Electrical Controls and Wiring

Chapter Introduction

This chapter covers all the knowledge required for the Red Seal exam concerning electrical control systems for oil-fired heating equipment. You must master the basic principles of electricity applied to burner circuits, control components, wiring compliant with the Canadian Electrical Code (CE Code) as well as the specific requirements of CSA B139 (installation and maintenance of oil-fired combustion equipment). Exam questions frequently focus on the operating sequence, troubleshooting, and identification of faulty components.

Fundamental Principles of Electricity Applied to Burner Circuits

Voltage, Current, and Resistance in Control Circuits

Oil burner control circuits typically operate at 120 V AC for the primary circuit and 24 V AC for the secondary control circuit (thermostat, aquastat). The relationship between voltage (E), current (I), and resistance (R) is governed by Ohm's law: E = I × R. Power (P) is calculated by P = E × I, expressed in watts (W).

For a typical control circuit, the total load does not exceed a few amps. Calculating current in a control circuit involves adding resistive loads (relays, transformers) and inductive loads (motors, solenoids). Inductive loads present a reactance that must be considered when calculating total impedance (Z), where Z = √(R² + X²).

Series and Parallel Circuits

In a series circuit, the current is identical through all components, and the total resistance is the sum of individual resistances: R_total = R₁ + R₂ + R₃. The voltage drop across each component is proportional to its resistance. In a parallel circuit, the voltage is identical across each branch, and the total resistance is calculated by: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.

For burner control circuits, the series configuration is the most common: the thermostat, aquastat, and safety switches are wired in series in the control circuit. If any one of these components is open (faulty or in the off position), the circuit is interrupted and the burner will not operate.

Power Factor and Inductive Circuits

Fan motors and the ignition coil of the ignition transformer are inductive loads. The power factor (PF) is the ratio of real power (W) to apparent power (VA): PF = W ÷ VA. A low power factor increases the current flowing in the conductors for the same useful power, which can cause overheating of wires and components.

Oil Burner Control Components

The Ignition Transformer

The ignition transformer steps up the voltage from 120 V AC to approximately 10,000 V to create the spark between the electrodes. The essential characteristics to know:

Parameter	Typical Value
Primary voltage	120 V AC
Secondary voltage	10,000 V AC
Secondary current	23 mA
Rated power	7,500 V (peak voltage)

The ignition transformer must be checked with a dielectric tester (megger) to ensure the internal insulation is not degraded. A current leak to ground can cause nuisance tripping of the circuit breaker or a lack of spark.

The Ignition Electrodes

The electrodes are positioned in the spray zone of the nozzle to ignite the air-fuel mixture. The critical adjustments are:

Electrode gap: 3.2 mm (1/8 in) typically

Horizontal distance from the nozzle: 6.4 mm (1/4 in) above the nozzle plane

Vertical distance: 3.2 mm (1/8 in) forward of the nozzle

An incorrect gap causes a weak or erratic spark, delaying ignition and creating an accumulation of unburned fuel in the combustion chamber.

The Flame Controller (Cadmium Sulfide or Photocell)

The flame controller uses a cadmium sulfide (CdS) cell or a photocell to detect the presence of the flame. The CdS cell presents a high resistance (approximately 100 kΩ) in darkness and a low resistance (approximately 1 kΩ) when exposed to the light of the flame.

The flame controller must:

29.Detect the flame within 10 to 15 seconds following the opening of the solenoid valve

30.Lock out the system if the flame is not detected

31.Shut off the fuel supply immediately in the event of flame loss during operation

The Primary Control Relay

The primary control relay is the brain of the system. It controls the start-up sequence, monitors the flame, and ensures safety. The main functions are:

34.Pre-purge period: the fan operates for 15 to 45 seconds before ignition

35.Ignition period: the ignition transformer is energized for 10 to 15 seconds

36.Post-purge period: the fan continues to operate after the burner shuts down

The primary relay can be CdS cell flame detection or thermocouple detection. The CdS cell model is more common in modern installations.

The Thermostat and Aquastat

The room thermostat controls the operation of the burner based on the room temperature. The aquastat controls the water temperature in the boiler. These two components are wired in series in the control circuit.

Component	Function	Normal Position
Thermostat	Opens/closes the circuit based on room temperature	Closed if T < setpoint
Aquastat (high limit)	Shuts off the burner if T > 90 °C	Closed if T < 90 °C
Aquastat (control)	Maintains the water temperature	Closed if T < setpoint

The Solenoid Valve

The solenoid valve controls the fuel flow to the nozzle. It is normally closed and opens when the coil is energized. The coil operates at 120 V AC and draws approximately 0.5 A. A faulty valve can remain open (fuel leak) or fail to open (no fuel supply).

Control Circuit Wiring

Conductor Identification and Standardized Colours

The Canadian Electrical Code, Part I (C22.1) mandates standardized colours for conductors:

Conductor	Colour
Grounded (neutral) conductor	White or natural grey
Protective grounding conductor	Green or green/yellow
Energized conductor (phase)	Black, red, blue (depending on phase)
Control conductor	Other colours (yellow, orange, violet)

In 24 V control circuits, the common conductor is often white and the energized conductor is red or yellow. It is imperative to respect these colour codes to avoid wiring errors during service work.

Wiring Diagrams and Standardized Symbols

You must be able to read and interpret wiring diagrams and hook-up diagrams. The standardized symbols according to CSA Z99 include:

Single-pole switch: line with a contact point

Relay: rectangle with coil and contacts

Motor: circle with the letter M

Transformer: two superimposed coils

Photoelectric cell: circle with incoming arrows

Typical Control Circuit Wiring

The control circuit of a standard oil burner includes the following elements wired in series:

Line (120 V) → Service switch → Primary relay → Aquastat limit → Thermostat → Primary relay (return) → Neutral

The primary relay then supplies:

The fan motor (120 V)

The ignition transformer (120 V)

The solenoid valve (120 V)

Grounding Requirements

Grounding is essential for safety. According to Rule 10-200 of the CE Code, all electrical equipment must be grounded. For oil burners, the burner chassis must be connected to ground by a grounding conductor of minimum 14 AWG copper. The resistance of the ground electrode must not exceed 25 Ω.

Burner Operating Sequence

Complete Start-Up Cycle

The operating sequence of a modern oil burner is as follows:

69.Heat demand: the thermostat closes, sending a signal to the primary relay

70.Pre-purge: the fan motor starts, the ignition transformer is energized (on some models)

71.Valve opening: the solenoid valve opens after 10-15 seconds

72.Ignition: the fuel is sprayed and ignited by the spark

73.Flame detection: the CdS cell detects the flame and the ignition transformer is de-energized

74.Operation: the burner operates normally

75.Shutdown: the thermostat opens, the valve closes, the fan continues during post-purge

Safety Times and Lockout

The safety time is the maximum period during which the burner can operate without flame detection before lockout. According to CSA B139, this time is 15 seconds maximum. After lockout, the system must be manually reset by pressing the reset button on the primary relay.

Common Causes of Lockout

Cause	Symptom	Remedy
Lack of fuel	Lockout after 15 s	Check the tank and supply line
Incorrectly adjusted electrodes	Weak or absent spark	Adjust the gap
Faulty CdS cell	No flame detection	Replace the cell
Blocked solenoid valve	No fuel	Clean or replace
Clogged filter	Insufficient pressure	Replace the filter

Electrical Calculations and Measurements

Electrical Load Calculation

To size the supply circuit for a burner, add the power of all components:

Component	Typical Power
Fan motor	150-300 W
Ignition transformer	50-100 W
Solenoid valve	60 W
Primary controller	10-20 W
Total	270-480 W

The total current is calculated by: I = P ÷ E = 480 W ÷ 120 V = 4 A. The circuit must be protected by a 15 A maximum circuit breaker according to Rule 14-402 of the CE Code.

Measuring CdS Cell Resistance

To check the CdS cell, use an ohmmeter:

In darkness: resistance > 100 kΩ

In light: resistance < 5 kΩ

Under flame: resistance between 500 Ω and 2 kΩ

A cell whose resistance is too high under the flame will cause nuisance lockout. A cell whose resistance is too low in darkness may prevent the burner from starting.

Checking Circuit Continuity

To troubleshoot a control circuit, use a voltmeter to check for the presence of voltage at the following points:

94.At the thermostat terminals: 24 V AC (or 120 V AC depending on the system)

95.At the primary relay terminals: 120 V AC at the input

96.At the primary relay output: 120 V AC during operation

97.At the motor terminals: 120 V AC during operation

Regulatory Requirements and Standards

Canadian Electrical Code, Part I

The CE Code, Part I (C22.1) applies to all electrical installations in Canada. The relevant rules for oil burners include:

Rule 8-200: Calculation of minimum circuit loads

Rule 14-402: Circuit protection by circuit breakers or fuses

Rule 26-700: Installation of heating appliances

Rule 26-702: Specific requirements for oil burners

Rule 26-702 notably requires that the burner be connected by a flexible cable or metal conduit, with an accessible service switch within 3 m of the appliance.

CSA B139 Standard

The CSA B139 standard (Installation and maintenance of oil-fired combustion equipment) specifies installation requirements, including:

The minimum distance between the burner and combustible materials

Ventilation requirements for the boiler room

Combustion testing procedures

Safety requirements for fuel storage

CSA B149.1 Standard

The CSA B149.1 (Natural gas and propane installation code) applies only to gas installations, but oil heating technicians must know the differences between the two types of systems to avoid confusion during inspections.

Troubleshooting Control Systems

Systematic Troubleshooting Procedure

Follow this procedure to diagnose a burner that is not operating:

119.Check the electrical supply: presence of voltage at the circuit breaker and service switch

120.Check the thermostat: continuity between terminals (closed if heat demand)

121.Check the aquastat: continuity between terminals (closed if temperature < setpoint)

122.Check the primary relay: indicator light on, reset button pressed

123.Check the motor: voltage at terminals, free rotation of the fan wheel

124.Check the ignition transformer: spark at the electrodes

125.Check the solenoid valve: voltage at terminals, audible click when opening

126.Check the CdS cell: resistance under the flame

Essential Diagnostic Tools

Tool	Use
Digital multimeter	Voltage, resistance, continuity
Clamp ammeter	Motor current
Dielectric tester	Transformer insulation
Pressure gauge	Pump pressure
Combustion analyzer	CO₂, O₂, flue gas temperature

Common Diagnostic Errors

Confusing a safety lockout with an electrical failure: always check the reset button

Ignoring the post-purge: the fan may continue to operate after shutdown, which is normal

Not checking polarity: reversed polarity can cause electric shocks and malfunctions

Replacing components without checking the root cause: a burned-out ignition transformer can be caused by incorrectly adjusted electrodes

Pitfalls to Avoid

136.Confusing the control circuit (24 V) with the power circuit (120 V): always check the voltage before touching conductors.

137.Forgetting to check the reset button: the most common cause of an inoperative burner is an unreset lockout.

138.Measuring CdS cell resistance with the circuit energized: always disconnect the cell before measuring its resistance.

139.Ignoring grounding requirements: an ungrounded burner is a deadly hazard and a violation of the CE Code.

140.Using inadequate wire gauges: the control circuit must use wire of at least 14 AWG for 120 V.

141.Not respecting the 15-second safety time: a primary relay that does not lock out within this time is faulty.

142.Confusing conductor colours: white is always the neutral, green is always the ground.

143.Forgetting the post-purge when troubleshooting: the fan may operate for up to 2 minutes after the burner shuts down.

144.Replacing a component without checking the adjustments: the electrodes must be set with a gauge after every service.

145.Not documenting service work: the Red Seal requires complete traceability of work performed.

Summary

Ohm's law (E = I × R) and power (P = E × I) are the foundations of all electrical calculations.

Burner control circuits are wired in series: thermostat, aquastat, and safety switches.

The ignition transformer produces 10,000 V to create the spark at the electrodes.

The CdS cell detects the flame and must present a resistance < 5 kΩ under the flame.

The primary relay controls the complete sequence: pre-purge, ignition, operation, post-purge.

The maximum safety time is 15 seconds before lockout.

The Canadian Electrical Code, Part I (Rule 26-702) and the CSA B139 standard govern installation.

Grounding is mandatory with a minimum 14 AWG conductor.

Troubleshooting must follow a systematic procedure: supply → thermostat → aquastat → relay → motor → ignition → valve → flame.

Conductor colours are standardized: white (neutral), green (ground), black/red (phase).

Review Questions

160.What is the typical secondary voltage of an ignition transformer?

161.What is the maximum safety time before lockout according to CSA B139?

162.What is the normal resistance of a CdS cell under the flame?

163.Which components are wired in series in the control circuit?

164.Which CE Code rule specifies the requirements for oil burners?

165.How do you calculate the total current of a burner whose components total 480 W?

166.What is the standardized colour of the grounding conductor?

167.What is the typical horizontal distance between the electrodes and the nozzle?

168.What happens if the CdS cell presents too low a resistance in darkness?

169.What is the minimum gauge of the grounding conductor for a burner?

Electrical Controls and Wiring