Heating Control Devices

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Heating Control Devices

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

Chapter 13 Heating Control Devices
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Heating Control Devices

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

• Explain the purpose of the electrical controls in
  warm air and hydronic heating applications that
  are necessary to safely operate and maintain the
  desired temperature in a conditioned space
• Describe the pilot safety controls and methods of
  ignition of the burners in a gas furnace
• Describe the operation of primary controls used
  to supervise the operation of an oil burner
• Draw a wiring diagram of an oil-fired, warm-air
  furnace
• Draw a wiring diagram of a gas-fired, warm-air
  furnace

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Heating Control Devices

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

• Explain the operation of an electric furnace or
  electric resistance duct heaters and the methods
  of control that are commonly use
• Draw the wiring diagram of an electric furnace
• Troubleshoot a gas furnace
• Troubleshoot an oil furnace
• Troubleshoot an electric furnace or electric
  resistance duct heater

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Key Terms

• Cad Cell
• Electrical Resistance Heater
• Fan Switch
• Gas Valve
• Hot Surface Ignition
• Ignition Module
• Pilot
• Pilot Assembly
• Primary Control
• Sequencer
• Spark Ignition
• Stack Switch
• Thermocouple

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Heating Fundamentals

• The basic heating appliance used in the heating
  and cooling industry usually heats air or water,
  or produces steam.
• Air is the most popular method of transferring
  heat from the appliance to the structure, and
  there are many different styles and designs of
  warm-air furnaces.
• Water is also a popular method of transferring
  heat from the appliance to the conditioned space.

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Basic Heating Controls

• In the basic warm-air furnace there are many
  controls that are applicable to warm-air furnaces
  regardless of the type of energy that is being
  used to supply the heat to the structure.

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Fan Controls

• In all types of forced-air heating equipment
  there must be some method of controlling the fan
  motor in order that the warm air is delivered to
  the conditioned space at the correct temperature.
• There are several types of fan switches used in
  industry Temperature Controlled, Time and
  Temperature Controlled, and Time Controlled.

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Temperature-Controlled Fan Switch

• Temperature-controlled fan switch is nothing more
  than a thermostat that close on a rise in
  temperature to start the fan motor when the
  furnace can supply warm air to the structure.
• The temperature-controlled fan switch must be set
  correctly in order to maintain the temperature of
  the air delivered to the structure and prevent
  overheating the combustion chamber.

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Limit Switches

• Limit switches on heating appliances are
  basically thermostats that open when an unsafe
  condition exists in the furnace, such as high
  furnace temperatures.
• Limit switches are used in case of flame rollout,
  which is flame extending outside the combustion
  chamber or heat exchanger.

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Control Circuitry of a line Voltage Limit Switch
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Control Circuitry of a line Voltage Limit Switch
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Gas Heating Controls

• The basic controls of a gas heating appliance is
  initiated when a switch, usually a thermostat,
  closes to call for heat.
• In a gas heating system this call for heat
  completes the heating control circuit, starting a
  chain reaction that results in lighting the
  burner.
• There are three basic types of gas burner
  controls standing pilots, where the pilot burns
  continuously intermittent pilots, where the
  pilot is automatically lit on a call for heat
  and direct ignition, where some method is used to
  light the main burner upon a call for heat.

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Proper Pilot Flame
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Power Distribution
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Intermittent Pilot Control System

• The intermittent pilot control system must light
  the pilot and control the main gas valve.
• The intermittent pilot only when there is a call
  for heating and remains off when there is no call
  for heat.
• There must be some method of igniting the pilot
  burner

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The Operational Sequence of an Intermittent Pilot
Burner Control System

• On a call for heat, some modules have a pre-purge
  cycle that occurs before the spark starts. During
  this pre-purge cycle, the combustion blower runs
  to clear the heat exchanger of any unburned gas.
  This cycle usually lasts 30 to 45 seconds.
• On a call for heat, the ignition module does a
  self-check, and if a failure is shown, the
  ignition wont start. If the checks are good, the
  module begins a safety lockout timing, powers
  the spark igniter, and opens the solenoid valve
  so gas can flow to the pilot. The pilot must
  light within a certain period of time or the
  module closes the valve.
• When the pilot lights, current flows from the
  ignition sensor through the pilot flame to the
  burner head and then to ground the ionized pilot
  flame provides a current path between the rod and
  burner head, rectifying the current. Because of
  the difference in size of the sensor and burner,
  current flows in only one direction. The current
  is a pulsating direct or rectified current, and
  it tells the module that a flame has been
  established. Ignition stops and the second main
  gas valve opens, allowing gas to flow to the main
  burner.
• As long as this rectified flame current remains
  above the minimum, the module keeps the main gas
  valves open. If the current drops below

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Diagram of Gas Furnace with Intermittent Pilot
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Direct Ignition Burner Control System

• The direct ignition systems use a spark igniter
  (direct spark ignition) or a silicon carbide
  igniter (hot surface ignition) to light the main
  gas burner directly.
• Ignition stops after a designated time or when
  the main burner flame ignition has been properly
  proved.
• The typical components of a direct ignition
  burner control system are the ignition module,
  igniter, sensor, gas control, and other common
  controls used on any type of gas furnace.

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The Operational Sequence of the Direct Ignition
Burner Control System

• On a call for heat, most modules have a pre-purge
  cycle that occurs before ignition. During this
  pre-purge cycle, the combustion blower runs to
  clear the heat exchanger of unburned gas. The
  cycle usually lasts 30 to 45 seconds.
• On a call for heat, the ignition module does a
  self-check, and if a failure is shown, the
  ignition will not start. If the checks are good,
  the module begins a safety lockout, powers the
  igniter, and opens the gas valve.
• Once ignition starts, the burner must light and
  ignition must be proved within the safety lockout
  timing. If the burner does not light, then the
  ignition stops and the gas valve closes. On a
  lockout, the system must be manually reset. Many
  modules allow for several attempts at ignition
  before locking out.

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The Operational Sequence of the Direct Ignition
Burner Control System

• When the flame lights, current flows from the
  sensor through the ionized pilot flame to the
  burner head and then to ground. The current is a
  pulsating, direct, or rectified current, and it
  tells the module that flame has been established.
  Ignition stops and the burner continues to run.
• As long as this rectified flame current remains
  above the minimum, the module keeps the gas valve
  open. If the current drops below the minimum or
  becomes unsteady, the module interrupts power to
  the gas valve, closing the valve and stopping gas
  flow. The module then performs the start safety
  check and, if its safe, the module attempts
  ignition again. Figure 13.30 shows a flowchart of
  the operation of a direct ignition system.

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Typical Diagram of Gas Furnace with Direct
Ignition Controller
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Oil Heating Controls

• The function of an oil burner control system is
  to turn the heating system on and off in response
  to the needs of the conditioned space.
• The control must also safeguard the operation of
  the heating appliance and oil burner.
• The primary control is the heart of an oil burner
  control system and supervises the operation of
  the oil burner.
• The primary control must control the oil burner
  motor, ignition transformer, and oil solenoid
  valve, if used, upon a call for heat.
• The primary control must safely control the
  operation of the oil burner.
• The primary control must ensure that the burner
  has lit and that the flame has been proved.
• Most primary controls will have to be manually
  reset once a flame failure has occurred in the
  oil burner.

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Cad Cell Oil Burner Primary Controls

• This primary control device cad cell mounted so
  that it its resistance according to cad cell
  decreases as the consists of a primary control
  and a light-sensitive views the oil burner flame.
  
• The cad cell changes the intensity of the light.
  The resistance of the intensity of the light
  increases

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Light Response Tolerance of Typical Cad Cell
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Correct Placement of a Cad Cell
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Stack Switch Oil Burner Primary Controls

• The stack switch is a heat-actuated control that
  uses the stack temperature to indicate that the
  oil burner has or has not established a flame.
• A bimetal element inserted into the stack
  actuates a push rod when the bimetal senses heat,
  signaling that the flame has been established and
  breaking the circuit to the safety switch.
• The correct location and mounting of the stack
  switch is in the center of the stack or vent in
  the direct path of the hot flue gases.
• The stack switch primary control starts the
  burner and supervises burner operation. When the
  thermostat calls for heat, the stack switch
  closes a relay, which starts the burner motor and
  ignition transformer and opens the oil solenoid,
  if used.
• At the same time, the safety switch heater starts
  to heat. If the oil burner establishes a flame
  and heat is felt in the stack, the bimetal in the
  stack switch will open a set of contacts, thus
  dc-energizing the safety switch heater.
• Stack switches are available with intermittent
  ignition, which stops the ignition transformer
  when the flame has been proven.

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Typical Diagram of an Oil-Fired Furnace with Cad
Cell Primary Control
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Typical Diagram of Oil-Fired Furnace with Stack
Switch
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Typical Wiring of Stack Switch
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Typical Diagram of an Electric Furnace Using a
Sequencer to Control Heater