Heat Cool Rooftop Air Conditioners

RTU air conditioners are the common system found in commercial applications and are also found in some residential applications. 

 Applications include small offices and low rise buildings where the owner or builder require low first cost and are willing to sacrifice energy efficiency.

 These package units contain all the components to produce cooled or heated air:

  • Compressor to compress the refrigerant.
  • Condenser coil to convert the compressed gas to a liquid.
  • Condenser fan to remove the heat in the condenser coil that was removed from the building.
  • Evaporator coil to remove the heat and humidity from indoor air.
  • Evaporator expansion valve or device to produce the cooling at the evaporator coil.
  • Supply air blower to move indoor air over the evaporator coil and through the ductwork.
  • Condensate pan to collect the condensation from the evaporator coil.
  • Some units operate as heat pumps and contain a reversing valve and associated controls.
  • Many systems have internal furnaces that operate on gas, propane, or electric.
  • A thermostat is used to operate the motors and compressors based on indoor temperature.  Humidistat is an option if humidity control is a priority.
  • Fresh air ventilation damper to control the amount of outside fresh air introduced into the system.
  • Economizer dampers and controls are an option.  If the system is calling for cooling and the outdoor air conditions can provide satisfactory air, the compressor is turned off and outdoor air is used for cooling.
  • In areas where hail is common, hail guards will save replacing the condenser coil and significant energy if the coil is damaged and not replaced.

 Advantages

The advantages of split system AC units and package units are:

  • Mass production resulting in low price.
  • Dozens of sizes, brands and hundreds of model variations.
  • Installed on the roof making floor space available for income producing uses.
  • Versatility in zoning and design.  Multiple units can be sized and installed to serve zones with different heat gain characteristics such as east and west building exposures.
  • Typically, the Btu per hour rating (Btu/h) of these units range from 30,000 to 240,000. (2.5 to 20 tons) Larger units are available.
  • Energy efficient units utilizing multiple compressors and variable speed compressors are available at additional cost.
  • Supply blowers are available in the standard constant volume, or more efficient variable speed, variable volume models.
  • Ventilation is build-in and does not require additional ductwork.
  • Combustion gas vents are built-in.
  • Filter racks can be located inside the unit or in the ductwork below.

 Disadvantages

  • Limited availability of high efficiency models.
  • Units have historically been installed using rule of thumb engineering.  Tons per hundred square feet as an example.  As building envelopes have improved units are typically oversized resulting in low energy efficiency and decreased comfort.
  • Ductwork installed is in unconditioned spaces such as attics and can account for 10% to 20% or more loss in energy efficiency due to leaks and heat gain into the duct.
  • Units must be properly sized to prevent short-cycling, which wastes energy and does not provide the desired temperature and humidity control.
  • Efficiency improvements have been accomplished by increasing the size of the outside condenser coil.  As this coil becomes dirty in normal use, significant loss in energy efficiency and unit capacity occurs.  Good maintenance is essential.
  • Water cooled condensers are not widely available or practical.
  • Roof leaks due to increased roof openings and traffic on the roof..
  • Roof environment is a terrible place to put an AC unit

 Cost

  • Cost is about $2,500 per ton (12,000 Btu/h) of cooling capacity for the builder model type units.