Heating Systems and Components
What the exam expects
Gas furnace knowledge is heavily tested: the sequence of operation from call for heat through inducer, pressure switch proof, ignition, flame proving, and blower delay; the role of the limit switch, rollout switch, flame sensor, and gas valve; and venting categories and combustion air requirements. Understand manifold pressure, gas derating at altitude, and the difference between a standing pilot and a direct-spark or hot-surface ignition system.
Also expect electric resistance heat (sequencers, limits, element resistance checks), heat pumps in heating mode (reversing valve, defrost, auxiliary and emergency heat), and basic hydronic concepts. Combustion safety is a recurring theme: a cracked heat exchanger, blocked flue, or inadequate combustion air can spill CO into the space.
Traps
- Assuming a furnace that lights is safe - verify combustion analysis and draft.
- Jumping a pressure switch to "make it run" and leaving it.
- Confusing a rollout switch trip (a symptom) with the actual cause (blocked flue or restricted air).
- Forgetting that flame rectification depends on a clean sensor and a good ground.
- Ignoring altitude derating, which reduces delivered Btu/h.
Worked example
Clock the meter: a 2-cubic-foot dial completes one revolution in 72 seconds. Rate = (2 x 3,600) / 72 = 100 ft³/h. With natural gas at about 1,000 Btu/ft³, input is roughly 100,000 Btu/h. A measured input well below nameplate suggests low manifold pressure or a partially blocked orifice.