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Packaged Rooftop Systems Offer More Options

Key Points
  • Packaged rooftop units provide space conditioning for a variety of commercial and industrial facilities.
  • These pre-assembled units contain all of the major components found in more complex systems.
  • Optional features can increase system performance but preventive maintenance is required.

Source: www.energy.gov
Packaged rooftop unit
Packaged rooftop heating, ventilating, and air-conditioning (HVAC) systems are found on the roofs of commercial and industrial buildings of all types and sizes. Single-packaged units can cover all the heating and cooling needs of a facility or they can be configured to handle cooling only. A packaged system contains all of the components found in larger HVAC configurations in a pre-assembled, self-contained unit. The advantages of these systems are their simplicity and lower installation cost.

System options

While packaged units are typically pre-assembled, they offer options which may be well-suited to the needs of your facility.

Heating. Natural gas and electric heating options are available in packaged rooftop units. The choice between the two is based primarily on the cost of the unit, as well as operating and installation costs. Natural gas heating units typically have higher overall installation costs when gas piping is included, but are generally less expensive to operate. A gas condensing option is now available. The added cost of piping natural gas to these units may be recovered quickly, depending on fuel costs and efficiency of the equipment.

Cooling. Commercial packaged rooftop cooling systems in the 5- to 30-ton range are often distributed in several locations along a rooftop, depending on the size of the building and the overall cooling load required. The efficiency of air-conditioning systems is measured using Energy Efficiency Rating (EER) and Seasonal Energy Efficiency Rating (SEER) metrics. A higher rating indicates a higher efficiency level. The most widely used energy-efficiency standard, ASHRAE 90.1-2010, recommends a minimum SEER rating of 13.0 for packaged units of 5 tons or less. Air-cooled units sized at 5 to 11 tons should have a minimum EER rating of 11.2, while 11.0 is recommended for systems ranging from 11 to 20 tons. For units over 20 tons, the minimum recommendation is 9.9 EER.

Additional features. Packaged units include a number of optional features for increasing performance and saving energy:

  • Temperature and time controls provide the flexibility to operate heating and cooling according to occupancy schedules and seasonal changes.
  • Economizers can be added to take advantage of free cooling. Dampers and controls allow for outside air to be used for cooling when outdoor temperatures are moderate. ASHRAE 90.1-2010 requires economizers for packaged rooftop units ≥4.5 tons capacity.
  • Variable air volume controls can save energy by matching fan speed to meet airflow demands.
  • Heat/energy recovery wheels transfer heat from summer supply air to the cooler exhaust air which decreases the air-conditioning load.

For facilities with multiple packaged units, comfort zoning can improve overall efficiency and increase flexibility to tailor temperature setting for specific building areas.

Maintenance and energy efficiency

A low-cost method for improving the efficiency of your current packaged rooftop unit is proper maintenance. These units are constantly exposed to the elements and their energy efficiency decreases over time if the coil and fan aren't cleaned, the insulation degrades, the compressor wears and/or the overall system deteriorates. The following maintenance activities help to ensure optimum system performance:

  • Change filters regularly (typically monthly)
  • Check refrigerant charge levels to determine if heat exchangers need cleaning
  • Inspect belts and replace when needed; annually is recommended
  • Check fan bearings and lubricate when appropriate
  • Inspect condensate drain for leaks and corrosion
  • Carefully examine the electrical system, including a visual inspection for contact wear
  • Check economizer louver motion and sealing
  • Apply a light-colored coating around the unit if the roof is black

A year-round maintenance program can pay dividends. For example, a unit with a badly fouled coil might operate at an efficiency of 1.6 kW per ton, rather than the designed 1.3 kW. For a 20-ton unit, that dirty coil could increase energy costs as much as $250 per month or more. To clean, soap and water will work if done regularly. You might also consider installing an ultraviolet light to help keep the coil clean between scheduled maintenance visits. ASHRAE Standard 180 offers checklists for maintenance tasks of rooftop units.

Right-sizing your system

When installing a new packaged system, size the unit to fit your current needs rather than matching the old design. Cooling systems are often oversized for a number of reasons:

  • Reliance on old nameplate data
  • Using rule of thumb formulas, which are only suitable for general estimating purposes
  • Lighting efficiency upgrades, which reduce cooling loads
  • Insulation upgrades, resulting in lower energy losses

Oversized equipment is more expensive to install and operate. Frequent compressor cycling often leads to premature equipment failure and higher electricity costs. Oversized fans and duct work also use more energy and cause greater variation in occupant comfort. Work with a qualified engineer or your system installer to make sure it is sized correctly.

Be proactive

Customers and employees enjoy the comfort a packaged rooftop unit provides. Facility managers and building owners appreciate lower operating costs and increased equipment performance. Careful planning combined with proper maintenance should help to make everyone happy. Work closely with your contractor during the installation process and schedule a regular maintenance program.

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