Split System Heat Cool Air Conditioner

Split system air conditioners are the common type of system found in residential applications. 

 Commercial uses are also numerous. Some applications include small offices and buildings or applications where rooftop mounted package units are not feasible.

 These units are also referred to as whole house cooling or central systems.

 Systems have these main components:

  • Compressor / condensing unit.  The compressor compresses the refrigerant gas, the condenser coil condenses the gas to a liquid, the condenser fan moves outside air over the condenser to discharge the heat removed from inside the building.  Humidity is controlled by condensation forming on the coil, collecting in a condensate drain pan and into drain piping.  Main electrical components include contactors and relays to start and stop the motors.  Heat pump condensing units have a reversing valve and associated controls to reverse the cycle and heat the building.
  • An air handling unit containing an evaporator coil fitted with an expansion valve or device to remove heat from the interior air. A supply air blower moves air across the coil and into ductwork to be distributed to the different spaces. Some units operate as heat pumps and provide both summer cooling and winter heating.
  • Thermostat to start and stop the condensing unit and indoor fan based on temperature.
  • Many systems have the cooling coil installed on a gas, oil, propane, or electric furnace.
  • Building codes require ductwork to bring in outside fresh air to the air unit.  The amount is based on the number of occupants and square footage of the building.
  • Air filters are included to keep the cooling coil from becoming fouled.  Fouled coils reduce performance and can cause expensive mechanical failures.  Filters can also have the added benefit of cleaning the air.

 The term, “split system,” is also described as a central air conditioner and heat pump.

 Advantages of Split Systems

The advantages over room AC units and package units are:

  • Mass production resulting in low price.
  • Dozens of brands and hundreds of model variations.
  • Efficiencies from 13 SEER to 20 SEER.
  • Versatility in zoning and design.  Multiple units can be sized and installed to serve zones with different heat gain characteristics such as upstairs and downstairs applications.
  • The most common Btu per hour rating (Btu/h) of these units range from 30,000 to 60,000. (2.5 to 5 tons) Larger units are available.
  • Air handlers can be installed in a closet, suspended in an attic or under floor in a basement or crawl space.
  • Energy efficient units utilizing dual compressors, multispeed compressors and variable speed compressors are available at additional cost.
  • Air cooled condensers are the standard.  More efficient water cooled evaporative condensers and the earth coupled geothermal condensers are available.
  • Air handling units are available in the standard constant volume, or more efficient variable speed, variable volume.

 Disadvantages

  • Units have historically been installed using rule of thumb engineering.  Tons per hundred square feet as an example.  As building envelopes have improved, units have been oversized, resulting in low energy efficiency and decreased comfort.
  • The term split refers to the split of the indoor unit and the outdoor unit.  Pipe to carry refrigerant between the units must be field installed.  This increases cost and the possibility of refrigerant leaks.
  • Spit systems require more refrigerant than package units.
  • Ductwork installed in unconditioned spaces such as attics 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.
  • Pipe is needed for drainage of condensate to outside the building.
  • Historically 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.

 Cost

  • Cost is about $1,500 per ton (12,000 Btu/h) of cooling capacity for the builder model type units.