Know when, how to specify a rooftop unit

There are some key components to recognize when designing air-cooled rooftop units

By Jon Silhol April 14, 2021


Learning Objectives

  • Learn about the many considerations for rooftop units and understand options in applied rooftop units.
  • Understand how ambient and interior temperatures can affect rooftop unit performance.
  • Discuss improvements in package rooftop units.

Rooftop units have long been used to satisfy building heating, ventilation and air conditioning requirements. Some of the benefits of rooftop units include lower first cost, ease of installation and reduction of area inside the building for mechanical equipment.

The maintenance of rooftop units also can be a benefit if proper roof access is available. All of the components are in one location to be worked on and maintenance personnel do not have to go throughout a building to find various units such as with a water-source heat pump system.

Rooftop unit efficiencies

Rooftop units are rated for different efficiencies based on the tonnage. Units rated for less than 65,000 Btu/hour are rated to a seasonal energy efficiency ratio, which “measures the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period.”

Units rated for 65,000 Btu/hour and higher are rated to energy efficiency ratio, which “measures of how efficiently a cooling system will operate when the outdoor temperature is at a specific level.” The energy efficiency ratio value is the efficiency at the peak cooling condition.

The integrated energy efficiency ratio is used on the larger tonnage units as a measure of efficiency at part load conditions. Table C403.3.2 in the International Energy Conservation Code outlines the minimum efficiencies required for different types of rooftop units, such as cooling only units or heat pumps and minimum efficiencies required based on the tonnage of the units.

Derating equipment

There are some general design parameters that apply to both package rooftop units and applied rooftop units. The ambient temperature will affect the performance of air-cooled equipment. All unitary air conditioners and heat pumps are tested and rated to AHRI Standards. AHRI Standard 210/240 is applicable for equipment less than 65,000 Btu/hour and AHRI Standard 340/360 is applicable for equipment from 65,000 Btu/hour to less than 250,000 Btu/hour.

The latest version of these standards is 2019 and 2017, respectively, which rate air-cooled equipment at 95°F dry bulb and 75°F wet bulb in the cooling condition, and 47°F dry bulb or 17°F dry bulb in the heating condition. These have long been the ambient temperatures air cooled equipment has been rated at.

These conditions apply to vast parts of the United States, but if a project is located in the Pacific Northwest, Southwest desert or upper Midwest, for example, the design ambient temperatures will be different from the standard testing temperatures.

Rooftop units that are installed in high-temperature locations, such as Phoenix, need to be derated due to ambient dry bulb temperatures that can reach 115°F. The daytime high temperatures can be 115°F with temperatures on a roof being near or exceeding 125°F. The rooftop unit is not able to reject as much heat to the atmosphere when the dry bulb is higher outside, so the total capacity and sensible capacity will be less than what is listed in a manufacturer’s catalog.

Many locations in the United States have a design heating temperature lower than 17°F. The building can still have a call for cooling, depending on the building use type. This can cause an issue with head pressure in the condenser and cause the refrigeration system to short cycle or have other problems. Manufacturers offer low ambient controls or kits that will enable the refrigeration system to operate properly at low ambient conditions. These are accessories and need to be specified by the design engineer. Engineers should be cognizant of the design ambient conditions and how they can affect the capacities of package rooftop units.

The entering air temperature is also a key component when designing air-cooled rooftop units. AHRI Standard 340/360 also defines the entering air temperature as 80°F dry bulb and 67°F wet bulb. This condition can be different depending on what climate a building is located in.

For example, in hot, dry climates a typical entering air temperature could be 78°F dry bulb and 63°F wet bulb. This will reduce the capacities of the rooftop unit from the listed catalog data. Air-cooled rooftop units are commonly used for office buildings and light commercial spaces. These spaces typically are 15% to 20% outside air and the entering dry bulb temperature is below 90°F.

It is important to keep the entering dry bulb temperature below 90°F to this type of equipment so the refrigerant system can operate properly. Most manufacturers rate this type of equipment to 90°F in their catalogs. This is due to not have the refrigeration circuit fail on high head pressure and to limit the compressor from cycling too often.

There is a limit to the temperature difference across the cooling coil that air-cooled equipment can produce because of the refrigeration cycle. These types of units can typically produce a 20°F to 25°F delta T in the cooling condition. An engineer should always calculate the entering air temperature to the equipment and ensure that the equipment will properly condition the space in both the cooling mode and the dehumidification mode. A rooftop unit will not dehumidify to below the standard room condition of 75°F and 50% relative humidity if the entering air temperature is too high. This will cause humidity to build up in the space and become uncomfortable to the occupants.

An engineer should look at different duct layouts associated with the rooftop units and shift rooms to adjoining rooftop units to lower the entering air  temperature or using specialty package rooftop units if an entering air temperature approaches 90°F dry bulb.

Equipment improvements

A common problem with air-cooled rooftop equipment is oversizing. Most engineers are conservative in nature and want to ensure that there is enough cooling and heating capacity. Engineers dread the phone call saying the space is too hot or too cold or that the controls aren’t working properly or that there are operations and maintenance issues. This can lead to several issues with package rooftop equipment.

Calculated peak load will occur for a few hours during the year, depending on which design criteria the engineer selects for a building. Typically, the 0.4% or 1% weather values are used from the ASHRAE weather data tables. That means the air conditioning equipment will need to operate at a reduced load 99% of the year.

Manufacturers, in response to stricter code requirements, have been able to improve the part-load operation of this equipment in recent years. Some of these improvements include variable speed compressors, electronically commutated motors and microchannel heat exchangers.

Equipment improvements allow air-cooled equipment to operate at part load much better than they have in the past and reduce energy consumption. These items replace single stage compressors and belt-driven fans that have long been used in package rooftop equipment, which had very limited turndown capabilities. These improvements help avoid the empty movie theater syndrome where the rooftop unit is at its minimum operating point, but there is no load in the space and people have jackets on in the middle of the summer.

Smoke control equipment

A potential issue when multiple package rooftop units are used with a common return plenum is whether smoke detectors are required. Section 606.2 of the International Mechanical Code requires smoke detectors on all units with a return airflow of 2,000 cubic feet per minute or greater. This would require smoke detectors on package rooftop equipment that are smaller than 5 tons.

Acoustical considerations

It is important to properly design the acoustics for any HVAC system. There are two key components when working with HVAC acoustics and those are sound power and sound pressure.  Sound power is the acoustical energy emitted by a source (i.e., a rooftop unit) and is a fixed value. Sound pressure is the level of the noise produced by the source. The sound pressure level can vary depending on how far away from the source and what acoustical treatments are between the source and the measurement point.

Where rooftop units are located is important when considering the acoustical impact these types of units can have on an occupied space. Chapter 49 of the 2019 ASHRAE HVAC Applications Handbook states the appropriate noise criterion level that spaces should be designed to be based on the use. It is always best to locate rooftop units over spaces with a higher NC level, such as storage rooms or corridors that have a 40 NC level instead of noise-sensitive spaces such as conference rooms, which have a 30 NC level. Additional design considerations are required when units are located over sound-sensitive spaces or the unit will create more noise than acceptable for the space below.

There are different pathways that have to be considered when dealing with the acoustics of rooftop units:

  • Airborne sound is the sound that travels down the ductwork.
  • Breakout sound passes through the walls of the ductwork.
  • Radiated sound is transmitted through the cabinet of the unit.

Acoustical treatments can be necessary to meet the required NC levels of the various spaces in a building. Typically, acoustical treatments are needed in the lower octave bands of 63, 125 and 250 hertz. These octave bands produce the low rumble sound from a mechanical unit. The other octave bands — 500, 1,000, 2,000, 4,000 and 8,000 hertz — are mid and high frequencies that produce the high-pitched sounds. Depending on the space requirements, the mid and high frequencies may not require as many acoustical treatments as the low frequencies.

Acoustical sounds treatments are usually required when the rooftop units have lower octave band ratings in the 80-decibel range. Larger rooftop units, such as 50-ton and larger units, will often have decibel ratings of 90 decibels or higher in the lower octave bands.

There are several ways accommodate these frequencies when designing HVAC systems. More distance from a unit’s supply and return opening before entering the space or a duct takeoff will allow for more sound reduction in the ductwork. Using horizontal supply and return ductwork connections to rooftop units will allow additional straight ductwork and elbows to be installed when compared to using vertical supply and return ductwork connections to rooftop units. This also allows for duct liner to be used, which can reduce sound levels in most duct configurations.

Additional ductwork elbows help reduce the sound levels by reflecting the sound wave. An engineer must balance sound performance with the pressure drop and fan performance with the type of ductwork elbows used. A smooth radius elbow will have different attenuation levels than a mitered elbow with turning vanes.

Another option to reduce sound levels is the use of sound attenuators that replace a section of ductwork. These come in rectangular, circular, T-shape or elbow configurations. There are several options for sound attenuators from the internal baffle type and length. Because the airflow is being restricted through a sound attenuator, there is a static pressure drop that an engineer needs to account for. The pressure drop is based on the velocity so the static pressure drop can vary greatly.

For example, a static pressure loss could be 0.10 inch for a sound attenuator design for a duct velocity of 500 fpm compared to a static pressure drop of 1 inch or more for a duct velocity of 2,000 fpm or greater. Good engineering practice is to design the systems for a static pressure of 0.25 inches or less.  This can be done through a combination of increasing duct velocity and selecting the appropriate type of sound attenuator.

It is important to also analyze the return air ductwork path for airborne sound transmission, as sound from the rooftop unit will travel back through the return duct and into the space. All of the airborne treatments mentioned above also need to be considered for the return air path from a rooftop unit. The return path can lead to noise issues in the space if the return ductwork has a direct path to the occupied space.

Supply ductwork breakout noise is often the critical path when dealing with rooftop unit sound levels. Some of these items can help with this such as duct elbows and duct liner. The ceiling type has a significant impact in reducing this sound path.

It is important to work with the architect when determining ductwork breakout noise levels. A ceiling with a higher sound performance can reduce or eliminate some of the HVAC sound mitigation requirements. Ceilings are rated for a noise reduction coefficient value. The ceiling materials are rated in the 250, 500, 1,000 and 2,000 hertz octave band in accordance with ASTM C423. The noise reduction coefficient value ranges from 0 to 1 with the higher the number meaning the more acoustic adsorption the ceiling provides.

Conference room ceilings are typically rated for a 0.9 noise reduction coefficient value. Duct lagging is a flexible mass-produced product that can be used to reduce the effects of ductwork breakout noise. While it is a high-end, expensive solution, it may be appropriate in some applications. Lagging is applied around the outside of the ductwork and typically is available in 1 pound per cubic foot densities.

Radiated sound from rooftop units can cause issues for the spaces underneath where the rooftop units are located. This is becoming more critical as open ceiling concepts have become more popular, as well as if the unit is located over sound-sensitive spaces such as conference rooms or private offices. The ceiling has a large effect on radiated sound. Without a ceiling, the sound mitigation becomes critical.

A mass form is often needed in and/or around the rooftop unit to mitigate the sound transmission from the rooftop unit through the roof. This is mostly dealing with the lower octave bands noted above. There are options on how this achieved and varies from building to building. A common solution is to provide additional concrete at the rooftop unit. This can cause issues with the roof structure, so this needs to be coordinated with the structural engineer.

An alternate solution to this is to provide a hollow concrete curb and use acoustical material to infill the curb. This can provide similar acoustical performance to a solid concrete mass as described above and significantly reduce the weight the roof structure must be designed for.

Rooftop units have rotating parts like all other mechanical equipment that produce vibrations. These include the supply fans, exhaust fans, condenser fans and compressors. It is critical to deal with these so the vibrations are not transmitted to the building structure. Several manufacturers internally isolate these parts, but additional means might be necessary. Vibration curbs isolate the entire rooftop unit through a series of spring isolators located around the curb. It is important to coordinate this with the rooftop unit manufacturer so spring isolators are not doubled up. This can lead to a resonance issue, which can be worse than not providing any vibration isolation.

Applied rooftop units

Although the available options between package rooftop units and applied rooftop units are getting less and less, there are still differences. Package rooftop units are limited in the components that are available. These include supply fan, exhaust fan, direct expansion cooling coil and a gas-fired or electric heating coil. These components are sufficient for a lot of building types, though not all.

An applied air-cooled rooftop unit maybe required, depending on the design requirements such as a low entering air temperature that would require a preheat coil. Additional components are available in applied air-cooled rooftop units as manufacturers make improvements that are used in a host of air handling units. These include return fans, chilled water-cooling coils, steam heating coils, energy recovery wheels, additional filtration levels and fan arrays. Return fans might be desired instead of the typical relief or exhaust fan that is only offered in package rooftop units.

Energy recovery systems are available in 30-ton units and larger, depending on the manufacturer. Applied rooftop units have a higher first cost, so it is important to know what the requirements are so there are no surprises with the project budget.

Author Bio: Jon Silhol is a mechanical engineer with SmithGroup. He is a member of ASHRAE and has 20 years of experience designing mechanical systems for various building types.