Specifying AHUs for health care facilities

Air handling units are central to most HVAC systems in all nonresidential building types. This article looks at the codes/standards that define correct specifications and outlines best practices for calculating an AHU's requirements in a health care facility.


This article has been peer-reviewed.

Learning Objectives:

  • Learn how to select the right type of air handling units (AHUs) for a health care project.

  • Consider budget considerations when designing and specifying AHUs.

  • Understand the different sections of the AHU and how they should be specified.

Air handling units (AHUs) are the most common means of air distribution in nonresidential buildings. AHUs are used to provide the airflow required for ventilation, heating, and cooling. AHUs in a health care setting are very important because they provide the required air change, filtration, and ventilation requirement to minimize the spread of airborne infectious diseases.

Figure 1: This 55,000-cfm custom air handling unit (AHU) installed at Dell Seton Medical Center at the University of Texas has two cooling coils in series to maintain a lower pressure drop through the air tunnel. All graphics courtesy: WSPThe mechanical engineer is responsible for the design and selection of AHUs for the critical spaces in a health care facility. Designing air distribution systems to serve hospital environments is a complex undertaking. There are specific requirements for ventilation and filtration to dilute and reduce contamination in the form of odor, airborne microorganisms and viruses, and hazardous chemical and radioactive substances.

Ventilation effectiveness is very important to maintain appropriate indoor air quality for patients and to minimize the risk of airborne hospital-acquired infections (HAI). There are additional design sophistication elements for the AHUs to minimize the risk of transmission of airborne pathogens and preserve a sterile and healing environment for patients. Choosing the right AHU plays a key role in minimizing HAI because the quality of the outside air and filtration effectiveness is controlled by the AHU design.

Several questions must be answered before the selection process begins. Some of the items are not unique to health care facilities and can be applied to most AHUs that serve high-performance buildings. Consider:

  • What type of spaces will the unit be serving?

  • What is the available budget?

  • Where will the unit be located?

  • What is the performance requirement?

  • What is the filtration requirement?

Figure 2: These two 45,000-cfm custom AHUs were installed at Winchester Medical Center, Winchester, Va., this unit had a 12-in. service vestibule in between the two units. Health care spaces

The spaces served determines the type of AHU selected. Health care spaces can be separated into critical and noncritical environments. The critical environments, such as operating rooms (ORs), intensive care units, post-anesthetic care units, neonatal intensive care units (NICUs), airborne infection isolation rooms, and protected environment rooms, will require an AHU that will provide the required ventilation per ASHRAE Standard 170-2017: Ventilation of Health Care Facilities. These units usually include added components like humidifier sections and filtration using high-efficiency particulate air (HEPA); in some cases, such as in ORs, it is considered good engineering practice.

Noncritical environments, such as general exam rooms, provider offices, general laboratories, and general patient rooms, do not require HEPA filtration and humidification. When selecting AHUs for health care spaces, the engineering best practice is to separate the units that are serving critical spaces from noncritical spaces.

Budget constraints may sometimes dictate that separate units maybe cost prohibitive, and for these instances, the AHU should be selected to meet the more stringent ventilation requirements of all the health care spaces that are served by the single unit. Alternatively, separate the ductwork for critical and noncritical spaces from a common AHU and provide the more stringent (i.e., more costly in first-cost and operations) spaces with duct-mounted supplementary systems.

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