Special report: Fan efficiency guidelines
New and proposed fan-efficiency provisions in commercial energy codes and standards are fostering cost-effective energy savings in HVAC systems.
Despite the fact that fans in commercial HVAC systems consume more than 1 Quad of energy (1015 Btus) annually in the U.S., they have not had explicit efficiency requirements in federal regulations or model codes and standards for energy efficiency and high-performance/green construction.
Those days are over.
The 2012 International Green Construction Code (IgCC) and ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings (2013 Edition) have requirements for minimum fan efficiency. These requirements are based on a standard published by the Air Movement and Control Association International (AMCA International), AMCA 205: Energy Efficiency Classification for Fans. AMCA 205 was first published in 2010, and its 2012 revision is ANSI accredited. The 2012 IgCC’s fan efficiency provisions are based on AMCA 205-10; ASHRAE 90.1-2013’s provisions are based on ANSI/AMCA 205-2012.
Additionally, AMCA and ASHRAE collaborated on fan-efficiency proposals for the 2015 International Energy Conservation Code (IECC), which were based on the ASHRAE 90.1-2013 language. The proposals cleared the first level of hearings in April 2013 and will undergo public review later in the year leading up to final-action hearings in October. AMCA International also is developing a proposal for ASHRAE 189.1: Standard for Construction of High-Performance, Green Buildings Except Low-Rise Residential Buildings.
Meanwhile, the U.S. Dept. of Energy recently initiated the development of a federal efficiency regulation for commercial and industrial fans, with completion of the regulation expected in 2015/2016 and enforcement beginning as early as 2019/2020.
This article describes the fan efficiency provisions that are in place in IgCC-2012 and ASHRAE 90.1-2013, and briefly discusses what could lie ahead in future codes, standards, and regulations.
Rating fan efficiency
AMCA 205 defines a metric, called a fan efficiency grade (FEG), that rates a fan’s ability to convert shaft power to air power, independent of motors and drives. FEGs are indices calculated from data taken at the peak total efficiency point on a fan curve developed during ratings certification tests.
AMCA developed FEGs as a dimensionless index to characterize the aerodynamic quality of a fan. The metric accounts for the reduced peak total efficiency that occurs for smaller fans compared to that of larger fans of the same type. This characteristic is due to nongeometric manufacturing tolerances, disproportionate bearing losses, and other aerodynamic factors that have a greater impact on smaller fans than on larger fans. When plotted as a graph, the differences in efficiency across fan sizes define a banana-shaped curve. The nature of this curve prohibits setting a straight-line efficiency requirement (e.g., all fans must have a minimum efficiency of 65%) because doing so would eliminate many smaller fan sizes of even the most efficient types of fans (see Figure 1). Smaller fans inherently have smaller efficiencies because bearing losses, manufacturing tolerances, and the fan structure all have a larger impact than they do for larger fans of the same design. The smaller fans, however, are designed for specific applications and duty points (airflow and pressure). Eliminating them wholesale via an efficiency standard would not serve the industry well.
Figure 1 also superimposes FEG curves defined by AMCA 205 on the straight-line efficiency of 65%. Setting a fan efficiency requirement based on FEGs is as simple as a straight-line efficiency requirement (e.g., all fans must have a minimum efficiency of FEG 67). Note in Figure 1 how the FEG curves penetrate the box that defines the smaller sizes that would have been prohibited by a straight-line efficiency requirement.
The FEG curves are defined in such a way that all fans of a particular design, having geometric proportionality, should have the same FEG, although there are sometimes exceptions to this rule.
If an energy code has a minimum fan efficiency requirement of FEG 67, any fan model with that rating or higher will comply. The FEG is a simple metric to segregate fans that do or do not meet a specific code requirement or regulation. Readers can learn more about AMCA 205 and FEGs at www.amca.org/feg/best-practices.aspx, and download a free copy of the standard at www.amca.org/feg/codes-and-standards.aspx.
Limiting sizing/selection practice
Commercial HVAC fans are usually sized and selected using software that yields a range of fan sizes for a specific fan model for given airflow (cfm) and pressure conditions. Construction budgets generally favor lowest-first-cost approaches, so the smallest fan size is generally selected. Although they may have the same FEG rating (as described earlier), the difference in actual efficiency and energy performance between the smallest and largest fan sizes is considerable.
Consequently, setting a minimum fan efficiency grade will not guarantee reduced fan-energy consumption unless care is taken to properly design the air distribution system and an appropriate fan selection is made. For this reason, AMCA 205 also prescribes that fans should be sized and selected to operate within 15 percentage points of the fan’s peak total efficiency. The sizing/selection window helps practitioners to right-size fans so they operate in their most efficient ranges of speed and pressure. The result is a higher first cost, but energy savings quickly recoup the higher cost. Table 1 shows the output of a manufacturer’s sizing/selection program for a double-width, double-inlet fan sized/selected for 80,000 cfm at 3-in. static pressure. The operating costs are based on a run time of 16 hr per day, 250 days per year, and electricity cost of $0.10 per kWh.