Electrical and Controls
This domain tests the low-voltage and line-voltage controls that start, monitor, and stop an oil burner. Expect thermostat and transformer circuits, primary controls, cad cells, ignition, limit and aquastat switches, and basic troubleshooting with a meter.
Control circuit
- The thermostat switches 24 V from the transformer to the primary control's T-T terminals; the primary control then powers the burner motor, ignition, and oil valve.
- The cad cell is a light-dependent resistor on the F-F terminals. In darkness its resistance is very high; when it sees flame it drops to roughly 500–1,600 Ω, which the primary control accepts as flame.
- If flame is not proven within the trial-for-ignition time (often 15–30 seconds), the primary control locks out and must be reset manually.
Safety switches
- Limit and aquastat switches break the burner circuit on high temperature; a low-water cutoff protects steam and some hot-water boilers. Never jumper a safety control to keep a call running.
Worked example
A burner runs 10 seconds, then locks out. The cad cell reads 50,000 Ω with the burner off and 900 Ω while flame is lit. Because 900 Ω is below the typical 1,600 Ω threshold, the primary should prove flame, so the fault is likely in the primary's flame-sensing circuit or a loose F-F connection. The reasoning: the cell proves light, the primary interprets it. If the cell stayed at 50,000 Ω with flame visible, the fault would be the cell, its lens, or its wiring.
Traps
- A cad cell sees light, not heat, so a dirty lens or misaligned cell looks like no flame.
- Measuring voltage with the meter in series blows the meter fuse.
- A transformer must be sized for the VA of the controls it supplies.
- Never bypass a limit or low-water cutoff.