Applied versus packaged rooftop units — does it matter?

The line between applied and packaged rooftop units is sometimes blurred, and ultimately the needs of the project will dictate whether applied or packaged should be recommended and specified based on the wide array of available options

By John Song, PE June 10, 2022
Courtesy: McGuire Engineers

 

Learning Objectives

  • Understand the latest technology, how they work and how they can impact energy.
  • Review pitfalls and benefits when specifying different rooftop unit options.
  • Differentiating between a packaged versus applied rooftop products shouldn’t matter when specifying equipment for your client with the many options available across manufacturers.

Rooftop units offer multiple opportunities to deliver robust designs with increased energy savings and have come a long way even within the past decade. Technological advances within the past 10 years of the industry have transitioned across multiple heating, ventilation and air conditioning systems to provide options for energy savings without breaking the bank.

With tons of options, each rooftop unit can be as unique as larger, more complicated HVAC systems. Traditional “packaged” rooftop units would typically comprise only the necessities — a heating coil, direct-expansion (known as DX) refrigerant coil, fan and filter.

“Applied” rooftop units comprise many different options above and beyond the traditional packaged such as energy recovery wheels, chilled water-cooling coils, hot water heating coils, hot gas reheat coils on DX units, fan arrays and more.

Many manufacturers provide packaged rooftop units as an economical option and, if not specified correctly, vendors often jump at the opportunity to substitute a bottom-end alternative as a cost savings. Specifier beware though; these lower quality alternatives may not live up to the needs of the client.

Recently, the higher end of packaged rooftop unit lineups from manufacturers has allowed for additional options previously offered only in the more costly applied market. The line between packaged and applied is blurred, but that shouldn’t stop you from specifying rooftop units with options to better benefit the requirements of your client.

If a higher-end packaged rooftop unit is the baseline, an applied rooftop unit with all the benefits and features may cost as much as two times a packaged unit with most features adding 10% to 20% and an energy recovery wheel adding approximately 50%. As specifying engineers, it is our job to educate our clients to the benefits of these added features along with the total cost of ownership. It may not make sense for short-term leases, but long-term owners may find benefits over the lifetime of the unit.

There are some common options that allow specifying engineers to select product to meet their client’s needs.

Figure 1: A simple packaged rooftop unit with refrigerant based cooling coil, natural gas heating and supply air fan on top of a roof in Chicago. Courtesy: McGuire Engineers

Rooftop unit cabinet construction

Given the name, rooftop units are commonly located outside and thus insulation can be a major factor in energy use of the unit. Cabinet construction becomes important with regard to leakage rates and heat transfer. Airflow through the unit is transient so the cabinet’s effect on heat transfer is often an afterthought and any derating of the unit is done by the manufacturer when selecting a unit to accommodate a building’s heating and cooling load.

More economical cabinets are constructed with insulation values as low as R-4 but better insulated units can use double-wall construction with R-13 foam construction. The higher R value will provide a slower heat transfer between the outdoors and within the unit during heating and cooling design days and everywhere in between.

Better designed thermal breaks reduce direct conduction paths and creative manufacturing techniques virtually eliminate seams and reduce air leakage to further reduce wasted energy. Reducing air leakage also improves indoor air quality by reducing unfiltered air from entering the air stream. This can be even more important for critical spaces which require air supplied through a high-efficiency particulate arrestance, commonly known as HEPA, filtration system, for example.

Figure 2: Scroll compressors work by using two Archimedean spirals where one is fixed and the other orbits and compressing fluid (refrigerant). Digital scrolls engage and disengage the orbiting spiral to control the amount of compression. Courtesy: McGuire Engineers

Compressor efficiency

Chilled water coils and water-cooled compressors are certainly an option for rooftop units and they each come with their own benefits and drawbacks. When specifying an air-cooled, refrigerant-based cooling system, compressors are a significant energy consumer.

The difference in compressor efficiency is the classic driving analogy we’ve all heard so much in the industry regarding variable speed drives. Slamming on the accelerator on your way to work only to hit the brakes and come to a screeching halt at the next stop light will still get you to work, but it’s hardly an efficient way to travel and much less comfortable than using the pedals to closely match the given driving requirements. Similarly, operating compressors to match building load requirements more closely is much more efficient with less wear and tear on system components.

Multiple fixed-speed compressor systems offer some turn-down capabilities. By staging compressors and loading and unloading individual compressors in the system, output can more closely match the load. For example, using a combination of four fixed-speed compressors offer 4:1 turndown capability and can provide 25%, 50%, 75% and 100% cooling capacities.

However, when cooling requirements are between stages, a compressor will inevitably cycle on and off to meet the cooling demand. Internal controls by the manufacturer can help reduce stress from cycling by alternating compressor operation, but efficiency is still lost compared to a system with greater turn-down.

Two-stage modulating compressors are an alternate option to reduce the number of compressors, improve part-load efficiency and reduce on/off compressor cycling. Two-stage compressor modulation in scroll type compressors is provided by bypassing a portion of the refrigerant gas and reducing volumetric flow. Doing so allows the compressor to continually operate but at a reduced capacity to reduce compressor cycling issues.

Digital scroll compressors or continuous modulating compressors, operate by continuously engaging and disengaging the “scroll.” Be forewarned though: acoustical care should be taken. Digital scroll compressors are not necessarily louder than scroll compressors, but the constant engagement and disengagement of the scroll provides changes in pitch that is much more noticeable. However, digital scroll compressors offer greater turndown by varying the time in which the scroll is engaged.

Specifying inverter (variable speed) compressors will provide the greatest efficiency by continuously operating close to the demand load point, reducing wasted energy. Energy efficiency ratings stay roughly the same of course as it is ratio of cooling capacity to energy, but the integrated EER increases dramatically.

Variable speed compressor output improves not only temperature control, but humidity control as well by continuously cooling. Cycling the compressor on and off reduces the run time of the compressor, which reduces humidity control as cooling turns on and off. The resultant dewpoint of the air is higher than with a continuously operating compressor.

Manufacturers have offered options to integrate multiple compressor options to keep both costs lower with the same turndown capabilities. For example, a variable speed compressor in combination with a standard scroll compressor can operate to maintain similar turndowns as full variable systems by operating the variable speed for the first 50% of capacity and then operating a fixed-speed compressor and then varying the initial compressor to reach capacities between 50% and 100%.

Figure 3: Fan array from 2007, which would eventually be outfitted with variable frequency drive control before electronically commutated motors became more readily available. Courtesy: McGuire Engineers

Hot gas reheat coils

Large assembly-type spaces and other spaces with larger outside air (known as OA) requirements in many geographic locations often fall victim to incorrectly specified systems and suffer high humidity issues. Many do not have dehumidification cycles and have heating coils located upstream of the cooling coil. In these instances where units have been specified for large load requirements but are in much lighter use, high relative humidity results in mold and bacterial growth problems.

Space temperatures are satisfied so compressors are cycled on and off, but moisture remains in the air. Traditional dehumidification scenarios require the air to dry out by lowering dewpoint (cooling) to wring out the moisture (condensation). Overcooling can become an issue when actual cooling load is low, so heating needs to be provided to maintain space temperature. This energy-intensive scenario is mitigated by reducing fan speed, but high OA units still suffer because of higher latent loads.

Providing a hot gas reheat coil in DX systems alleviate these issues by operating a solenoid valve (i.e., three-way valve) and diverting hot refrigerant gas from the compressor to reheat the air. The evaporator coil is still used to drive down the dewpoint temperature and dehumidifying the air, while the hot gas reheat coil operates to maintain space temperature.

Modulating hot gas reheat valves can optimize the system even further by closely matching the coil requirements to prevent overheating.

Fan arrays for rooftop units

Specifying fan array systems increases fan efficiency while also increasing fan redundancy by using multiple, smaller, more efficient fans with electronically commutated, direct-drive motors. EC fan arrays also allow for a reduction in fan section length.

Initial offerings of fan array technology used variable frequency drives to operate the fans and, if individual fan control was required, each fan would be provided with its own VFD, which could take up significant space. EC motors allows for additional space savings not only within the cabinet, but outside the cabinet as larger or multiple VFDs are not required. A motor control panel is still needed.

With smaller fans and motors, the energy draw is less than what was previously used in belt-driven centrifugal fans or even single direct-drive fans. Fan array systems also allow for fan redundancy, so should a single fan fail, the remaining fans can increase their speed to prevent minimal loss in air volume. Downed fans can even be replaced — while the other fans are still operating — with wiring harnesses that allow for quick power connections if continuous operation is required for critical environments.

Backdraft or isolation dampers can be added to each fan to prevent backflow through an inoperable fan. An increase in pressure drop may not be desirable and for a building with a full maintenance team a blank off panel installed for a down fan may be the more preferred option.

Smaller EC fans with their use of permanent magnetic, brushless motors equates to higher efficiency and better control. Replacing smaller, 7.5- or 10-horsepower fans are much easier than replacing a single 50-horsepower fan and motor.

EC fan arrays also allow for shorter cabinet lengths as they require less physical room due to their design. Fan arrays also naturally allow for more uniform distribution of the air, which helps with airflow over heating and cooling coils.

Beware: There may be other factors that begin to creep in and cause issues. If the airflow doesn’t enter the fan section correctly or there is too much internal stratification, the fans can operate unbalanced and can cause whirring noises. Air blenders are a relatively inexpensive option to mitigate these issues.

Adding EC motors to condenser fans also increases control capabilities and energy efficiency in the condenser section and the overall unit.

Energy recovery systems in rooftop units

Increasing energy efficiency requirements and measures are continuously added to the International Energy Conservation Code. Most notably, the 2012 version of the IECC began to require energy recovery systems if installation met certain requirements which then propelled to be more widely used in the market. Energy recovery wheels and ventilators (ERW, ERV) are more common now and can be specified as an option on rooftop units.

ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality limits air leakage between the exhaust and OA streams at 10% for Class II air and 5% for Class I air. When using desiccant wheels, most of the cross contamination occurs around the outer edge of the wheel and at the center. If lower leakage rates are required, rooftop units can also be outfitted with fixed plate ERVs.

Added cost of energy recovery components have longer payback periods but depending on local energy codes they may be required.

Packaged controls

Prepackaged controls are a large benefit to owners using rooftop units because they have been engineered and tested at the factory. End users can then be less reliant on outside sources to provide and install sensors and control programming within the packaged unit during construction. More time spent in a closed environment in the factory means less time spent in the field. Additional cost savings can be realized by the client with the reduction in potential troubleshooting exercises.

For example, dehumidification becomes much easier to manage with prepackaged controls for the operation of the hot gas reheat coil. ERWs are much easier to control as the rooftop unit can take in information from many different sensors to efficiently operate the speed of the wheel.

OA monitoring stations installed directly in the OA stream of the packaged unit allows for better monitoring and control of the dampers to reduce excess air. As a result of better OA damper control, energy usage is further reduced as tighter controls allow the system to provide only what is required.

Coupled with a carbon dioxide (CO2) sensor, demand-controlled ventilation can be implemented to reduce energy costs. Measuring both CO2 and OA, OA dampers can be controlled to provide reduced amounts OA while keeping within reasonable CO2 concentration levels. Only time will tell how desirable energy conservation measures such as DCV will be viewed with the recent COVID-19 pandemic, but we calculated a conservative payback of approximately two years for a 15-ton rooftop unit on a recent museum project.

ERWs, ventilators and their bypass operation when economizing can be difficult with lots of different moving components and sensors. Relying on custom control systems can be difficult to program, commission and troubleshoot if there are problems. ERWs require defrost control, depending on the temperature and enthalpy between airstreams. Prepackaged controls more easily mitigate issues during operation of these complicated components.

A higher education client of McGuire Engineers requested a study to determine the approximate difference in installation cost when using a system with controls packaged at the factory versus a custom control solution installed in the field. The savings were found to slash two-thirds of the cost in controls as the packaged control system allows for installation in a factory-controlled environment rather than in the field. More complicated programming at the BAS was virtually eliminated as only integration of the new unit into the BAS was required with the packaged controls.

Using packaged controls comes with its own drawbacks and does not absolve the specifying engineer from reviewing the manufacturer provided controls. In VAV systems, some rooftop unit vendors will push to have the duct static pressure sensor installed close to the discharge of the unit versus the typical two-thirds downstream. There is inherent difficulty in maintaining proper VAV control when pressure sensors are located at the discharge as its measurements will not accurately reflect duct static at the end of the system.

Acoustical concerns in a rooftop unit

Technological advances have reduced some typical concerns from the past, but others are still prevalent and sometimes even more so than before. Fan acoustics are significantly reduced with EC fan array as there is less vibration than their belt-driven counterparts, but compressors are still the largest noise producing component.

Sound attenuation can be added to the compressor section, but can become quite costly if the unit is to be located near a property line or worse, above a sound-sensitive space (e.g., conference rooms or private offices).

Tenants can hear continuous pitch changes with digital scroll compressors and generally the biggest offender is usually the low-end octave bands that are more difficult to absorb with their longer wavelengths. Additional sound deadening material is highly recommended between the unit and the roof. An acoustic consultant is, of course, highly recommended for sound-sensitive scenarios.

Some rooftop units have the option to add sound absorption in the cabinet. Roof curbs with sound and vibration deadening material can decrease transmission through the roof structure.

Figure 4: Integrated energy efficiency ratings increase when selection fan array options and variable speed compressors options. Combining both options further increase efficiency. Courtesy: McGuire Engineers

Equipment efficiency

Air-cooled air conditioners in the 2021 edition of IECC require minimum EER ratings and have increasing IEER ratings depending on the year. IEER, arguably the more important efficiency rating as it factors in part-load use instead of total capacity, is not difficult to meet by implementing any of the aforementioned options.

Reviewing 30-ton unit selections with three different compressor options from one manufacturer, yielded IEER ratings with increasing value. Multiple options were viewed (see Figure 4):

  • Four single-stage compressors with two ECM fans, IEER = 16.2.
  • Variable lead compressor and two single-stage compressors with two ECM fans, IEER = 17.4.
  • Variable lead compressors and two single-stage compressors with four ECM fans, IEER = 18.0.
  • Three variable compressors and four ECM fans, IEER = 18.6.

EER values stayed relatively the same, but a noticeable increase in IEER numbers were found. Operating equipment that more closely matches load requirements increases operating efficiency by reducing unneeded energy usage.

Figure 6: An existing packaged rooftop unit for a confidential theater in Chicago. This theater also had issues with mold and high humidity with an incorrectly specified unit. Courtesy: McGuire Engineers

Total brake horsepower is reduced when using a four-fan array system rather than two fans. While working through a rooftop unit selection with 12,000 cubic feet per minute of air movement, two supply fans provided approximately 10.1 total BHP while only provided 60% of the airflow with one fan failure. Four supply fans provided 8.70 total BHP while providing 100% redundancy in case of one fan failure.

Many standard options available with packaged rooftop units continue to blur the lines between packaged and applied rooftop unit products. When specifying rooftop units, as with any product, it is important to understand the needs of the client. The name “packaged” or “applied” shouldn’t matter.

 


John Song, PE
Author Bio: John Song is a mechanical engineer and project manager at McGuire Engineers with 10 years of experience in the HVAC industry.