How to select an air handling unit
Air handling units come in all shapes and sizes. Learn to balance and prioritize all of the choices related to performance, efficiency, maintainability, and space constraints.
- Know the different types of AHUs, and their basic anatomy.
- Understand the codes and standards that govern AHU specification.
- Learn about how energy can be saved in HVAC systems that use AHUs.
A basic definition of an air handling unit (AHU) might be “a box with a fan, coils, and filters.” From there it gets considerably more complicated. Proper selection of an air handler requires answering myriad questions ranging from “what capabilities are required?” to “will it fit?” Only after establishing these basic project constraints can the art of evaluating and selecting an AHU begin.
Before starting this process, it’s important to realize that there will not be a “perfect” selection for any AHU as many competing criteria, not the least being cost, will force compromises. It is the engineer’s job to balance and prioritize all of the decisions related to performance, efficiency, maintainability, and space constraints to select a unit that has the lowest lifecycle cost for a given application.
This article provides general information and guidance on the selection of various AHU components, starting with a brief description of the major categories of AHUs. While much of the discussion in the remainder of the article relates primarily to large AHUs, the general considerations can be applied to any size.
Types of AHUs
Fan coils/blower coils are the smallest and simplest category of AHU and, as the names imply, they typically consist of little more than a fan and a heat transfer coil(s). To keep the coils from getting dirty too quickly, a simple filter is also included. They generally have simple controls and serve a single temperature zone. While they have their applications, they are typically less efficient than larger AHUs and have difficulty providing tight temperature and humidity control.
Packaged AHUs are very common in smaller buildings and commercial applications, particularly as rooftop units. Packaged units generally contain fans, coils, filters, and dampers in a single casing. Often the casing includes its own air conditioning compressors and means for heating such as gas burners, electric heating coils, or heat pump coils. They often serve single temperature zones, but large variable air volume (VAV) AHUs serving multiple terminal boxes (zones) are available. Because of their compactness and lower initial costs, packaged units have a reputation for being inefficient and maintenance intensive, but performance and reliability are improving. They are available in sizes from a few thousand cfm to more than 30,000 cfm, but their standardization can be limiting in some applications.
Modular AHUs allow users to select individual components housed in modules having consistent construction and cross sections. The user can select the type of casings, fans, filters, coils, and accessories from a variety of different options. Modules are assembled at the factory or can be shipped individually and assembled on-site. Modular units generally allow great flexibility and can meet most air processing requirements.
Custom AHUs are available in nearly any configuration that a user might require. They generally have the highest quality construction and are most commonly used in institutional or industrial applications where high flow rates, very close control, and harsh conditions exist. They may also be applied in irregular spaces that would not conform to a modular line. Custom units can be configured to include virtually any combination of air processing components. They also can include walkways and service areas within them and can even accommodate space for skid-mounted equipment like pumps or heat exchangers. They are the most costly of all of the types of units discussed, but can be expected to have the longest lifespan.
Anatomy of an AHU
Casing and construction: A quality AHU can last more than 30 years with proper maintenance. Double-walled construction is now standard for all but the smallest units, but the application of the insulation between the walls also is important. Injected foam insulation with no through-metal connections (no thermal bridging) is available from a variety of manufacturers and generally has better thermal and acoustical performance than fiberglass insulation. If the unit is to be installed outdoors, extra insulation is recommended and corrosion-resistance should be a top priority.
Mixing box: Most AHUs supply some percentage of outside air for ventilation. The mixing box is the place where outside air is combined with return air from the building. Control dampers are used to proportion the incoming airstreams and relief air.
Filters: Air filters remove contaminants from the airstream and significantly improve air quality (IAQ). Rating systems for air filters, such as ASHRAE Standard 52.2-2007 Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, define the mean efficiency reporting value (MERV) providing a comprehensive and consistent indication of a filter’s capture performance with a range of particle sizes.
Newer rating systems, such as the European Committee for Standardization EN779:2012, rate air filters based not only their ability to capture particles, but also on their predicted annual energy use. It is expected that a similar approach will adopted in the United States in the coming years.
The lifecycle cost of filters should be carefully considered during the design and subsequent purchasing of the filters. Overall, the first cost of filters can be as little as 4% of the lifecycle cost of the filters when considering energy use, changeout, and disposal costs. It’s also extremely important not to skimp on access space. If an AHU is difficult to access and filters are difficult to reach, they won’t get changed, and the unit will use excessive energy and underperform.
Supply and return fans: Fans are the heart of any AHU and can represent a significant portion of the building’s total annual energy usage. The supply fan pushes or pulls the air through filters and coils and then distributes it through ductwork directly to spaces or to terminal boxes. Not all AHUs require a return fan, but units serving multiple spaces or using air-side economizers typically require a fan to return air to the AHU and to relieve air from the building.
AHUs in which the supply fan is installed after the heating and cooling coils are referred to as draw-through units since the supply fan draws the air through the unit.
In blow-through AHUs, the supply fan is located prior to the coils. This arrangement allows fan heat: which can be significant—to be removed from the airstream without having to subcool the supply air as is necessary for draw-through units. Although much less common than draw-through, blow-through units do have applications, particularly in healthcare. They also are seeing increased application in low-temperature air systems.
There are many types of fans applied to air handlers; the primary differences among them relate to blade configuration and whether the fan wheel is fully housed or open. (An open wheel arrangement is referred to as a plenum fan.)
The energy required for any fan is a function of the amount of air to be moved together with the air pressure the fan must generate. ASHRAE Standard 90.1-2010, Energy Standard for Buildings Except Low-Rise Residential Buildings, provides maximum fan power restrictions on HVAC systems based on the flow rate and a variety of factors related to application and filtration level. Future versions of Standard 90.1 will be incorporating a minimum fan efficiency grade (FEG) as described in AMCA 205-2010. Fans more than 5 hp will be required to have a minimum FEG of 67 and will need to operate within 10% of their peak efficiency. (This may not apply to packaged units, which are certified in their entirety.
A critical part of any fan selection is acoustic performance. It’s always important to know the maximum acceptable noise level on a project. Proper selection and specifying of fans and AHU casing can reduce the need for silencers and other costly noise mitigation techniques. Because the best way to reduce fan sound is to reduce the fan power, efficient fans frequently have the best acoustic performance.
Coils: Coils are used to heat, cool, and dehumidify air. The heat source can be from hot water, steam, electric-resistance, or hot refrigerant vapor (as with a heat pump). Cooling and dehumidification can be provided via expansion of refrigerant (referred to as Direct eXpansion, or DX), or indirectly through the circulation of chilled water or glycol. In dry climates, cooling also can be effected by spray coils that reduce the dry bulb temperature of the air, but increase the air’s humidity.
Access: Access sections are frequently omitted from AHUs either through designer oversight or intentionally due to space or budget restrictions. But skimping on access can prove shortsighted as each component within an air hander will require routine service, repair, or replacement many times over the life of the unit. Coils must be cleaned frequently to maintain proper heat transfer, and they must be accessible from front and back to do so. The more difficult it is to reach a component, the less likely maintenance will be performed, which will result in lower overall efficiency and reduced longevity.
Humidifiers: There are numerous methods for delivering humidification, including steam, ultrasonic dispersion, infrared heating, and atomization of water. Careful consideration is necessary to determine which method is best suited for a given project, but humidifiers in general are maintenance intensive. They must therefore be installed in easily accessible locations since serious damage and IAQ issues can arise if humidifiers are not operating properly for extended periods.
Codes and standards
The International Building Code (IBC) and International Mechanical Code (IMC) provide requirements for, among other things, equipment location, disposal of condensate, and minimum outside air quantity. Energy efficiency requirements for individual components and packaged units are provided within the International Energy Conservation Code (IECC), ASHRAE 90.1, and California Energy Commission’s Title 24. Each state and locality determines the applicability of these codes and standards.
There is significant pressure to go beyond code-minimum performance, and many mandates are in place for federal projects requiring new buildings to operate with much less energy than minimally code-compliant ones. The U.S. Green Building Council’s LEED rating system requires new buildings to have at least 10% less annual energy costs than a code-compliant building and awards points based on incremental savings above 10%. The International Green Construction Code (IgCC) and ASHRAE Standard 189.1 also tighten energy performance and are being increasingly adopted by states and localities.
Multiple organizations provide standards for the testing, rating, and installation of AHUs and associated components. Some examples of these include Air Movement and Control Assn. (AMCA), International, Air-Conditioning, Heating, and Refrigeration Institute (AHRI), ASHRAE, and Sheet Metal and Air Conditioning Contractors National Assn. (SMACNA). These organizations produce testing and rating standards that can be used by manufacturers and specifiers to gauge performance.
Large institutional users typically have their own standards in addition to codes to ensure consistency and ease of maintenance for air handling equipment.
Emerging trends in AHUs
Although much of the technology in AHUs has remained relatively unchanged for decades, some relatively new components and practices are being incorporated that can be useful in the right application.
Direct-drive fans couple the fan wheel directly to the motor shaft and are typically applied with variable-frequency drives (VFD). This eliminates the drive losses associated with belts and can result in higher efficiency and lower overall noise.
Fan arrays use multiple small, direct-drive fans in lieu of a single large fan. Applied properly, the fan array can reduce the overall space required for the AHU while providing redundancy and energy-efficient operation. Depending on the number of fans, they can be controlled in unison by one or more VFDs. Like so many other things related to AHUs, care must be taken when applying a fan array to ensure the goals of the project are met as efficiently as possible.
Energy recovery is increasingly applied in AHUs and may be required by energy codes in certain applications having high percentages of outdoor air.
Energy recovery enables incoming air to exchange heat and moisture with building exhaust air via desiccant-coated wheels or special materials in a flat-plate, counter-flow arrangement. Successful application is dependent on many factors, most importantly climate. It is generally easiest and most cost effective to apply when dedicated outside air units are used.
Dedicated outside air units
Dedicated outside air units are increasingly being applied in lieu of traditional air units that mix outdoor air and return air. For many climates, the toughest part of an AHU’s job is treating outdoor air. Coils must be sized to handle to most extreme ambient temperatures. In humid climates, dehumidification requires air to be cooled below the mixed air’s dewpoint even if the occupancy of the building might not require such low temperatures to meet space temperature setpoints. Additionally, the mixing dampers and their associated control sequences in AHUs are common modes of failure, which together with sensor drift can result in over-ventilation (higher energy use) or under-ventilation (poor IAQ).
A better approach for many buildings is consolidating all of the outdoor air treatment into dedicated AHUs that supply 100% outdoor air. Treated air (dehumidified or humidified) from these units can be supplied directly to occupied spaces or can be injected into mixing boxes into other AHUs dedicated to temperature control.
While the addition of dedicated outdoor AHUs might at first sound like a far more expensive approach, their use may add little to the overall cost of a job as they can allow simplification of other AHUs.
Condensate collection from cooling coils can save a considerable amount of water and money. The air conditioning process removes water from the air, which is then typically sent to a drain. Humid climates, including much of the eastern half of the United States, are generally good candidates for recovery of condensate. The recovered water may be collected in a cistern together with rainwater or grey water, or may be used as make-up for cooling towers.
UVC lights (ultraviolet light in the C band) reduce the growth of bacteria, mold, and algae on coils and drain pans. Keeping coils clean and free from deposits improves heat transfer and can contribute to overall IAQ, particularly in critical environments such as hospitals. The UVC emitters must provide proper coverage of the wet side of the child water coil, and care must be taken to ensure that plastic and rubber components within the AHU are not exposed to the ultraviolet light as they will be degraded. Additionally, the emitters require periodic replacement as they lose output power and efficacy within a few years.
Commissioning is a must for all AHUs, regardless of size and complexity. Although the commissioning process is far from new, it has only achieved widespread use in HVAC systems in the past decade with the increased adoption of sustainability rating systems which require it.
AHUs that aren’t completely commissioned are almost guaranteed to not operate properly. Periodic re-commissioning is also necessary since sensors and dampers drift over time. Better yet is continuous commissioning in which the HVAC system’s key parameters are baselined and continuously metered and monitored to give an early indication of lagging performance.
Training is also an important component of commissioning. System operators must be properly trained to understand all operating modes of each piece of equipment. Training materials must be left on-site so that new personnel can come up to speed easily.
Energy use comparison
As a final note related to the energy use of AHUs, a comparative annual energy simulation was made for a typical new office building meeting or in some cases improving upon minimum prescriptive requirements of ASHRAE 90.1-2007 (see Table 1). The building is 175,000 sq ft and is located in Richmond, Va. The HVAC system is comprised of four large VAV AHUs, each supplying 32,000 cfm to single-duct terminal boxes with hot water reheat.
All systems in the model are held constant except for the AHUs. The base case represents a decent AHU meeting ASHRAE 90.1 while the Alternate Case uses an improved FEG, premium efficiency motor, and a static pressure reduction of 0.5 in. wc—easily achievable through careful AHU and duct system design.
The results show a total energy reduction of nearly 2% for the building and an energy cost reduction of greater than 3%, which could earn the project at least one incremental LEED point for credit EA1 – Optimize Energy Performance.
The preceding information is necessarily general and is no way a comprehensive guide to proper selection and application of AHUs. Every project carries with it a unique set of criteria which must be balanced to arrive at the best (not perfect) solution. Design engineers will do well to organize these criteria early in a project and economic analysis is usually required to support the ultimate path forward.
Each individual component within an AHU must be selected with a combination of research, analysis, and experience. By keeping energy efficiency and maintainability firmly in mind throughout the selection process, it is less likely there will be regrets when project is complete.
Even the best AHUs and installations, however, require a strong commitment from the building’s operators to keep them running well. Preventive maintenance programs together with continuous commissioning will help ensure the lowest possible ownership cost for any system.
Follow these design and troubleshooting tips when specifying air handling units (AHUs).
- When beginning a project, spend the time to list all the various goals and constraints including proposal requirements, applicable codes and standards, energy goals, and owner preferences. Create a matrix of each system option and document their relative strengths and weaknesses.
- Try hard to sell energy efficiency and maintainability at the beginning of a project. First cost is hugely important, but many owners will be willing to come up with extra money up front if they can be shown the benefits of lower total ownership costs.
- Consider having control dampers, electrical disconnects, and VFDs installed at the factory. More up-front coordination can be required, but the result is often a higher-quality installation and can also be less expensive when considering the savings in on-site electricians and controls contractors.
- Pay close attention to duct design and limit the pressure losses both inside and outside of the AHUs. Decreasing the required pressure in a fan system by just a few tenths of an inch of water can result in thousands of dollars a year in fan savings.
- When selecting and scheduling fans for AHUs, work closely with the AHU manufacturer to ensure that all losses associated with internal components are considered. This is especially important for plenum fans, which will have casing exit losses that can be significant.
- Don’t forget to leave space for maintenance including the space to remove and change coils in the future. Many facilities require a clear space for service equal to the width of the AHU.
- Never skimp on commissioning an AHU.
Air Movement Control Association, AMCA Standard 205-2010 (Rev. 2011): Energy Efficiency Classification for Fans.
2012 International Green Construction Code. International Code Council (ICC), Inc., Country Club Hills, Illinois.
ASHRAE/ANSI/IESNA Standard 90.1: Energy Standard for Buildings except Low-Rise Residential Buildings. ASHRAE. Atlanta, 2007.
Thomas Lawrence, Jason Perry, Tyler Alsen, “AHU Condensate Collection Economics” ASHRAE Journal, May 2012.
Mark Stutman, “Using Field Measurements of Air Filter Performance and HVAC Fan Energy Measurement to Select Air Filters with Lowest Lifecycle Cost,” Strategic Planning Energy and the Environment, Volume 32, No. 1, 2012, Taylor and Francis Group.
Rob McAtee is vice president and mechanical department head and has more than 20 years experience in energy and building systems. Evan Riley has more than 10 years experience in design, commissioning, and modeling of HVAC systems.