Table of Contents
- Introduction
- Description of Technology
- Key Components
- Safety Issues
- Maintenance of Technology
- Maintenance Checklist
- Performance Monitoring
- O&M Costs
- Additional Support
- Sources of Information
Introduction
The term “economizer” is used to refer to several different devices within the field of heating, ventilation, air conditioning, and refrigeration.1 This operations and maintenance (O&M) summary focuses specifically on air-side economizers (outdoor-air economizers). These devices supplement compressor-based mechanical cooling with “free cooling” provided by fresh air when delivering this fresh air requires less energy than conditioning the building’s return air. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) 90.1-2019 defines an air-side economizer as:
A duct and damper arrangement and automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather.
In most climates, the cost of installing air-side economizers in both new and existing buildings is easily recovered in the resulting energy savings. Their cost effectiveness has made air-side economizers common throughout the United States. However, realizing the potential savings requires that these devices are operated correctly and maintained adequately. This guide provides an overview of key concepts that will help building operators and managers maintain air-side economizing equipment.
Description of Technology
An air-side economizer is a duct and damper arrangement with a control system that enables the heating, ventilation, and air conditioning (HVAC) system to use outdoor air to meet the cooling load when outdoor conditions are favorable. Buildings typically require cooling to maintain comfortable indoor conditions even during mild conditions (e.g., when the outdoor temperature is 50–60 °F). Without an economizer, these buildings will typically use a fixed outdoor-air damper that provides a fraction of the ventilation’s total air volume to maintain indoor air quality. This outdoor-air damper is configured to meet the space’s calculated ventilation requirements (set as low as possible) to limit the amount of outdoor air that must be cooled in warm and hot conditions or heated in cold conditions. Thus, even in mild conditions when the outdoor air would require very little cooling, the air stream delivered to the cooling coil is predominantly returned from the building’s interior. A properly functioning economizer will supply various percentages of outdoor air, ranging from the minimum ventilation requirement to 100%, to the cooling coil (see Figure 1) when cooling the outdoor air requires less energy than cooling the return air. The economizer therefore reduces the mechanical cooling required.
The control algorithm and setpoints are an important part of proper air-side economizer operation. If controlled incorrectly, the economizer can supply too much outdoor air when outdoor conditions are not favorable (CERTs, n.d.). This will lead to increased energy consumption during both hot and cold conditions and may place hydronic systems at risk of freezing when the outdoor air is below 32 °F. When determining which conditions are favorable for economizing, the outdoor humidity may be considered in addition to the dry-bulb temperature. Economizers can therefore be controlled using either enthalpy (calculated using both temperature and humidity) or the dry-bulb temperature (ASHRAE 90.1). The intended operation and control of the economizer must be understood to perform proper maintenance. Regardless of whether enthalpy or dry-bulb temperature is used to control the economizer, achieving the potential energy and cost savings requires that the economizer be set up correctly and maintained for sustainable performance.
Key Components
An air-side economizer is considered part of the air-handling equipment in a commercial HVAC system and relies on air-handling systems (e.g., the ductwork, relief/exhaust dampers, and blowers/fans) to function correctly. However, this Best Practice will only address those components that are directly part of the economizer. The O&M Best Practices for other systems should also be consulted to maintain the HVAC system as a whole (Unitary HVAC Equipment; Variable Air Volume Systems). For the purposes of this guide, the air-side economizer consists of dampers and associated actuators, sensors, and the controller that uses the sensor information to determine the damper position. Figure 1 depicts a typical air-handling layout and labels the components of the economizer. However, these components vary considerably across the industry depending on equipment type (e.g., packaged rooftop units vs. built-up air-handling units), equipment size, and building type. The remainder of this section will discuss economizer components and highlight different component configurations that may be encountered.
Dampers and Actuators. Air-side economizers use an outdoor-air damper and a return-air damper to meet the building cooling load when outdoor conditions are favorable. As Figure 1 shows, the return-air damper controls how much air is returned from the conditioned building spaces and cycled again through the building. The outdoor-air damper controls how much outdoor air is allowed to mix with this return air before it is conditioned and delivered to the building. When outdoor conditions are favorable, the economizer will open the outdoor-air damper and simultaneously close the return-air damper. To accomplish this, both dampers must be controlled with one or more actuators that receive the same signal or have signals that are synchronized with each other. The actuator has historically been a motor or actuator and linkage assembly; although in the last two decades, positive, gear-driven dampers have become more of the status quo. Gear-driven dampers have less slop in the linkage connection and perform better, ensuring a fully closed position. The linkage or gears move the damper blades in response to the actuator movement so that the damper opens and closes as the actuator rotates clockwise or counterclockwise. Some actuators include a mechanical spring that will cause the damper to return to its fail-safe position. The fail-safe position for outdoor-air dampers is the normally closed position when the electric actuator is not powered. Packaged rooftop units typically have an economizer damper accessory that links the outdoor-air and return-air dampers together and allows them to be controlled with a single actuator. On the other hand, built-up air handlers will typically use two separate actuators. In addition to the outdoor-air and return-air dampers, most systems include a relief or exhaust damper that operates in parallel to the outdoor-air damper. The relief damper prevents the building from becoming over-pressurized and allows more outdoor air to be mixed with the return air. During operating hours, the outdoor damper is usually set up to remain slightly open, even when outdoor conditions are not favorable for economizing, to provide the required minimum level of outdoor ventilation air in the building (ASHRAE 62.1). The intended minimum damper position specified in the design of the economizer must be understood to verify the economizer’s performance. If the outdoor-air minimum is excessive, this will result in excessive heating and cooling energy used to condition the outdoor air when the economizer is not active.
Controller and Sensors. Air-side economizers may be configured to use one of several potential control strategies (PNNL, 2020a; PNNL, n.d.), and the control strategy determines which sensors are needed. Two basic control functions are required: (1) activate the economizer only when there is a call for cooling and when outdoor conditions are favorable to provide free cooling; and (2) modulate the economizer dampers so that the air supplied is not so cold that comfort complaints or freeze conditions result. The most basic limit control requires an outdoor dry-bulb temperature sensor, and the controller will enable the economizing mode when the outdoor temperature falls within an allowable range. If a return-air temperature sensor is installed, then the controller may enable the economizing mode when the outdoor temperature is lower than the return temperature. In addition, a mixed air temperature sensor or supply air sensor may be used so the controller can modulate the outdoor-air damper to achieve a desired mixed air or supply air temperature. For the outdoor-air and return-air streams, a humidity sensor may be installed in addition to the dry-bulb temperature sensor. The dry-bulb and humidity values are used to calculate an enthalpy value that may be used to control the economizer instead of the temperature. An economizer controller can either be “integrated” or “non-integrated;” an integrated controller refers to a control algorithm that is programmed into the HVAC controllers, and a non-integrated controller refers to a dedicated hardware module that houses the control algorithm.
When humidity sensors are used to determine outdoor or return-air enthalpy, they require regular calibration. If humidity sensors are not regularly calibrated, then large errors can result in improper economizer operation. If maintenance resources are not available for regular humidity sensor calibration, then reverting to a dry-bulb temperature economizer limit strategy may improve operation.
Safety Issues
Operation, maintenance, and diagnostics of air-side economizers require knowledgeable and experienced personnel. Standard electrical, mechanical, and other safety practices apply to these systems. Hazards to maintenance personnel include the electrical and mechanical dangers inherent in mechanical systems powered by high-voltage electricity, as well as environmental hazards associated with the location of this equipment (e.g., accessing rooftops, mechanical rooms, and basements).
Maintenance of Technology
Without proper maintenance, air-side economizers may result in poor indoor air quality or significant energy waste and costs. However, these issues may be left unnoticed or undiagnosed for an extended period of time. Therefore, a run-to-failure maintenance approach is not appropriate for air-side economizers, and regular preventive maintenance must be performed.
The air-side economizer is only one part of a much larger HVAC system and therefore depends on the proper function of HVAC components, such as ductwork, exhaust/relief dampers, and fans. Excessive vibration, rust, mold, fan failure, clogged intake-air screens, and clogged filters can all affect economizer operation. Maintenance guides for HVAC units and variable air volume systems, as well as ASHRAE 180-2018, provide guidance for maintaining these HVAC systems. Proper documentation of maintenance activities and observations is essential. Large buildings typically have many air-handling units or rooftop units, and each may have its own economizer. An organized documentation system such as a computerized maintenance management system, makes sure preventive maintenance is performed in a timely manner and that the issues with specific systems may be referenced later.
Maintenance Checklist
A sample maintenance checklist for air-side economizers is provided in Table 1. However, these activities should be adjusted to meet the needs of specific equipment. Consider the following examples of how these activities may vary across the industry:
- Validating sensor measurements will be simpler when the sensors are integrated in a building automation system (BAS). If not integrated in a BAS, the local controller may need to be accessed to obtain the measurement. Recently, building codes have required that a basic level of fault detection be included with both dedicated local and central building management system economizer controls. These can speed operational review and alert maintenance staff to sensor or damper control problems.
- The return-air damper and outdoor-air damper may be physically linked in a rooftop unit and the adjoining linkage should be maintained. In a built-up air handling unit, each damper may have its own actuator and linkage to maintain.
- Systems located in marine environments are especially susceptible to rust and other forms of corrosion. Maintenance activities may need to be scheduled more regularly in such an environment.
The checklist in Table 1 provides a foundation and a starting point. However, the checklist does not replace manufacturer recommendations and contracted O&M services. All maintenance should be performed by trained and authorized personnel.
Table 1. Sample maintenance checklist for air-side economizers.
Performance Monitoring
Monitoring all HVAC systems is important to make sure proper operation. The most common method for performance monitoring involves trending data points using the BAS. The most important data points include the following:
- Outdoor-air temperature (and humidity);
- Return-air temperature (and humidity);
- Mixed-air or supply-air temperature;
- Outdoor damper position;
- Return damper position; and
- Zone CO2 levels, if available.
O&M Cost
ASHRAE offers a web-based public database with current information on service life and maintenance costs of typical HVAC equipment: (https://xp20.ashrae.org/publicdatabase/default.asp). While this database does not address air-side economizers specifically, the database does provide insight into how O&M costs vary by building type and geographical location. The database also provides insight into the expected service life of typical HVAC systems, such as unitary systems and air handling units.
Additional Support
Because economizer systems are part of a larger HVAC system, specific support comes in the form of training opportunities for larger HVAC systems. To encourage quality O&M, building engineers can refer to ASHRAE/ACCA Standard 180.
Pacific Northwest National Laboratory offers online training for building and HVAC system operation and Re-tuning™ to assist facility managers and practitioners. This training covers many system types but specifically addresses air-side economizer systems, how they work, and opportunities for efficiency.
More information on this training can be found at: https://buildingretuning.pnnl.gov/
Sources of Information
ASHRAE 62.1-2019. Ventilation and Acceptable Indoor Air Quality. The American Society of American Heating Refrigeration and Air Conditioning Engineers, Atlanta, GA. https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2
ASHRAE 90.1-2019. Energy Standard for Buildings Except Low-Rise Residential Buildings. The American Society of American Heating Refrigeration and Air Conditioning Engineers, Atlanta, GA. https://www.ashrae.org/technical-resources/bookstore/standard-90-1
ASHRAE 180-2018. Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems. The American Society of American Heating Refrigeration and Air Conditioning Engineers, Atlanta, GA. https://www.ashrae.org/technical-resources/bookstore/standards-180-and-211
CERTs – Clean Energy Resource Teams. Preventative Maintenance for Heating, Cooling and Refrigeration Systems. Clean Energy Resource Teams, St. Paul, MN. https://www.cleanenergyresourceteams.org/sites/default/files/FSELP_PreventativeMaintenance.pdf.
PNNL – Pacific Northwest National Laboratory. n.d. Building Re-Tuning Training Guide: Air-Side Economizer Operation. PNNL-SA-86706. Pacific Northwest National Laboratory, Richland, WA. https://buildingretuning.pnnl.gov/documents/pnnl_sa_86706.pdf
PNNL – Pacific Northwest National Laboratory. 2020a. Re-tuning Commercial Buildings. Pacific Northwest National Laboratory, Richland, WA. https://buildingretuning.pnnl.gov/retuning_measures.stm
PNNL – Pacific Northwest National Laboratory. 2020b. Best Practices for Unitary HVAC Equipment Operation and Maintenance. PNNL-SA-141468. Federal Energy Management Program (FEMP) O&M Program. https://www.pnnl.gov/projects/best-practices/unitary-hvac-equipment
PNNL – Pacific Northwest National Laboratory. 2020c. Best Practices for Variable Air Volume (VAV) Systems Operations and Maintenance. PNNL-SA-141526. Federal Energy Management Program (FEMP) O&M Program. https://www.pnnl.gov/projects/best-practices/variable-air-volume-systems
1The term “economizer” is used in reference to vapor compression systems, boilers, condensate energy recovery, hydronic plants, and air distribution. An economizer within a vapor compression refrigeration system increases both the cooling capacity and efficiency of the system. A boiler economizer improves efficiency by preheating the boiler feedwater using flue gas. A water-side HVAC economizer allows heat to be transferred directly from the condenser water loop to the chilled water loop. An air-side HVAC economizer increases the fresh air intake when outdoor conditions are suitable.
Actions and activities recommended in this Best Practice should only be attempted by trained and certified personnel. If such personnel are not available, the actions recommended here should not be initiated.
Published May 2021