14 Tips for Reducing Wasted HVAC Fan-System Energy

In 2010, Air Movement and Control Association International (AMCA) hosted a meeting with consulting engineers from the Chicago area and asked them where energy efficiency fell as a priority when sizing/selecting fans. They answered that what they really care about is whether a fan fits in the box and meets acoustical requirements. Efficiency, they said, was not a priority, but would become one if required by code.

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As described in the inaugural Associations Solutions column in the January 2014 issue of HPAC Engineering, codes are beginning to adopt fan-efficiency requirements. But codes alone won’t do much to save fan energy—even the most efficient fan will perform poorly if not sized properly. A few sharp turns immediately before or after a fan will kill performance, even if the fan is sized and selected properly.

This article provides design and specification tips for reducing wasted energy in air systems and for substantially reducing total cost of ownership.

Check air-delivery requirements against the owner’s project requirements. Consult with the owner and architect about how much airflow is needed, where it’s needed, and when. Minimizing airflows when possible can save substantial amounts of energy.

Minimize total pressure drop. Reducing system total pressure drop (fan pressure rise) also can save a lot of energy. This could mean larger ducts, larger air handlers, and more air handlers serving smaller spaces. The “90.1 User’s Manual”¹ describes fan power limits that depend in part on system pressure drop and has related guidance on their application.

Eliminate or greatly reduce system effects. For a fan to achieve its rated performance, the inlet airflow must be fully developed, symmetrical, and swirl-free. Ducting needs to be designed so that the asymmetrical flow profile exiting the fan can diffuse and approach fully developed flow. The effect on fan performance when these conditions are not met is called “system effect.” For example, if an elbow is placed close to the fan inlet, performance will be compromised. See “AMCA Publication 201-02 (R2011): Fans and Systems”² to learn more about system effects and how to account for them in fan-sizing/selection calculations.

Minimize the number of fittings, especially close-coupled ones. Engineers know that every bend or turn causes a total pressure loss, and coefficients for calculating these losses for standard fittings are available. These coefficients, however, are based on having fully developed airflow entering an individual fitting. When placed close together, the upstream fitting disrupts the airflow entering the downstream one, so the published coefficient does not correctly represent the pressure-loss change attributed to close-coupling. ASHRAE recently commissioned a research project to experimentally quantify the effects of close-coupling to provide guidance on determining associated coefficients. AMCA 201 provides guidance on proper use of air straighteners and turning vanes to reduce pressure losses in fittings.

Specify low-leakage systems and require post-installation leakage testing. Much has been published about duct leakage, but leakage from components such as air handlers, variable-air-volume (VAV) boxes, terminal units, and access doors also needs to be considered. Combining all sources is called “system leakage.” 2012 ASHRAE Handbook—HVAC Systems and Equipment³ and 2013 ASHRAE Handbook—Fundamentals⁴ offer fresh thinking on system-leakage impacts, requirements, and testing.

Right-size the fan. Because of fan and connected-system characteristics and associated aerodynamic effects, fans have a wide range of efficiencies over the range of system airflow and pressure operating points.

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TABLE 1. Output from fan-sizing/selection software offers a range of sizes to meet airflow and pressure requirements at a desired operating point. Note all sizes have the same fan-efficiency grade (FEG), but there’s a considerable difference in energy consumption. The yellow-highlighted row shows a typical fan selection. The green-highlighted row indicates more efficient and long-term cost-effective fan selection. The energy savings of the 60-in. fan will repay the higher first cost in less than two years.

A fan rated at 85 percent peak efficiency sometimes can operate at 50 percent efficiency. Fan-sizing/selection software will provide four to six different fans at the design point of operation. These options usually are a particular model of fan at different sizes. Smaller fans cost less, but consume more energy because they operate at higher speeds to attain the required airflow and pressure (Table 1). Part-load operation of variable-flow systems also should be considered: The goal is to minimize fan-system power during normal operation (by definition, design conditions almost never occur).

Know your tradeoffs and think of the future. Fan shaft power varies with the cube of speed for simple return and exhaust systems. As a result, running these types of fans with 10 percent more flow has an energy penalty of 33 percent. Energy impacts are somewhat smaller for VAV supply fans because of system-efficiency variations related to the presence of coils and filters (pressure-drop variation with flow is more linear than quadratic) and the use of a non-zero duct static-pressure set point. Selecting a smaller fan may be necessary to fit a configuration. But if the test-and-balance contractor has to increase speed to meet the airflow you specify, then a larger motor often will be required. It’s far more efficient to upsize the fan in your design to reduce energy use.

Be wary of VFD specifications. It’s well known that variable-frequency drives (VFDs) can save energy at part loads. However, if the test-and-balance contractor must set a VFD to higher speeds to get more airflow because either the fan was sized too small or the fan pressure rise needed was much greater than planned for, then the system operating range will be reduced. VFD-related energy and cost savings also might be significantly reduced, thus, changing VFD cost-effectiveness.

Use direct-drive motors/fans where possible and where belt maintenance is questionable. Belt drives are not always a bad thing—they can provide flexibility for adjusting fan speeds to match field conditions. But if a fan is located where adjusting the belt periodically will be difficult, it’s likely that it will rarely get done. In such cases, going with a direct-drive motor could be a good investment.

Specify the fan operation schedule for controls, especially for exhaust-only fans. Direct test-and-balance contractors and commissioning providers to check the fan operation schedule and include it in system and operation-and-maintenance documentation.

Use duct static-pressure set-point reset on VAV systems. At lower loads, design-condition static pressure is not needed. Reset duct pressures to satisfy the zone with the strongest call for cooling (so that its VAV box is wide open). Reset strategies are available now, even for older pneumatically controlled systems without zone-level DDC control.

Ensure energy-code compliance. ANSI/ASHRAE/IES 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings, and the 2015 International Energy Conservation Code have requirements for fan efficiency. Documenting fan-efficiency grades and peak total efficiency in fan schedules will document compliance.

Ensure design intent is followed through by the contractor. Specify the operating-point brake horsepower in your fan schedule and specify that fans must be licensed to bear the AMCA seal for air performance. Evaluate contractor and value-engineering substitutions. It’s not unheard of for smaller, lower-cost fans to be slipped into a project to reduce first costs.

Get out into the field. Visiting construction sites and occupied buildings to witness test-and-balance testing will demonstrate how your designs are being constructed, from duct runs to air handlers and fan installations. Check controls programming. Talk to owners and operators, contractors, and commissioning providers about how designs are impacted by reality throughout their life cycle.

References
1) ASHRAE. (2011). 90.1 user’s manual. Atlanta: ASHRAE.
2) AMCA. (2011). AMCA publication 201-02 (R2011): Fans and systems. Arlington Heights, IL: AMCA.
3) ASHRAE. (2012). ASHRAE handbook—HVAC systems and equipment (pp. 19.2-19.6). Atlanta: ASHRAE.
4) ASHRAE. (2013). 2013 ASHRAE handbook—Fundamentals (pp. 21.15-21.17). Atlanta: ASHRAE.

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Michael Ivanovich, AMCA

develops and advocates consensus positions on energy efficiency and green construction codes among Air Movement and Control Association International (AMCA) member companies. AMCA is a not-for-profit manufacturers association with more than 320 member companies in North and South America, Europe, Asia, and the Middle East.