Gas Heating Systems
Gas furnace operation, ignition sequence, combustion safety, heat exchangers, gas pressures, and temperature rise.
- Describe the complete ignition sequence of a modern gas furnace
- State typical natural gas manifold pressure and temperature rise ranges
- Explain the function of limit switches, flame rollout switches, and pressure switches
- Identify symptoms of a cracked heat exchanger and associated safety hazards
- Calculate airflow using the temperature rise method and verify proper furnace operation
Leçon 1
Furnace Types & the Ignition Sequence
Types of Gas Furnaces
Gas furnaces are classified by efficiency and air flow direction:
| Type | Efficiency | Venting | Condensation |
|---|---|---|---|
| Standard (80%) | 78-82% AFUE | Category I, natural draft or induced | No |
| Mid-efficiency | 80-83% AFUE | Category I, induced draft | No |
| Condensing (90%+) | 90-98% AFUE | Category IV, PVC/CPVC vent | Yes |
| Upflow | N/A | Air flows up through furnace | N/A |
| Downflow | N/A | Air flows down through furnace | N/A |
| Horizontal | N/A | Air flows horizontally | N/A |
Condensing furnaces extract so much heat from the combustion gases that water vapor in the flue gas condenses. This additional heat extraction is what pushes efficiency above 90%. The condensate is acidic and must be properly drained.
The Modern Ignition Sequence
Most modern furnaces use a hot surface igniter (HSI) instead of a standing pilot. The ignition sequence follows a precise order controlled by the integrated furnace control (IFC) board:
Pre-Purge and Post-Purge
Pre-purge: The inducer runs before ignition to clear any residual gas from the combustion chamber. Post-purge: The inducer and/or blower continues running after the flame goes out to clear remaining combustion products from the heat exchanger. Both are critical safety features.
The modern gas furnace ignition sequence is: thermostat call, inducer start (pre-purge), pressure switch proves draft, hot surface igniter glows, gas valve opens, flame sensor proves flame, blower starts after time delay. Each step must complete before the next begins. The hot surface igniter has replaced standing pilots on most modern furnaces.