Heating Control Devices
Electricity for Refrigeration, Heating and Air
Conditioning 7th Edition
Chapter 13 Heating Control Devices
Heating Control Devices
Upon completion of this chapter the student will be able to:
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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
Heating Control Devices
Upon completion of this chapter the student will be able to:
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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
Key Terms
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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
Heating Fundamentals
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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.
Basic Heating Controls
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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.
Fan Controls
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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.
Temperature-Controlled Fan Switch
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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.
Limit Switches
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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.
Control Circuitry of a line Voltage Limit Switch
Control Circuitry of a line Voltage Limit Switch
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.
Proper Pilot Flame
Power Distribution
Intermittent Pilot Control System
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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
The Operational Sequence of an Intermittent Pilot
Burner Control System
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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 won’t 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
Diagram of Gas Furnace with Intermittent Pilot
Direct Ignition Burner Control System
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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.
The Operational Sequence of the Direct Ignition
Burner Control System
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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.
The Operational Sequence of the Direct
Ignition Burner Control System
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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 it’s safe, the module attempts
ignition again. Figure 13.30 shows a flowchart of the operation of a direct ignition system.
Typical Diagram of Gas Furnace with Direct
Ignition Controller
Oil Heating Controls
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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.
Cad Cell Oil Burner Primary Controls
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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
Light Response Tolerance of Typical Cad Cell
Correct Placement of a Cad Cell
Stack Switch Oil Burner Primary Controls
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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.
Typical Diagram of an Oil-Fired Furnace with Cad
Cell Primary Control
Typical Diagram of Oil-Fired Furnace with Stack
Switch
Typical Wiring of
Stack Switch
Typical Diagram of an Electric Furnace Using a
Sequencer to Control Heater