Replacement rooftop units such as these at the Atlanta Motor Speedway, offer maintenance, service, humidity control, and direct digital control advantages over their predecessors. (Photo courtesy of Lennox.)

DALLAS - According to Lennox, humidity control and system servicability in rooftop units (RTUs) go a long way toward keeping commercial customers happy. They can also make HVAC contractors look like heroes by keeping energy costs down.

Humidity control is vital to achieving acceptable IAQ in commercial buildings, but it can be a challenge for many systems, especially on mild days, said the manufacturer.


The goal is to keep the relative humidity (rh) level below 60 percent, the recommendation of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).

According to literature from Lennox, when the weather is hot and there is a high sensible load, a rooftop unit is required to run most of the day.

During the normal cooling cycle, a standard rooftop unit will remove high levels of moisture from the outdoor air that enters the building, as well as return air from the occupied space. In many cases, the manufacturer said, this will keep the building below the recommended 60 percent rh level.

A hot gas reheat system initiates dehumidification operation based on the actual rh level of the space.

The Humiditrol® system, for example, can remove moisture by operating the refrigeration cycle like a standard unit in cooling mode, then using a hot gas reheat coil to reheat the cooled and dehumidified air to a neutral temperature before returning it to the occupied space.

During mild-temperature and high-humidity conditions, like rainy days and humid nights, hot gas reheat units can remove more moisture because they start operation based on a need to dehumidify the space, instead of a need to control temperature, Lennox pointed out. Units with hot gas reheat can run until the humidity set point is satisfied.

For example, when the outdoor air is 65°F, a standard L Series® 3-ton rooftop unit can remove approximately 0.18 gallons of water per hour. The same unit equipped with hot gas reheat dehumidification can remove 1.44 gallons per hour.

A Lennox replacement rooftop like this was used at the Atlanta Motor Speedway. (Photo courtesy of Lennox.)


Improving the efficiency and speed of RTU service and maintenance can also help a contractor save time and increase profitability, by helping the customer save time and money. More of today’s RTUs offer more designs that allow technicians to work faster, reducing the chance of error and simplifying facility management requirements, said Lennox. Today’s RTUs offer the following improvements that facilitate service and maintenance:

• Toolless access panels. Some door handles don’t require any tools to open them, nor screws and bolts to be removed.

• Hinges on critical access doors. Hinges on doors to the air filter, economizer, compressor, and controls can make a lot of service and maintenance easier.

“Components should be located around the perimeter of the unit so they can be easily seen and reached during inspection and repair,” the manufacturer said.

“In cases where performance requirements prevent this, look for features such as a slide-out supply fan, which slides out for service but remains in the optimum position during operation.”

• Choose a direct digital control (DDC) system that provides a high level of diagnostic and operating information, to allow technicians to quickly diagnose problems and determine the service or maintenance needed, said Lennox. In general, make sure your DDC has an easy-to-use interface. “Alphanumeric error codes are easy to read and quickly understood.”

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Sidebar: Improve Energy Leaks

DALLAS - Lennox Industries and Wal-Mart Stores Inc., are working together to help reduce energy usage and take advantage of opportunities to provide environmental sustainability. Lennox has designed a new line of high-efficiency, packaged rooftop units (RTUs), and Wal-Mart expects to start installing them during third-quarter 2007.

These RTUs feature efficiencies greater than 12 EER and advanced airflow operation. The units were specifically designed to achieve a high level of energy efficiency during part-load operation, which typically accounts for a majority of operating hours, the manufacturer said.

The line’s efficiency is said to be higher than the stricter energy-efficiency standards set to take effect in the United States and Canada over the next few years.

“Reducing energy usage is an essential part of our sustainability initiatives. These new, cutting-edge, high-efficiency units will help us meet those goals,” said Jim McClendon, chief engineer, Prototype and New Format Development, Wal-Mart Stores. “As the 2006 Vendor of the Year award winner, we were confident that Lennox was up to the task of helping us meet this sustainability objective. They have definitely delivered an outstanding new rooftop unit.”

The new units also help improve the indoor environment for customers and associates through comfort control and IAQ capabilities. A unit control system helps speed and simplify maintenance and installation work. The system also monitors and controls unit performance.

“This new rooftop unit product line builds on that strong tradition of leadership and provides a great solution for our customers,” said Susan Herndon, vice president, National and Global Accounts, Lennox Industries.

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Publication Date:06/18/2007