Electrical Knowledge for Gas Equipment
Core concepts
Gas-fired equipment depends on electrical controls, so the exam expects basic electrical literacy: Ohm's law, series versus parallel circuits, and the behavior of switches, thermostats, and safety interlocks. You should understand 24-volt control circuits, transformers, and how a call for heat travels from thermostat to control board to gas valve. Know the function of flame sensing (thermocouple, flame rectification), limit switches, and rollout switches, and why each is wired in series as a safety device. Basic motor theory for inducer and blower motors, capacitor function, and the meaning of normally open versus normally closed contacts are all fair game.
Traps to watch
Questions frequently test whether a safety switch is wired in series (it must open the circuit on fault) and whether a component is load or control. Another common trap is confusing voltage and current measurements: voltage is measured across a load, current in series with it. Be careful with transformer VA ratings and with the difference between a thermocouple (millivolt) and a flame rectification sensor. Do not assume a blown fuse means the board is bad; trace the circuit.
Worked example
A 24-volt control circuit draws 0.5 amps. Using Ohm's law, resistance equals voltage divided by current: 24 / 0.5 = 48 ohms. If the transformer is rated 40 VA, the available current is 40 / 24 ≈ 1.67 amps, so the 0.5-amp load is within capacity. Rule of thumb: safety devices go in series; measuring devices go in parallel.