined from AMCA Standard 205-12. (Figure 2)

Figure 2: Example of AMCA FEG Curve

Limitations of The FEG Metric That Resulted In It Being Replaced By The FEI Metric

Referring to Figure 2, a 24.5-inch diameter fan with a Peak TE of 69% would be classified as an FEG71. Note that a 12-inch diameter fan with a Peak TE of 60% is also FEG71. In this example, the smaller fan is the same efficiency as the larger fan, when as noted earlier, smaller fans would inherently be less efficient. Another significant shortcoming of the FEG metric is that the highest FEG fan value does not necessarily result in the lowest energy consumption. Table 1 below illustrates this point. Notice that the 72-inch fan requires the least energy (lowest BHP). Yet, the 48-inch fan has a greater total efficiency (66% vs. 60%) and a higher FEG (71 vs. 63).

So how can the fan with a higher efficiency consume more than twice the power? First, the FEG is based on fan total efficiency and fan total pressure. Total pressure is used because it is a measure of the total energy imparted to the air. However, the velocity pressure exiting a fan can only be used when it is contained in a duct – and is lost on non-ducted fans. This makes FEG an inappropriate and often misleading metric for many fan applications, such as sidewall propeller fans, powered roof ventilators (PRVs), and plenum fans. Static efficiency will correlate to power consumption for fans without a discharge duct.

For these reasons, the FEG metric cannot be applied universally to all fan selections to minimize power consumption and needs to be replaced with another metric in ASHRAE 90.1-2019.

Our next blog post will discuss the revised fan energy metric of ASHRAE 90.1-2022: FEI-Fan Energy Index.