The Next Phase of Rooftop Units

Rooftop units have functioned in much the same way for a long time. They are packaged HVAC units that contain the necessary components to provide temperature-conditioned air to a space. These units typically contain a fully packaged direct expansion (DX) refrigeration system. In most climates, they also contain a heating element such as an indirect gas-fired furnace or electric heater. Rooftop units (RTUs) are used in commercial buildings, such as retail stores, warehouses, and offices, and historically have been controlled by a basic on/off thermostat like what you find in your home. To minimize the HVAC footprint within the building, these units are installed on the rooftop (hence their name) and connect to the building’s ductwork.

However, RTU technology has recently improved to offer much more than basic air tempering and ventilation. Today’s newest RTUs can dehumidify, modulate their energy usage, and deliver a greater range of mixed air percentages than the range offered by a traditional unitary RTU. This allows for RTUs to support a wider variety of HVAC applications and opens up more possibilities for building design.

Unitary RTUs

Traditionally, RTUs provide three functions. The first is air conditioning, accomplished via refrigerant in the evaporator coil. The unit receives warm return air from the space and delivers cooled supply air. The second is heating, whereby air from the space is returned to the unit, mixed with outdoor air, and heated with either an electric heater or gas furnace. The third is ventilation, during which a typical RTU draws in and conditions a small amount of fresh air to mix with recirculated air.

These functions, while important, leave out several features that are now increasingly common in a new HVAC system. One is dehumidification, which most unitary RTUs do not do; they may increase cooling in a warm room when what is really needed is less humidity. This can result in a less comfortable space for occupants. Another is that unitary RTUs tend to provide an outdoor air (OA) percentage of less than 20%. Finally, it is rare for the compressor, gas furnace, or condensing fan motor in a unitary RTU to modulate, meaning to adjust heating or cooling output depending on space and occupancy conditions.

Recent RTUs

Recent advances in engineering allow for a greater range of capabilities from a rooftop unit. An example of a more advanced RTU component is an inverter scroll compressor, which uses an inverter to control the compressor’s motor speed. This allows for speed to be adjusted during part load conditions, reducing the unit’s energy consumption. Another example is the inclusion of an electronically commutated, or EC, motor in the condensing fan which can modulate to control head pressure. A modulating gas furnace that controls temperature without the need for cycling on and off can also now be designed into an RTU.

RTUs can also include modulating hot gas reheat (HGRH) which dehumidifies at the same time as it cools, making for more comfortable spaces. They can even now incorporate an air-source heat pump (ASHP) that draws energy from ambient air to sustainably support all-electric heating and cooling.

Why Choose an Optimized RTU?

Specifying an RTU with these enhanced features has three main benefits:

• Occupant comfort. Using hot gas reheat to dehumidify in addition to heating or cooling optimizes space comfort and prevents the air from feeling either clammy or humid. The elimination of furnace cycling via a modulating gas furnace also contributes to comfort by tightly controlling temperature and reducing temperature swings.
• Energy efficiency, which translates to lower operating costs. EC motors are a highly efficient technology, as are inverter scroll compressors. Air-source heat pumps are an important component of many decarbonization initiatives due to the energy savings they provide. ASHPs can provide heating capacity in below-freezing ambient conditions down to 0ºF by using boost speed control.
• Outdoor air delivery. Higher percentages of outdoor air contribute to better indoor air quality (IAQ) and higher comfort for occupants. Choose an RTU capable of a wide range of OA percentages, from higher than 35% to as low as 5%, so that you can adjust it to your desired application.

Learn More

Greenheck now offers rooftop units with many configurable options. Your local Greenheck representative can discuss applying RTUs in a building project. You can also check out our product page to read about specifications and features.