HVAC engineers balance operating costs versus initial costs in their designs every day. One area where this occurs is ducted air systems and their associated fan motors. An air distribution system could be designed with relatively small ductwork and equipment to keep initial costs lower, but the result is higher static pressures and a higher operating fan horsepower. On the other hand, an oversized air distribution system may have lower operating costs, but comes with a higher initial price tag. For many years, ASHRAE Standards have included fan power limitation guidelines to help engineers balance these competing goals. In 2009, Pennsylvania adopted the 2009 International Energy Conservation Code (IECC) that reflects the ASHRAE Standard. This code requirement has become more of a challenge over the last few years with the growing popularity of airside devices, like higher level filtration media and energy recovery, along with economic pressures to keep initial costs low.
The fan power limitations in the 2009 IECC may have noticeable impacts on your HVAC system designs.
Larger Equipment and Ductwork
The largest air pressure drops in the system occur within the air handling equipment. Heating and cooling coils may have larger face areas than in the past in order to keep air pressure drops lower. The result is equipment with larger cabinet sizes, but lower internal static pressure drops.
The code also restricts the nameplate horsepower of the selected fan so that it is no larger than the next available motor size greater than the calculated power consumption. This prevents fan motors from being oversized and less efficient at their operating point.
There are two fan power limitation calculation methods that can be used. The first restricts the nameplate horsepower based on an equation with airflow quantity as the only variable. The second method limits the system brake horsepower but allows adjustments or credits for a variety of typical devices such as filtration, heat recovery devices, and sound attenuation.
The code states that all fans in a system are included in the total power limitation including supply, return, and even fan-powered terminal units. Exceptions exist for individual exhaust fans below one horsepower and for fume hood exhaust fans. Helpful calculation adjustments are included for hospital and laboratory systems were pressure relationships are required.
The overall goal of the fan power limitation is to reduce operating power consumption and increase system efficiency. However, every design needs to be functional and consider initial costs. If you need assistance in understanding more about fan power limitations or have questions for your next project, please do not hesitate to contact Douglas C. Barnhart, PE at (717) 845-7654 or firstname.lastname@example.org.