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All-Air Rooftop

General

These systems are commonly applied to low-rise buildings and have the bulk of the equipment on the roof, either as a factory package or in a built-up penthouse. They usually use reciprocating or screw compressors with air-cooled condensers. Unitary Systems have all components on the roof. Split Systems have the compressor and condenser on the roof and the evaporator coil and air handling components in separate packages located inside the building.

rooftop heating cooling

Unitary systems require large roof penetrations for the ducts. Split systems have only small penetrations for the piping but do require field refrigeration piping and system evacuation and charging with refrigerant.

Rooftop systems can utilize single-zone, multizone, or VAV cycles.

Their advantages include:

  • Ease of installation as roofs are readily accessible and large
  • Reduced first cost due to elimination of indoor machinery rooms
  • No loss of rentable space
  • Factory-packaged guaranteed performance units available from the manufacturer
  • Each tenant can own, operate and maintain a separate unit, or owner can individually meter and bill each tenant for energy.
  • Each tenant can set its operating hours without regard to others (while a central plant must operate whenever any tenant requires cooling or heating).

Their disadvantages include:

  • Because of the factory set sensible heat ratio in packages, oversized units may be needed to achieve both the required sensible and latent capacity for an individual zone.
  • Step controls on cooling and heating limit the quality of temperature and humidity control
  • Difficult trouble-shooting and maintenance during inclement weather
  • Problems meeting exhaust building code requirements under fire conditions
  • Problems locating air intakes away from condenser air discharges and plumbing vents
  • Failure to install adequate access walkways to each unit, or of personnel using them, can result in roof damage and cause water leaks into the tenant space.

Links to Related Topics

Ventilation
Indoor Air Quality
Filtration
Outside Air Control
Controls
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Dual Duct
All-Air Central Multizone
Air-Water Central Systems
All Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System