Best Practice Guide

Local Governments

Introduction

Planning an Energy Program

Funding and Approval

Education

Lighting

Central HVAC System

Heating Subsystem

Natural Ventilation and Air Infiltration

Optimization

Condenser/Chilled Water Subsystem

Controls and Load Reduction

Air Handling Subsystem

Procurement Policy

Building Envelope

Demand Response

Distributed Generation

Water Use

Motors

Food Service

Resources and Appendices

Lessons Learned


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Central HVAC System

Air Handling Subsystem

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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.

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.


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.