Electrical Wiring, Schematics, and Sequence of Operation
What the exam tests
Reading ladder and wiring diagrams, tracing 24 V and line-voltage circuits, understanding series safeties, and following the sequence of operation.
Core rules
- Line voltage (120 V) feeds the transformer, inducer, igniter, and blower; the transformer steps down to 24 V for the thermostat and gas valve.
- In a ladder diagram, power flows left to right; series switches must all close, parallel paths are alternatives. Safety switches are series; the thermostat is a switch between R and W.
- Typical sequence: call for heat → inducer on → pressure switch closes → igniter energizes → gas valve opens → flame proven → blower on after delay → call satisfied → gas off → blower off delay.
- Color code: R 24 V hot, C common, W heat, G blower, Y cooling, and common safety/limit wires per manufacturer.
- A blown low-voltage fuse usually means a shorted wire, valve, or thermostat wire, not a bad board.
Worked example
No heat; thermostat calls W. At the board, R to W has 24 V, but the inducer does not start. The inducer is wired through a high limit and a board relay. Check for 120 V at the inducer. If 120 V is present, the inducer motor or capacitor has failed; if absent, trace the series limit and board output.
Traps
Do not assume a wire color is universal across brands. Do not measure 24 V with a low-impedance tester that loads the circuit. Do not overlook the common wire: a thermostat that has R but no C may need batteries or a different configuration.