Manufacturers Best Practice Guide

Central HVAC System: Subsystems

Condenser Chilled Water Subsystem
Chillers are the most expensive piece of the HVAC puzzle and tend to have a long life. Because of this, owners and plant managers should decide whether it is more cost-effective to retrofit an existing chiller or replace it entirely with a new, more efficient model.

Older systems tend to have oversized chillers that waste energy without necessarily providing greater comfort. Older, oversized systems are inefficient, consuming as much as twice the energy as their newer, right-sized counterparts. Chillers that have been poorly or irregularly maintained waste even more energy, increasing operation and maintenance costs. High efficiency chillers can also cut peak load by as much as 40% over low efficiency units.

Chillers less than 10 years old can usually be cost-effectively retrofitted rather than replaced. The exception is if cooling loads have been significantly reduced through other upgrades, such as a lighting retrofit. Operational and maintenance savings resulting from a downsized chiller equipped with VFDs in most cases will make up for the higher initial cost of replacement. Look closely at lifecycle costs and potential energy savings to help to determine the most cost-effective option.


Air Handling Subsystem
Fans, ducts and dampers that move air through a building are 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, more than half of all buildings have oversized fans that waste as much as 60% 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%.

Air handling subsystems are fixed at either a constant air volume or variable air volume (VAV). As the name implies, VAV systems enable fans to adjust according to actual heating and cooling loads, rather than running full-on or full-off. Installing variable-speed drives (VSDs) on VAV fans and motors makes sense for motors that run long hours or have fluctuating loads - the larger the motor, the bigger the possible savings. Adjust the air handling subsystem to the lowest point possible without compromising occupant comfort or product safety, and while continuing to meet local codes and requirements for air quality. This reduced flow can result in energy savings of up to 50%. 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 replacing the unit is not cost effective, consider improving efficiency by rightsizing fans and replacing fan V-belts with synchronous belts that help reduce slippage and wear.

Some air handling subsystems include an economizer that provides free cooling by automatically drawing in outside air when it is cooler than return air. When outside 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.