Design of RTUs: Best practices

Learning Objectives

RTU insights

• Rooftop unit (RTU) filtration and humidification requirements, energy efficiency, ease of maintenance and aesthetics are major considerations.
• RTUs are popular for their space efficiency and modular designs, cooling around 60% of commercial floor space in the U.S.
• Design teams must select from four categories of RTUs based on project needs and energy efficiency considerations.

Rooftop units (RTUs) are popular choices for heating, ventilation and air conditioning (HVAC) systems because they optimize space, provide efficient climate control and feature modular designs. They save valuable floor space compared to air handling units (AHUs), which are situated inside a building. Design teams clearly recognize these advantages: Today RTUs cool approximately 60% of floor space in commercial buildings within the U.S., according to the U.S. Department of Energy.

The technology, in short, is proven. But design teams face challenges in terms of choice: With so many options now available, how do you specify the right RTU for a particular project? Following are several considerations to guide these decisions.

When it comes to HVAC equipment on rooftops, there are specific code requirements to ensure safety, accessibility and proper installation. The International Mechanical Code Section 306.5 mandates that if equipment requiring access (such as HVAC units) is located on a roof with a slope of three units vertical in 12 units horizontal (25% slope) or greater, a level platform must be provided on each side of the appliance or equipment. This platform ensures safe access for service, repair or maintenance of rooftop equipment. Equipment must also be installed at least 10 feet from the edge of the roof or proper fall protection must be provided along the perimeter of the roof.

Furthermore, equipment must also be positioned so that water, snow or ice from the roof or eaves cannot fall directly on the unit or be ingested into the outside-air intake. Maintaining sufficient distance between fresh air intakes and exhaust fans is also important to prevent contamination of the fresh air being delivered to the unit.

When installing HVAC equipment on new or existing roofs, careful consideration of weight-related factors is essential to ensure safety and prevent damage to the building. Smaller packaged HVAC units typically weigh around 600 pounds, while larger semi-custom or fully custom units can reach up to 65,000 pounds. Understanding the weight of the specific unit you’re installing is crucial for proper structural support. Additional structural reinforcement is often necessary to accommodate the weight of large HVAC units. Therefore, HVAC engineers should coordinate carefully with structural engineers to assess the new or existing roof structure and determine if modifications are needed. They should also consider factors such as snow load, wind load and dead load (permanent weight) when calculating the total load on the roof.

Energy efficiency is yet another consideration. ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings serves as a benchmark for energy-efficient building design. It has been updated regularly since its inception in 1975 and remains the energy efficiency guide for most commercial buildings.

The entire HVAC system, including RTUs, variable air volume boxes, building automation systems, ductwork configurations and fans, must meet the requirements outlined in ASHRAE 90.1. Section 6.8 of Standard 90.1 provides specific guidelines related to the minimum efficiency requirements of electrically operated unitary air conditioners and condensing units. These minimum efficiencies must be met for all new equipment installations.

Figure 1: This photo shows the hot water and chilled water piping within the service vestibule of a semi-custom unit used on the Hackensack Meridian hybrid OR project. The piping and equipment within the service corridor was designed to allow the maintenance staff to walk within the unit and access all the components. Courtesy: Syska Hennessy

Types of RTUs

The first step for a design team is to select among four overarching categories, which include dedicated outside air systems (DOAS), packaged units, semi-custom units and custom units. Below is an overview of each:

DOAS are used to provide fresh air for ventilation; there is no recirculation. Therefore, they are typically used in conjunction with another type of system that handles heating and cooling loads. Common choices for these supplemental systems are fan coil units, chilled beams terminals and variable refrigerant flow/volume systems, which offer energy efficiency and individual temperature control for each zone.

DOAS are especially advantageous in places like airports, classrooms and conference rooms, where there are higher requirements for outside air used for ventilation. The downside, however, is that they are usually more costly because there is extra piping and equipment involved.

Packaged units are simple, self-contained units used for light commercial buildings where precision temperature control is not required. They typically range in size from 2 to 150 tons and each unit comprises direct expansion cooling, an electrical or gas heating coil and a single air filter. Additional components and features are available as options, but these units lack the customization available in semi-custom and custom units.

The plus of packaged units is cost-efficiency; they’re off-the-shelf. The minus is a dearth of customization options, for the same reason. Furthermore, packaged units are not the most energy-efficient choice. They meet the basic criteria but are unlikely to exceed more ambitious targets for decarbonization.

Semi-custom units are good fits for projects with strict guidelines for filtration and humidification, although they are more expensive than packaged units and DOAS. These units are more robust and provide up to 250 tons of cooling. Because there are more options associated with semi-custom units, they are often used in health care and pharmaceutical environments. In these cases, facility owners and managers understand that the precision associated with the equipment leads to higher costs.

Custom units are, not surprisingly, more costly than semi-custom units. These units are the most versatile and have the capability to incorporate their own heating and cooling equipment, like a small mechanical equipment room within the unit. Expense is one downside; another is lead time. It takes longer to procure the units because they are fully customizable.

But they offer many benefits: Custom units can meet stringent criteria for footprints and filtration, they can be built to withstand higher static pressures and lower supply-air temperatures and they can be painted with custom exterior finishes or cladding for aesthetic coordination.

Figure 2: Example of a simple packaged rooftop unit at a branch bank facility. The unit comprises a supply fan, direct expansion refrigerant cooling coil, gas heating, a single air filter and an airside economizer. Courtesy: Syska Hennessy

Factors that influence RTU choices

With simpler projects, such as an office or retail space, cost is usually the key consideration and packaged units suffice. These units comply with minimum code requirements across most jurisdictions.

But if a project has higher filtration or humidification requirements, the solution is often a semi-custom or custom unit. Below is a case study that exemplifies the decision-making process:

Syska Hennessy, which served as the mechanical engineer for the fit-out of two hybrid operating rooms at Hackensack Meridian Hospital in Hackensack, New Jersey, chose a semi-custom unit for the project.

That was the logical choice in this instance. The hospital intended these operating rooms (ORs) to be used for both imaging and cardiac catheterization. Since the ORs were relatively large, had a high air-change-rate requirement and required high-efficiency particulate air filtration, they needed fans capable of delivering high airflow and static pressure.

A fan array comprising multiple fans was chosen to increase energy efficiency as well as redundancy in the fan system. The team needed an RTU, therefore that could provide higher static pressure and proper humidification and be equipped with pre-filters and final filters. The unit was constructed with a large maintenance vestibule to house the humidifier system, piping and provide the maintenance staff with protection from the weather while maintaining the unit.

Similar criteria influence choices for other health care projects. But regardless of the unit type, design teams must also consider access for future maintenance and efficiency. Engineers should work closely with vendors to select the correct fans. Otherwise, they face the risk of oversizing, which leads to an excess power draw.

It is difficult to compare custom and semi-custom units with packaged units and DOAS in the realm of energy efficiency because the former two are used with chiller plants, which means that the cooling process happens elsewhere. That said, engineers can choose fans for the semi-custom and custom units that offer higher efficiency — assuming that a client is willing to pay extra.

Once a unit is installed, proper maintenance is required. For example, facility staff should replace filters at proper intervals, because as debris accumulates on the filters, it adds resistance to the air stream, which translates into more energy usage. The manufacturer typically suggests replacement intervals. But with semi-custom and custom units, designers can install a gauge to monitor pressure drops across the filters. Once the gauge measures a certain setpoint, the filters should be changed. It’s a more scientific approach toward maintenance.

Energy efficiency also extends outside the design of the AHU. Ductwork systems must also be rightsized to minimize the amount of static pressure required by the RTU to deliver the proper airflow to the building.

Figure 3: Design of individual components for the semi-custom unit used on the Hackensack Meridian hybrid operating room project. The unit was designed with a service corridor to protect the maintenance staff from the weather and houses the piping, electrical components and the steam generator serving the unit. Courtesy: Syska Hennessy

Emerging technologies

Design teams should also think about future needs and strategies to integrate emerging technologies into their RTUs.

For instance, it is possible to equip semi-custom and custom units with energy recovery features. Energy recovery systems use air that is exhausted from a building to pre-treat the air coming into the building, which decreases demand for extra cooling and heating. Energy recovery wheels are becoming more efficient, allowing for better energy transfer.

Figure 4: This is one of the operating suites on the Hackensack Meridian hybrid operating room project. Ductwork was routed below the roof from the semi-custom rooftop unit to the two operating rooms. An array of laminar high-efficiency particulate air diffusers were used at the ceiling to supply air to the room, while lower return grilles were used to return air back to the rooftop unit. Courtesy: Syska Hennessy

New refrigerants are being developed every few years that are slowly but surely replacing chlorofluorocarbon-type refrigerants, which are detrimental to the environment. Current regulatory requirements are now phasing in the use of mildly flammable A2L refrigerants with lower global warming potential, such as R-400 and R-500 series blends. And the new refrigerants can be included in packaged and semi-custom units.

Additional methods of promoting energy efficiency include upgrading the control systems on basic packaged RTUs with features such as multispeed or variable speed supply fans, modulating outdoor air control dampers, modulating heating coils and employing demand-controlled ventilation.

Figure 5: Shown is an example of a semi-custom unit used for a health care facility. The unit comprises supply and return fans, a direct expansion refrigerant cooling coil, gas heating, pre- and final air filters and an air-side economizer. Courtesy: Syska Hennessy

When it comes to designing RTUs, engineers must consider filtration and humidification requirements, energy-efficiency, the ease of maintenance, the potential for integration with emerging technologies and even aesthetics, while ensuring that solutions do not exceed budgets. Decisions require input from several trades; they cannot be made in a vacuum. The process may be time-consuming, but the end-result is an RTU that satisfies specific facility needs efficiently and lasts for a long time.

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