Controls, Safety Devices, and Electrical Interface

Chapter Introduction

This chapter covers all the controls, safety devices, and electrical interfaces that every Class B gasfitter must master for the Red Seal exam. You will find operating principles, regulatory requirements from the Canadian Electrical Code, Part I (CE Code) and CSA B149.1, as well as installation, testing, and troubleshooting procedures. Approximately 15 to 20% of exam questions cover these systems. A thorough understanding of this chapter is therefore essential.

1. Fundamental Principles of Control Circuits

1.1 Voltage and Current in Control Circuits

Control circuits for residential and commercial gas appliances typically operate at 24 V AC, obtained from a step-down transformer. This low voltage is chosen for safety and simplicity. Power circuits (fan motors, pumps) operate at 120 V or 240 V AC, depending on the load.

The fundamental relationship you need to know is Ohm's Law: E = I × R (voltage = current × resistance). In alternating current, impedance Z is used rather than pure resistance: E = I × Z. Total circuit impedance includes resistance, inductive reactance, and capacitive reactance.

Electrical Power: P = E × I (watts) for a direct current circuit. For a single-phase alternating current circuit: P = E × I × power factor (cos φ). For a three-phase circuit: P = √3 × E × I × cos φ.

1.2 The Control Transformer

The step-down transformer converts line voltage (120 V or 240 V) to 24 V AC for the control circuit. Essential points:

The primary is connected to the power source; the secondary supplies the control circuit.

The power rating of the transformer is expressed in volt-amperes (VA). It must be sufficient to supply all loads in the control circuit (thermostat, valves, relays, etc.).

An overloaded transformer overheats and can cause a fire. The total load of the control circuit must not exceed 80% of the transformer's rated capacity.

Typical calculation: A control circuit includes a thermostat (0.5 A), a gas valve (0.8 A), and a relay (0.3 A). Total current = 1.6 A. Required power = 24 V × 1.6 A = 38.4 VA. A 40 VA transformer would be acceptable, but a 50 VA model is recommended for safety margin.

1.3 Wiring Diagrams and Symbols

You must be able to read and interpret ladder diagrams and wiring diagrams. Standardized symbols include:

Symbol	Component
L1, L2	Power supply lines (120 V, 240 V)
T1, T2	Transformer terminals (24 V)
R (rectangle)	Resistance / heating element
S (switch)	Normally open (NO) or normally closed (NC) contact
Coil (circle)	Relay, contactor, valve
Contact (line with angle)	Relay contact, NO or NC

Golden rule: In a ladder diagram, the control circuit is drawn between two vertical rails (L1 and L2 or T1 and T2). Loads are placed on the right rail. Each rung represents an independent circuit.

2. Thermostats and Temperature Control Devices

2.1 Mechanical Thermostats

The mechanical thermostat uses a bimetallic element or a bellows filled with gas or liquid. The expansion or contraction of the element actuates an electrical contact. Key points:

Heat anticipator: a small heating resistor that warms the bimetallic element while the burner is operating, causing the contact to open slightly before the room temperature reaches the setpoint. This prevents excessively long cycles and temperature overshoot.

Differential: the gap between the cut-in and cut-out temperatures. A differential that is too narrow causes short cycling; a differential that is too wide causes uncomfortable temperature variations.

2.2 Electronic Thermostats

Electronic thermostats use thermistors (temperature-sensitive resistors) or thermocouples to measure temperature. They offer superior accuracy (±0.5 °C) and programmable features. Essential points:

Power supply: batteries, 24 V AC from the transformer, or backup battery.

Electronic heat anticipator: digital adjustment, often automatic.

Compatibility with heating systems: conventional (gas, oil, electric) or heat pump.

2.3 Heat Anticipator Adjustment

Heat anticipator adjustment is a common exam procedure. The anticipator value must match the total control circuit current when the thermostat is closed. This value is measured in amperes and is set on the anticipator dial (typically 0.1 A to 1.2 A).

Adjustment procedure:

34.Measure the control circuit current with an ammeter (clamp-on ammeter) in series with the thermostat.

35.Set the anticipator to the measured value.

36.Verify the operating cycle: the burner should start and stop normally, without short cycling.

Common trap: An anticipator set too high causes long cycles and temperature overshoot; an anticipator set too low causes short cycles and excessive room cooling.

3. Gas Valves and Safety Devices

3.1 Main Gas Valves

The main gas valve (combination valve-regulator) controls the gas supply to the burner. It includes:

An integrated pressure regulator that maintains a constant outlet pressure (typically 3.5 inches water column for natural gas, 11 inches for propane).

A manual shut-off device (handle).

A solenoid or servomotor for automatic opening and closing.

A pressure tap port (manometer) for measurement.

3.2 Solenoid Valves

The solenoid valve opens when the coil is energized with 24 V AC. The plunger is attracted by the magnetic field, opening the gas passage. Important points:

Minimum holding voltage: the valve must remain open with reduced voltage (typically 85% of rated voltage).

Opening time: typically 1 to 3 seconds.

Closing time: must be fast (less than 1 second) to shut off gas immediately in the event of flame loss.

3.3 Safety Shut-off Device (Thermocouple)

The thermocouple is a safety device that detects the presence of a pilot flame. It is made of two different metals (typically copper and constantan) welded at one end (the hot junction). When heated by the pilot flame, it generates an electromotive force (emf) of approximately 25 to 30 mV DC. This voltage energizes the coil of the magnetic gas valve (electromagnet) that holds the pilot valve open.

Operation:

54.The pilot is lit manually; the thermocouple heats up.

55.The generated emf holds the pilot valve open.

56.If the pilot flame goes out, the thermocouple cools, the emf drops to zero, the valve closes and shuts off the gas.

Thermocouple tests:

Measure the voltage at the cold junction (terminals) with a multimeter set to mV DC: it must be at least 10 to 12 mV to hold the valve open.

A dirty or oxidized thermocouple produces insufficient voltage.

Tightening of the fitting at the valve is critical: excessive tightening torque can crush the cold junction and reduce voltage.

3.4 Flame Safety Shut-off Device (Flame Rectification)

Modern electronic systems use flame rectification. A sensing rod (electrode) is placed in the flame. The flame acts as a rectifier: it allows current to pass in only one direction (positive or negative half-cycle). The control module detects this rectified current (in µA) and holds the gas valve open.

Key points:

Typical flame current is 1 to 10 µA DC.

Polarity is important: the sensing rod must be connected to the appropriate terminal on the module.

A yellow or unstable flame produces weak current.

The sensing cable must be insulated and routed away from power cables to avoid interference.

3.5 Safety Pressure Switches

The pressure switch is a safety device that verifies gas or air pressure before ignition. Two main types:

Gas pressure switch: verifies that supply pressure is within the acceptable range (minimum and maximum). It is normally closed (NC) and opens if pressure is too low or too high.

Air pressure switch (on forced-draft appliances): verifies that the fan is operating correctly and that draft is sufficient. It is normally open (NO) and closes when airflow is adequate.

Adjustment: The pressure switch is set according to manufacturer specifications. Incorrect adjustment can prevent ignition or cause nuisance shutdowns.

4. Ignition Systems

4.1 Standing Pilot Ignition

The standing pilot is a small flame that remains lit continuously. It is held by the thermocouple (or thermopile). Ignition of the main burner occurs from the pilot when the main valve opens.

Advantages: simplicity, reliability, no electronics.

Disadvantages: continuous gas consumption (approximately 3 to 5 ft³/day), energy loss.

4.2 Intermittent Pilot Ignition (IPI)

Intermittent Pilot Ignition uses a pilot that only lights when the thermostat calls for heat. An electronic module generates a high-voltage spark (10,000 to 20,000 V) to light the pilot. Once the pilot flame is detected (by rectification), the main valve opens.

Typical sequence:

82.The thermostat calls for heat.

83.The module energizes the pilot valve and generates sparks.

84.The pilot flame is detected (rectification).

85.The main valve opens; the burner ignites.

86.In the event of flame loss, the module locks out after 3 attempts.

4.3 Direct Spark Ignition (DSI)

Direct Spark Ignition eliminates the pilot. The spark is generated directly at the main burner. The module opens the main valve and generates sparks simultaneously. The flame is detected by rectification.

Advantages: gas savings (no pilot), fast ignition.

Disadvantages: more complex electronics, risk of lockout in the event of a problem.

4.4 Flame Detection by Rectification

Flame rectification is the most common detection method in electronic systems. The module applies an alternating voltage to the sensing rod. The flame rectifies this voltage (diode effect). The module measures the rectified (DC) current to confirm the presence of the flame.

Typical values:

Minimum flame current: 1 µA (some modules require 2 to 3 µA).

Normal flame current: 5 to 10 µA.

If the current is below the threshold, the module locks out.

5. Relays, Contactors, and Timers

5.1 Control Relays

The relay is an electromechanical switch controlled by a coil. When the coil is energized (24 V AC), the magnetic field attracts the armature and actuates the contacts. Contact types:

NO (normally open): closes when the coil is energized.

NC (normally closed): opens when the coil is energized.

SPDT (single-pole double-throw): one common contact, one NO contact, and one NC contact.

Applications: control circuit isolation, switching of significant loads, interlocking.

5.2 Contactors

The contactor is a power relay used to switch significant loads (fan motors, compressors). It is designed to handle high currents (15 to 100 A) and voltages from 120 V to 600 V. Contacts are made of silver alloy to withstand electrical arcing.

5.3 Timers

Timers are used for:

Delaying ignition (pre-purge).

Limiting operating duration (safety timer).

Controlling defrost cycles.

Types: mechanical (synchronous motor), electronic (RC circuit), digital (microprocessor).

6. Canadian Electrical Code, Part I (CE Code) Requirements

The Canadian Electrical Code, Part I (CE Code) (C22.1-21) applies to the electrical installations of gas appliances. The following rules are essential for the exam:

6.1 Rule 8-200: Circuit Capacity

Rule 8-200 states that circuit capacity must be calculated based on the total load. For 24 V control circuits, the load is generally low, but the transformer must be properly sized.

6.2 Rule 10-204: Grounding

Rule 10-204 requires the grounding of all non-current-carrying metal parts that could become energized. Gas appliances must be connected to ground by a grounding conductor.

6.3 Rules 26-700 to 26-728: Heating Appliances

These rules cover the electrical installation of gas heating appliances. Key points:

Rule 26-702: appliances must be connected with approved cable or conduit.

Rule 26-704: appliances must have an accessible disconnecting means.

Rule 26-706: motors must be protected against overloads.

6.4 Rule 2-024: Conductor Identification

Rule 2-024 requires conductor identification by colour or marking. The grounding conductor is green or green with yellow stripes. The neutral conductor is white or grey. Phase conductors are black, red, blue, etc.

7. CSA B149.1 Requirements

CSA B149.1 (Natural Gas and Propane Installation Code) contains specific requirements for safety devices and controls.

7.1 Article 5.8: Safety Devices

Article 5.8 requires that all gas appliances be equipped with safety devices that shut off the gas supply in the event of:

Flame loss (thermocouple, rectification).

Abnormal gas pressure (pressure switch).

Insufficient ventilation (air pressure switch, draft switch).

7.2 Article 5.10: Temperature Controls

Article 5.10 requires that heating appliances be equipped with a thermostat or temperature control device that maintains temperature within prescribed limits.

7.3 Article 5.12: Testing

Article 5.12 requires that all safety devices be tested after installation and during maintenance. Tests must be performed in accordance with manufacturer instructions.

7.4 Article 6.4: Ventilation

Article 6.4 addresses ventilation of spaces where gas appliances are installed. Inadequate ventilation can cause carbon monoxide (CO) accumulation and trigger safety devices.

8. Testing and Troubleshooting Procedures

8.1 Voltage and Current Verification

Tools: digital multimeter (DMM), clamp-on ammeter, manometer.

Voltage verification procedure:

146.Turn off the electrical power supply.

147.Set the multimeter to the AC voltage function (V~).

148.Connect the probes to the circuit terminals.

149.Restore power and measure.

Flame current measurement:

151.Turn off the power.

152.Insert the multimeter (µA DC setting) in series in the flame sensing circuit.

153.Restore power and observe the reading.

8.2 Thermocouple Testing

155.Measure the voltage at the cold junction (terminals): it must be at least 10 mV (typically 25 to 30 mV).

156.Heat the hot junction with a lighter: the voltage should increase.

157.Check the tightness of the fitting: excessive tightening can damage the thermocouple.

8.3 Pressure Switch Testing

159.Check gas pressure with a manometer (in inches of water column, in. w.c.).

160.For an air pressure switch: verify fan operation and measure differential pressure.

161.Compare measured values to manufacturer specifications.

8.4 Ignition System Troubleshooting

Symptom: The burner does not ignite.

Checks:

165.Electrical power supply (120 V at the transformer primary).

166.Secondary voltage (24 V).

167.Thermostat: continuity, setting, anticipator.

168.Pressure switches: condition, setting, connections.

169.Ignition module: diagnostic light, lockout.

170.Ignition electrode: position, gap, cable.

171.Sensing electrode: position, contamination, cable.

172.Gas valve: coil voltage, continuity.

Symptom: The burner shuts off after a few seconds.

Checks:

175.Flame current (rectification): must be above the module threshold.

176.Sensing electrode position: must be in the flame.

177.Grounding: must be correct and continuous.

9. Reference Tables

9.1 Typical Voltages and Currents

Component	Voltage	Typical Current
Control transformer	24 V AC	1 to 5 A
Thermostat	24 V AC	0.1 to 1.2 A
Gas valve (solenoid)	24 V AC	0.5 to 1.5 A
Relay	24 V AC	0.1 to 0.5 A
Thermocouple	25 to 30 mV DC	0.1 to 0.5 A
Flame sensing (rectification)	24 V AC	1 to 10 µA DC
Ignition electrode	10,000 to 20,000 V	1 to 5 mA

9.2 Typical Gas Pressures

Gas Type	Supply Pressure	Outlet Pressure (regulator)
Natural gas	7 to 14 in. w.c.	3.5 in. w.c.
Propane	11 to 14 in. w.c.	11 in. w.c.

9.3 Unit Equivalencies

Unit	Equivalence
1 in. w.c.	249 Pa (0.249 kPa)
1 kPa	4.02 in. w.c.
1 psi	27.7 in. w.c.
1 ft³ natural gas	1,000 BTU (approx.)
1 BTU	1,055 J

10. Common Pitfalls to Avoid

188.Confusing NO and NC: A NO contact is open at rest; a NC contact is closed at rest. Always read the diagram before testing.

189.Forgetting thermocouple polarity: The thermocouple is polarized; reversing the connections prevents the valve from opening.

190.Neglecting grounding: Poor grounding causes nuisance shutdowns and electrical shocks.

191.Undersizing the transformer: The control circuit load must not exceed 80% of the transformer capacity.

192.Ignoring pressure switches: A misadjusted or faulty pressure switch prevents ignition or causes shutdowns.

193.Confusing voltages: The control circuit is 24 V, but the power circuit is 120/240 V. Always turn off the power before working.

194.Forgetting to purge: After a gas shut-off, the system must be purged before attempting ignition.

195.Not checking flame current: Insufficient flame current (less than 1 µA) causes repeated lockouts.

196.Using non-certified parts: Safety devices must be CSA or ULC certified.

197.Ignoring manufacturer instructions: Settings and procedures vary from one appliance to another.

11. Summary

Control circuits operate at 24 V AC, obtained from a step-down transformer. Ohm's Law (E = I × R) and power (P = E × I) are fundamental.

The thermostat regulates temperature; the heat anticipator must be set to the control circuit current.

Gas valves are controlled by solenoids; the thermocouple (25 to 30 mV) and flame rectification (1 to 10 µA) provide flame detection.

Pressure switches verify gas and air pressure before ignition.

Ignition systems include the standing pilot, intermittent pilot ignition (IPI), and direct spark ignition (DSI).

The Canadian Electrical Code, Part I (CE Code) (Rules 8-200, 10-204, 26-700 to 26-728) and CSA B149.1 (Articles 5.8, 5.10, 5.12, 6.4) impose specific requirements.

Testing procedures include measuring voltage, current, pressure, and flame current.

Common pitfalls include confusing NO/NC, thermocouple polarity, grounding, and transformer sizing.

12. Self-Assessment Questions

210.What is the typical voltage of a control circuit? (24 V AC)

211.What is the emf generated by a heated thermocouple? (25 to 30 mV DC)

212.What is the minimum flame current for rectification? (1 µA)

213.What is the typical outlet pressure of a natural gas regulator? (3.5 in. w.c.)

214.Which rule of the Canadian Electrical Code addresses grounding? (Rule 10-204)

215.Which article of CSA B149.1 addresses safety devices? (Article 5.8)

216.What is the recommended maximum capacity for a control transformer? (80% of rated load)

217.What happens if the heat anticipator is set too low? (short cycling)

218.What is the role of the air pressure switch? (verify fan operation)

219.What is the difference between IPI and DSI? (IPI uses an intermittent pilot; DSI ignites the main burner directly)

This chapter prepares you for Red Seal exam questions on controls, safety devices, and the electrical interface. Review the diagrams, typical values, and code rules. Good luck with your preparation!

Controls, Safety Devices, and Electrical Interface