Air-Conditioning Control Systems

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Air-Conditioning Control Systems

• Electricity for Refrigeration, Heating and Air
  Conditioning 7th Edition

Chapter 15 Air-Conditioning Control Systems
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Air-Conditioning Control Systems

• Upon completion of this chapter the student will
  be able to

• Understand the electrical circuitry of a
  residential condensing unit
• Make all electrical connections to install a
  condensing unit in a residential application
• Troubleshoot a residential condensing unit
• Understand the basic control systems used in
  residential air-conditioning control systems
• Understand the control systems used in light
  commercial air-cooled and water-cooled packaged
  units
• Make all electrical connections for a complete
  residential installation
• Understand the control systems used In gas heat
  electric air-conditioning packaged units
• Troubleshoot residential air conditioning system

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Key Terms

• Air-Cooled Package Unit
• Condensing Unit
• Factory-Installed Wiring
• Field Wiring
• Gas Pack
• Indoor Fan Relay Package
• Internal Pressure Relief Valve
• Packaged Air-Conditioning Unit
• Short-cycling
• Water-Cooled Packaged Unit

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Basic Condensing Units

• A condensing unit is the portion of a split
  air-conditioning system that is mounted outside
  and contains the compressor, the condenser, the
  condenser fan motor, and the necessary devices to
  control these components.
• A split system is one that is divided into two
  parts, usually a condensing unit (outside) with a
  fan coil unit (inside).
• In most cases, the condensing unit is used with
  some type of equipment that will produce the air
  flow, and a coil must be mounted in the air flow.
  
• Most condensing units used in the industry today
  are air-cooled condensing units.

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Condensing Unit Components for a Simple Control
System

• Most modem condensing units pick up their 24-volt
  power supply to operate the control system from
  the furnace transformer or from an indoor fan
  relay package.
• The evaporator fan motor usually is controlled by
  a relay when the system contains a furnace or fan
  coil unit and the condensing unit.
• The compressor and condenser fan motor in a
  condensing unit are usually controlled by a
  contactor.
• The simplest control system used on condensing
  units today is a contactor that controls the
  operation of the entire condensing unit with the
  exception of the internal overloads in the
  compressor and condenser fan motor.
• Most condensing units without a high-pressure
  switch will have an internal pressure relief
  valve in the compressor that opens if the
  discharge pressure exceeds an unsafe level.

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Condensing Unit Components for Complex Control
Systems

• The more complex systems incorporate a
  high-pressure and a low-pressure switch.
• If any of the safety controls open, the
  compressor and condenser fan motor stop.
• Several manufacturers also use a device that
  protects the system from short-cycling.
• This device maintains a certain period of time
  between the cycles of the equipment so the system
  does not cut on and off in rapid succession
  (short-cycling).
• This control system adds a relay and a time clock
  mechanism for this purpose.

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Wiring

• All condensing units come from the manufacturers
  with a wiring diagram and, in some cases, with an
  installation wiring diagram.
• Usually it is not difficult to follow the
  installation instructions and wire the condensing
  unit correctly.

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Troubleshooting

• Any troubleshooting of the condensing unit can be
  done from the schematic because of the simplicity
  of most control systems.
• By using the schematic diagram, and having an
  understanding of the components, you should have
  no trouble with diagnosing a residential control
  system.

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Packaged Units

• A packaged air-conditioning unit is built with
  all the components housed in one unit.
• In most cases, packaged units are complete except
  for the power connections and the control
  connections.
• Air-cooled packaged units are the most widely
  used types of packaged units.
• The units are used on applications ranging from
  small residential cooling units to large
  commercial and industrial air-cooled units.
• The smaller residential systems may be installed
  when the structure is built or at a later date.
• The air-cooled packaged unit contains the
  compressor, evaporator fan motor, condenser fan
  motor if not of remote design, and all the
  necessary controls.
• Water-cooled packaged units are usually used on
  commercial and industrial systems.
• The water-cooled unit requires a cooling tower
  or some means of supplying water to the
  condenser.
• If a water-cooled unit is used, there must be
  interlocks to ensure that the cooling tower pump
  is operating when the compressor is operating.

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Control System for Packaged Air-Conditioning
Systems

• Water-cooled packaged units utilize a line
  voltage control system with some type of
  interlock to start the cooling tower pump or
  other accessories that are required.
• Air-cooled packaged units can incorporate some
  form of heating as well as cooling.
• The smaller packaged units usually have some easy
  means of installing electric resistance heat into
  the equipment.
• The larger air-cooled packaged units make
  electric and gas heat available to the customer.
• Water-cooled packaged units and air-cooled
  packaged units usually use a hot-water coil,
  steam coil, or resistance heat for heating
  purposes.

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Small Air-Cooled Packaged Units

• The small air-cooled packaged units usually have
  a simple and easy-to-follow control system.
• The control system uses a contactor to operate
  the compressor and the condenser fan motor and an
  evaporator fan relay to control the evaporator
  fan motor.
• All necessary safety components are also included
  in the control system.
• The installation of the air-cooled packaged unit
  is relatively simple because the only necessary
  connections are from the power source and the
  control source or thermostat.

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Gas-Electric Air Conditioners

• The basic control system for the air conditioning
  is the same as the one for most small residential
  units.
• However, several components are added to take
  care of the heating control system.
• The installation instructions should be followed
  when installing this type of system.

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Rooftop Units

• The air-cooled packaged units used in rooftop
  applications are complex because they usually
  have some type of heating to go along with the
  air cooling and, in some cases, some type of zone
  control.
• These units are usually installed on the roof of
  a structure.
• But they could just as easily be installed on
  ground level when the necessary changes are made
  in the ductwork connections.
• Often the electric connections of these systems
  are hard to install because of the number of
  wires required for the control circuit and the
  power wiring.

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Field Wiring

• Wiring that must be installed by the installation
  technician and is called field wiring.
• The factory-installed wiring is the wiring
  installed at the factory.
• The factory wiring has been sized, color-coded,
  and installed in the control system to operate
  the equipment properly.
• The remainder of the wiring, whether it be the
  power wiring or control wiring, must be connected
  in the field.

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Power Wiring

• There are two power connections that must be made
  on a split air-conditioning and heating system
  the connections to the condensing unit and the
  connections to the evaporator fan motor or
  furnace.
• The electric supply wiring of a commercial and
  industrial system is somewhat more complex than
  the simple residential systems.
• The use of three-phase current in commercial and
  industrial structures does not add to the
  complexity of the system, only to the number of
  power wires that must be supplied to the system.

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Sizing Wires and Fuses

• The distance that the power wiring must be run is
  important because of the voltage drop that can
  occur on long circuit runs.
• The installation instructions usually give a
  wiring chart with the length that is allowable
  for each size of wire.
• It is also important to follow the manufacturers
  recommendations for the fuse or breaker size.
• Before any heating, cooling, or refrigeration
  system can be expected to operate properly, it
  must first be supplied with the correct size wire
  and fuse to deliver the proper voltage to the
  system.

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Control Wiring

• The control wiring of heating and
  air-conditioning systems is just as important as
  the supply power wiring.
• Low-voltage control systems are used on
  residential and small commercial applications.
• On large commercial and industrial applications,
  line voltage control systems are often used.
• Many large commercial and industrial systems are
  controlled by special control systems electric,
  pneumatic, or electronic.

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Residential

• All residential packaged air conditioners use a
  low-voltage control system that connects the
  thermostat to the unit.
• Residential split systems are usually easy to
  install.
• They all incorporate a 24-volt control system,
  with the low-voltage power being supplied from an
  indoor fan relay package, the furnace, or the
  condensing units.
• The control system of a residential split system
  can be connected several different ways, and the
  manufacturers installation instructions should
  be followed.
• The more modern condensing units are usually
  designed without the low-voltage transformer
  but with two low-voltage connections instead.

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Industrial

• Control systems for relatively small commercial
  units are similar in design to the residential
  low-voltage system.
• The only exception is that the system uses a
  control relay that supplies the condensing unit
  with 240 volts while using a normal 24-volt
  control system.
• Two-stage heating or two-stage cooling can also
  be used with a low-voltage control system.

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Sizing Wire

• Control wiring in most cases is a small wire (No.
  18 to No. 20) that is usually covered by a rubber
  jacket when using a low-voltage control system.
• Thermostat wire can be purchased single-stranded
  or multi-stranded, with or without a rubber
  jacket protecting the small thermostat wires.
• The wiring of air-conditioning and heating
  systems is one of the most important factors in
  the installation of equipment.
• The life of the equipment can be drastically cut
  by undersized wire or by having loose
  connections.
• The installation instructions are the best place
  to look for the proper method of wiring and the
  proper connection points.

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Schematic Diagram of a Simple Air-Cooled
Condensing Unit
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Schematic Diagram of a Condensing Unit with
Safety Controls
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Schematic Diagram of a Small Air-Cooled Packaged
Unit Used in a Residence
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Schematic Diagram for Electric Air-Conditioning
and Gas-Heating Unit
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Control System of a Furnace