Best Practice Guide

Commercial Office Buildings

Central HVAC System

Air Handling Subsystem

Windows at NRDC are fully operable, allowing for natural ventilation. Photo courtesy of NRDC.View Details.

Fans that move conditioned air consume about seven percent of total electrical energy in a building. Along with the ducts and dampers, this equipment is collectively known as the air handling subsystem of the HVAC system.

As with chillers, older central HVAC systems tend to have oversized fans. In fact, nearly 60 percent of all buildings have oversized fans that waste as much as 60 percent of the energy consumed. This money is literally blown out the door. Rightsizing fans removes excess capacity and lowers energy consumption of the air handling subsystem by an average of 50 percent.

Irvine Center Towers. Photo courtesy of Transwestern Commercial Services.View Details.

Air handling subsystems are fixed at either a constant air volume or variable air volume (VAV). As the name implies, VAV subsystems enable fans to adjust according to the actual needs of building occupants. Additional energy savings can be made by installing variable speed drives (VSD) on VAV fans and motors. VSDs make sense for motors that run long hours or have fluctuating loads - the larger the motor, the bigger the savings possible. Adjust the air handling subsystem to the lowest point possible without compromising occupant comfort and while continuing to meet local codes and requirements for air quality. This reduced flow can result in energy savings of up to 50 percent. Preset two or more pressure settings, e.g., daytime/nighttime or summer/winter, to maximize savings throughout the day and year.

Unfortunately, it can be quite expensive to replace a constant volume unit with a VAV. If it is not cost-effective to replace, improvements can still be made by rightsizing fans and replacing fan V-belts with synchronous belts, which help reduce slippage and wear.

Some air handling subsystems include an air economizer that automatically intakes air when the air is cooler than the return air, taking advantage of this free source of cooling. When outdoor temperatures rise, a damper closes vents so that only the minimum amount of outdoor air needed to maintain air quality is allowed to enter. Air economizers can also be used to pre-cool a building in the evening and early morning hours, further reducing the need to run cooling equipment.

Business Examples

Even though the Natural Resources Defense Council relies on natural ventilation instead of an air handling system, sensors are installed to monitor CO2 levels at its Santa Monica offices. When more ventilation is needed, a warning light alerts staff to open a window so that fresh air can circulate.

Transwestern Commercial Services installed variable frequency drives on the fan system at Irvine Center Towers as part of a wider HVAC retrofit. This allows facility staff to take full advantage of HVAC upgrades.

Air economizers are outfitted on all buildings at Adobe Towers in San Jose. This allows the air conditioning and distribution systems to maximize the use of outside air most of the year, improving indoor air quality while reducing heating and cooling costs. At no time is less than 20 percent outside air used; more typically, this is in the 80 to 90 percent range. The air circulation system has a minimum setting as well as carbon dioxide sensors that monitor air quality in office towers and parking garages. As such, air circulation changes-out air within the building about five times each hour.


Cushman & Wakefield, the facilities management company, oversees air-sampling testing conducted by a third-party environmental testing service two times annually. Findings to date have shown that indoor air quality at Adobe Towers is superior to ASHRAE and Cal/OSHA standards, and cleaner within the buildings than outdoors for substances tested, including: carbon dioxide, carbon monoxide, ozone, fungi, and formaldehyde, the most common indoor air contaminants.


A variable frequency drive was added to the fan system in one tower, so that now all three office towers are capable of adjusting air volume and fan power to actual building loads. This retrofit cost Adobe $126,960 to install, but received a $63,500 rebate, achieving payback on investment in less than 10 months. Energy cost savings are estimated at more than $78,000 per year for this single measure.


Resources

Flex Your Power Product Guide: Ducts and Duct Sealing

U.S. Environmental Protection Agency, ENERGY STAR Building Manual: Fan System Upgrades.

ClimateQuest, downloadable calculators that 1) analyze cost savings from an economizer, and 2) analyze ventilation loads. Website also has articles related to HVAC and heat recovery as well as historical weather data.

Air Infiltration and Ventilation Center, international technical and information forum covering the areas of ventilation and air infiltration in the built environment with respect to efficient energy use, good indoor air quality and thermal comfort.