Chapter 13Heating Controls

7/30/2019 Chapter 13Heating Controls

1/30

Heating Control Devices

Electricity for Refrigeration, Heating and Air

Conditioning 7th Edition

Chapter 13 Heating Control Devices


2/30


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

Explain the purpose of the electrical controls in warm air andhydronic 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 supervisethe 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


3/30


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

Explain the operation of an electric furnace or electricresistance 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 ductheater


4/30

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


5/30

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.


6/30

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.


7/30

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.


8/30

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 thetemperature of the air delivered to the structure and prevent overheating the

combustion chamber.


9/30

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.


10/30

Control Circuitry of a line Voltage Limit Switch


11/30

Control Circuitry of a line Voltage Limit Switch


12/30

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 heatingcontrol 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 somemethod is used to light the main burner upon a call for heat.


13/30

Proper Pilot Flame


14/30

Power Distribution


15/30

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


16/30

The Operational Sequence of an Intermittent PilotBurner 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 to45 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 f lows 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


17/30

Diagram of Gas Furnace with Intermittent Pilot


18/30

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.


19/30

The Operational Sequence of the Direct IgnitionBurner 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 thechecks 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.


20/30

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.

The Operational Sequence of the DirectIgnition Burner Control System


21/30

Typical Diagram of Gas Furnace with DirectIgnition Controller


22/30

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 andsupervises 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 flamehas been proved.

Most primary controls will have to be manually reset once a flame failure

has occurred in the oil burner.


23/30

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


24/30

Light Response Tolerance of Typical Cad Cell


25/30

Correct Placement of a Cad Cell


26/30

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 thehot 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.


27/30

Typical Diagram of an Oil-Fired Furnace with CadCell Primary Control


28/30

Typical Diagram of Oil-Fired Furnace with StackSwitch


29/30

Typical Wiring ofStack Switch


30/30

Typical Diagram of an Electric Furnace Using aSequencer to Control Heater

Chapter 13Heating Controls

Documents

Transcript of Chapter 13Heating Controls