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.
By Matt Chandler, PE, LEED AP BD+C, BEAP; IMEG Corp., St. Louis March 20, 2018

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.

Performance

Having met the performance requirements of governing codes and authorities and properly specifying the equipment, designers still need to ensure the AHU meets the performance expectations of the building owner or client. This is important to note because not all requests can be accommodated in the final design due to varying opinions, limited time, and most significantly, limited budget.

Because no one likes to be surprised about this—and unmet expectations are often processed by the disappointed party as blatant failure—it’s important to consider these expectations throughout the design process and clearly communicate which features will and will not be included. Note, these decisions are not to be made solely by the engineer, but in collaboration with the key stakeholders and decision-makers to appropriately allocate limited project resources.

Heating

Heating capacities must be sufficient to properly condition the supply air with adequate outside-air quantities on the coldest days. Using ASHRAE weather data for the project area is a good starting point, typically with the minimum extreme annual mean dry-bulb temperature. Slight adjustments from these values to the design conditions used in the calculations may be warranted, depending on the goals and expectations of the owner or local code requirements.

Heat in a hospital must be provided by a redundant source. If, for example, your hospital doesn’t have a redundant boiler with fuel-oil backup, additional considerations will be needed to provide redundant heating capabilities at the AHU itself. Air blenders, pumped heating coils, or glycol systems will need to be considered for freeze protection.

For AHUs serving more than one space, the unit is typically only expected to deliver around 55°F supply air in the winter, so the specific needs of each zone are handled by the terminal heating equipment (reheat coils, perimeter radiation, etc.). For single-zone AHUs, all of the heat for the space may be provided from the unit, requiring a much higher discharge temperature.

Cooling

The ASHRAE weather data is a good starting point for the cooling design day calculations as well, typically by using the monthly design dry-bulb and mean coincident wet-bulb temperature 1% data for the warmest month of the year. Project goals and local codes can, once again, warrant further adjustments.

Required outside-air quantities on the design day are another critical component of the evaluation. In addition to outside conditions, interior expectations play a large role in cooling calculations. Unlike when in heating mode, all the cooling is frequently expected to be delivered by the AHU unless a fan-coil unit, dedicated outside-air unit, or similar supplemental cooling equipment is provided to handle a portion of the load.

In addition, relative humidity (RH) expectations within the space can also lead to additional burdens on the AHU. In spaces where interior temperatures of 75°F are required, maintaining space RH in the (generally acceptable) range of 55% to 60% RH can be accomplished with the AHU providing temperatures of approximately 55°F leaving the cooling coil.

However, where lower space temperatures and/or lower space RH levels are desired, there must be a method provided to accomplish this. Often, this method is to provide cooler (thus drier) supply air from the cooling coil. Not only does this require additional (or colder) chilled water, but it may require a different coil selection or unit design. Desiccant wheels, direct expansion (DX) equipment, glycol chilled-water systems, and supplemental cooling strategies all offer pros and cons to be considered. Alternative equipment arrangements, such as separately conditioning return air and outside air, can also lead to lower discharge humidity levels while minimizing reheat.

Humidity control

In winter, low humidity can cause problems. Humidifiers are required in most health care HVAC systems in climates with a heating season to maintain humidity levels above the minimum thresholds (typically somewhere between 20% and 30% RH). In addition to code requirements, it’s important to consider occupant comfort, equipment operating conditions, and storage requirements for drugs and other supplies. For steam humidifiers, adequate dispersion distance must be provided downstream of where the steam is introduced to avoid condensation. Adequate monitoring and controls must be provided to maintain a safe and effective operation.

Redundancy

While often not mandated by the code authorities, redundancy in the supply and/or return fans is a worthwhile consideration. With the widespread availability of fan-array unit designs, there are now several reasonable approaches that can be adopted.

For example, sizing each fan in a four-fan unit for one-third of the total airflow would allow one fan to fail while maintaining design airflow rates. Even sizing each of the four fans for one-quarter of the total would mean the failure of a single fan would only result in a 10% to 15% reduction from design values due to the reduced static pressure, especially if the nonfunctional fan is blanked off or equipped with a backdraft damper. Combining this with minor controls adjustments can provide options to continue to meet space requirements, especially if the failure doesn’t occur on a design day. The number of variable frequency drives (VFDs), bypasses, etc., will also come into play and are evaluated on a project-by-project basis.

Another popular method of providing partial redundancy while minimizing cost impact is to cross-connect the supply and return ductwork of adjacent units. This cross-connect is typically not sized to provide 100%-capacity redundancy (although it could), but to provide some transfer of conditioned air from the functioning unit to the areas served by the failed unit in an emergency. Motorized dampers in the cross-connected ductwork accomplish the changeover when needed. Manual dampers can be used in smaller systems or where reduced cost is desired. Equipment sizing and compatibility of spaces served by each unit should be considered before proceeding with a cross-connect design.

Air filtration

Minimum air-filtration levels are mandated by code, but the appropriate level to provide is an often-disputed topic among infection-control specialists, energy managers, facilities staff, and design teams. While higher levels of filtration come with some desirable benefits, these aren’t always supported by the available research or budget—and come with a higher replacement cost and a penalty in energy usage due to the increased static pressure.

Where current and/or future requirements are unclear, designing the unit to handle the higher of the requirements may be a wise choice in some cases. For example, a hospital may choose to install HEPA filters where only 90% filters are required. In that case, having the appropriately sized filter housing and fan capabilities will allow the installation of the higher filtration level while allowing the unit to operate at design flow rates.

Efficiency

Because the equipment will be in operation for many years, an energy-efficient design should be pursued. Both the International Energy Conservation Code and ASHRAE Standard 90.1: Energy Efficiency for Buildings Except Low-Rise Residential Buildings have incorporated fan-horsepower limitations into the HVAC design requirements. These limitations impose restrictions on how much power can be consumed to move the air through the system. Adjustments are made to the limit by acknowledging the project type, system components, etc.

However, some fundamental design elements provide a good starting point for any design. Sizing the AHUs so that the air velocities are in the 350- to 400-fpm range will make a substantial reduction in pressure drop (therefore, the horsepower required) through the unit, as compared with more traditional design standards of 500 fpm for velocities through the unit.

Sizing duct mains and long runs with lower velocities, thus lower pressure drops, will also have a significant impact. Maximum velocities of 1,800 fpm for medium-pressure ductwork and 1,500 fpm for low-pressure ductwork are good starting points. Managing unit and duct velocities is a critical first step in any design where energy is to be minimized and fan-horsepower limitations must be met.

The use of appropriate duct fittings, minimizing system effect factors, and selecting efficient fans are also key design elements. In addition to managing energy used by the fans, energy codes also dictate several other energy-related design elements including the use of economizers, energy recovery, minimum equipment efficiencies, etc.

Other considerations

It is beyond the scope of this article to provide a comprehensive design guide for all aspects of AHU specification, but here is brief list of additional items that should be considered: supply-fan configuration (blow-through versus draw-through), return fan versus relief fan, sources of heat (hot water, gas, electric), sources of cooling (chilled water, DX), condensate design (trap height, discharge to sanitary or storm, recovery for cooling tower make-up), ultraviolet (UV) lights, energy-recovery systems, controls instrumentation and control strategies, noise/vibration mitigation, size/location of access doors, inward- versus outward-swinging doors, unit construction, damper selection and locations, air mixing, duct connections, access platforms, maintenance vestibules, pipe cabinets, screen-walls or other concealment methods, and seismic forces. Incorporating these design elements makes the balancing act that much more complex.

When specifying AHUs, there are numerous requirements to be considered from multiple sources. The challenge is to satisfy the mandatory items and optimize the remaining elements to align with the project goals. One of the key takeaways is that there is not a one-size-fits-all solution, even for buildings or projects of similar type. Some owners may choose to accept a reduction in performance or maintenance access for the sake of expediency or project budget. Others may have the funds to invest heavily in the current installation to support enhanced performance and future flexibility. Budget is not always the driving factor, either.

As a result, communication is an essential part of the specification process. It is important for the owner and key stakeholders to understand what is in the project and what is not. These decisions must be made as a team, but it is the role of the consultant to spearhead the communication and drive these discussions and the decision-making processes.


Matt Chandler is a project manager and senior mechanical engineer for IMEG Corp., where he leads complex projects for health care and research laboratory facilities.

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