Gas Furnace Controls, Safety Devices, and Limits
What the exam tests
Control boards, thermostats, pressure switches, limit/rollout switches, flame sensors, gas valve safety circuits, and lockout behavior.
Core rules
- Safety switches are wired in series with the gas valve circuit: high limit, rollout, pressure switch, and flame-proving circuit. Any open switch must stop gas flow.
- High-limit switches typically open at 180–200°F and reset automatically; rollout switches are manual-reset and open around 200–250°F. A tripped rollout means flame escaped the heat exchanger.
- The pressure switch proves inducer airflow/draft; it does not prove flame. Its tubing must be clear and routed correctly.
- The control board sequences inducer, ignition, gas valve, blower, and shutdown; it also enforces ignition trials, retries, and lockout.
- A cracked heat exchanger, blocked flue, or dirty filter can cause limit cycling or rollout. Never bypass a safety to "test" operation.
Worked example
A furnace runs 10 minutes, the burner shuts off, the blower continues, and it restarts after 5 minutes. The high limit is opening. Check filter, blower speed, and temperature rise. If rise is 90°F against a 40–70°F nameplate, airflow is low. Correcting airflow restores normal limit operation.
Traps
Do not replace a limit for nuisance tripping before measuring temperature rise and static pressure. Do not confuse a pressure-switch fault with a flame-sensor fault; the sequence and fault code tell them apart. Manual-reset rollout switches must not be reset repeatedly without finding the cause.