Today, everything has changed. Gone is the concept of one size fits all. Buyers have more options, including practically all of the same features found in other types of building HVAC systems, from economizers for energy conservation and prewired interfaces for building automation systems to heat wheels and other heat recovery devices. No longer can the systems be labeled as bare bones.
What has contributed to this change has been a shift in focus from first costs to life-cycle costs by both manufacturers and users. The result has been the development of new rooftop systems that include features to make them more efficient and easier to maintain.
The typical rooftop unit today comes in one of two configurations: single-zone or VAV. Single-zone systems remain the more widely used. They are low-pressure systems that provide a constant volume of airflow to a single zone controlled by a single thermostat. Since they do not include controls for multiple zones, this type of rooftop unit is best suited for use in the areas with fairly uniform heating and cooling loads.
The VAV configuration is gaining popularity, primarily as a result of its ability to serve multiple areas, each with different heating and cooling loads. The typical VAV rooftop unit supplies air at a constant temperature to a distribution system that includes VAV boxes to regulate airflow into each different area. The VAV configuration makes the units more flexible while improving their operating efficiency.
Rooftop units are available today in a wide range of capacities and configurations. The most common cooling configuration uses a DX system, with air or water cooling. Operating efficiencies for air-cooled units range between 1.0 and 1.5 kW per ton, and between 0.80 and 1.0 kW per ton for water-cooled units, including all fan and pump energy use. Units can also be configured to use externally generated chilled water. Today’s typical rooftop unit is 30 percent more efficient than those of earlier generations.
Rooftop units can use hot water, steam, natural gas or propane for heating, with capacities ranging from 40,000 to more than 2 million Btu for natural gas and propane units, and up to 4 million Btu for steam units.
Some of the most common complaints about rooftop units have come from maintenance personnel. Early generation systems simply were not easy to maintain. Access panels were held in place with sheet metal screws. Panels often were not replaced properly and screws were lost, exposing the interior of the unit to the elements. With time, the protective finishes on the exposed panels broke down, resulting in rusting and further exposure of unit components to the elements. Removable panels were often dropped during removal or reinstallation, resulting in damage to the building’s roof. But perhaps the most serious drawback was that the units were not designed to make maintenance easy. As a result, proper maintenance was rarely performed.
New rooftop unit designs facilitate maintenance. Exterior panels are protected by an epoxy finish that resists the elements, practically eliminating rusting problems. Machine screws have been replaced with hinged panels and latches, allowing easier access and more secure fastening of panels. Electronic diagnostic panels have been included in some system designs to ease system setup and troubleshooting. The net result is that rooftop systems offer the same advantages as earlier generation systems — including low first costs, factory assembly and ease of installation — without sacrificing performance or efficiency.
Rooftop units are suited for use in practically any low-rise application, such as in retail and institutional facilities. They are particularly well suited for applications where flexibility is required, such as frequent reconfiguration of interior spaces and functions.
