Specifying AHUs is a balancing act on every project

Engineers face a dizzying number of codes, standards, and project-specific requirements, limitations, and expectations for air handling units (AHUs). This article provides a roadmap for navigating the most critical of these considerations.


This article has been peer-reviewed.Learning objectives:

  • Be aware of the various codes, standards, guidelines, and owner or client expectations that affect the specification of air handling units (AHUs).

  • Understand other common factors that affect AHU design including equipment location, space constraints, and flexibility.

  • Know the typical criteria to consider when specifying AHUs with respect to heating and cooling capacities, humidity control, fan redundancy, air filtration, and efficiency.

Correctly specifying air handling units (AHUs) is a challenging task on any project due to the numerous, and often conflicting, project requirements. These requirements include codes and standards that dictate various components and performance, space constraints that typically squeeze the equipment into the smallest space possible, energy efficiency goals, and budget constraints to allow money to be freed up for other aspects of the building.

Figure 1: All controls, instrumentation, piping, and surrounding ductwork and equipment are arranged to provide clear access to the indoor air handling unit in the penthouse of SSM Health St. Joseph Hospital. All graphics courtesy: IMEG Corp.Numerous codes and standards affect proper design and selection of AHUs, and their applicability will vary by location, project type, building type, and areas served. The local building codes should be consulted first, and they will generally dictate the version of the International Building Code, International Mechanical Code, and other codes that apply, along with any local amendments.

State codes also should be consulted, especially for health care projects where the state health department and code reviewers will have additional requirements that must be met. In health care settings, Centers for Medicare & Medicaid Services (CMS) Conditions for Coverage & Conditions of Participations requirements also must be considered during design. This accreditation may be provided by a third-party reviewer, in which case the requirements of the third-party reviewer need to be met as well.

All this can add up to a staggering number of requirements. In addition to codes, standards and guidelines such as those published by the Facility Guidelines Institute (formerly AIA Guidelines) describe a standard of care that is often referenced for requirements. Many states have even adopted these standards as code.

Beyond these codes and standards are requirements that apply to specific departments or spaces served. For example, equipment serving a pharmacy will need to meet the requirements provided by the United States Pharmacopeial (USP) Convention in the federally enforceable standard USP 797 and soon-to-be-enforceable USP 800.

Figure 2: The return-fan array and relief air damper (air moving left to right) are shown in the mechanical penthouse at SSM Health St. Joseph Hospital in Lake Saint Louis, Mo. Vibration isolation is included in the fan’s support structure. Space constraints

In new projects, owners and architects typically want to minimize the space (therefore, the project cost) associated with the mechanical, electrical, and plumbing (MEP) systems. However, this desire to minimize the space allocated must be balanced with the space required for proper installation, efficiency, operational clearances, and maintenance access.

Maintenance personnel typically prefer AHUs to be located indoors. However, roof space is often more readily offered by architects looking to maximize the use of building interior spaces. While there can be benefits associated with the initial location of equipment, and potentially easier access when replacing the equipment, there are also drawbacks to such placement. Maintenance is more difficult when access must be obtained through a roof hatch, or even when accessed horizontally through a man-door, and will be even more challenging and less appealing on days that are hot/humid, rainy, snowy, cold/windy, or icy. In addition, AHU components exposed to the weather typically will have a shorter lifespan as compared with indoor equipment. The reduced accessibility and increased potential for degradation provide a double-whammy negative effect on maintainability.

If equipment must be located on the roof, it must be placed a specific distance from plumbing vents, building exhausts, and other items that have a negative effect on indoor air quality (IAQ). The same requirement applies to the outside-air intakes of equipment located indoors, usually ducted to a louver or hood.

Regardless of location, maintenance clearance and replacement access are critical to maintaining each unit in optimal operational condition over the long term. Units with heating and cooling coils will need service clearance that is sufficient to pull the coils (typically through the side of the unit) for maintenance and replacement. Many manufacturers offer equipment in variable aspect ratios to help manage this issue. The clearance required can be further managed, to some extent, through the thoughtful specification of vertically or horizontally split coils to reduce individual coil dimensions.

Another creative way to accommodate this requirement is to align the coil-pull space with a pair of double doors leading to a corridor or similar adjacent space; the doors can be opened when the access space is needed. Similarly, the adjacent interior wall can be constructed of metal studs and gypsum so that the wall can be removed and rebuilt at a low cost if pull space is required. Installing access panels, removable wall panels, or even faux louvers can accomplish the same purpose.

A more frequent occurrence is the replacement of the air filters. This requires significantly less space, but consideration should be given to how the filters will be transported to the AHU location (sometimes quite challenging) and how the maintenance staff will accomplish the filter replacement (again, this can be more difficult in a large unit without an access section adjacent to the filters that is large enough for staff to enter). If the unit has carbon filters, which are quite heavy, it is important to provide a clear delivery route that can be navigated by maintenance staff while carrying the filters. Coil cleaning, strainer cleaning, and overall unit operational verification all benefit from adequate access.

Figure 3: Air handling unit sections are shown being lifted seven stories to be assembled in the penthouse at BJC HealthCare’s Missouri Baptist Medical Center in St. Louis. These units were coordinated to fit in the penthouse.Calculations

AHUs are typically expected to last 15 to 25 years, according to various industry standards, although it is not uncommon to see units pressed to continue operating to 30, 40, or 50 years. With this potentially long lifespan and a substantial upfront investment, it can reasonably be expected for these units to perform well and provide some level of flexibility for most future applications.

However, “performing well” requires a clear understanding of what the AHU is expected to accomplish. For example, an AHU that serves hospital patient rooms is expected to provide cooled and dehumidified air in the summer and warmed and humidified air in the winter, with the proper outside airflow rate, filtration level, air changes, and pressure relationships. If this unit is expected to serve office space in the future, it will usually be sufficient for the task.

The opposite scenario would likely pose a greater challenge (unless the potential situation was accounted for ahead of time) because a unit designed for office space would likely not have adequate heating, cooling, and airflow capacity and may lack features such as humidification. Even a unit designed to serve patient-care areas would have difficulty if the space it served was renovated into operating room suites. The expectation of HEPA filtration, low-temperature supply air, and the ability to maintain 25 air changes per hour (ACH, or the applicable rate required by local codes) may be expected in an OR but unavailable in the existing equipment.

An understanding of current and future potential needs is important. The starting point in most cases is to understand how the building envelope, occupants and activities, and equipment will impact the cooling and heating loads in each space. In hospitals, it’s important to note that airflow to many spaces is driven by air-change requirements rather than space heating or cooling loads. A wise strategy for hospitals is to apply the code-required air-change rates and cross-check each space to verify whether heating- and cooling-load requirements fall below that threshold.

This process works well and proves generally useful in moderate climates. Once the airflow requirements are known, there is still the all-important task of determining the static pressure losses from the unit to the air devices to ensure the equipment is capable of delivering the air to its intended location.

Once again, considering the future and providing for flexibility can prove valuable. If building conditions, or the installing contractor’s prerogative, lead to a slightly different duct size, routing, or fitting arrangement, it could have a devastating impact on the static pressure calculations. Thus, ensuring the unit has sufficient capability to overcome mild increases in static from the onset is prudent, because options to increase the static pressure capabilities later are often limited and expensive.

Similarly, the needs of most facilities will change over time, so providing some flexibility to serve future needs can prove extremely valuable. Additionally, in the hospital-accreditation process, many components are of the pass/fail type; e.g., air-change requirements, pressures, temperatures, humidity range, etc., are either in range or out—and out means fail. If your unit is maxed out and out of range, the results could prove very costly.

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