GAS FURNACE COMPONENTS

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GAS FURNACE COMPONENTS

Gas Furnace Components: Functions & Overview

1. Thermostat

  • Function: Controls furnace operation by signaling when to heat.
  • Key Parts: Temperature sensor, control board interface.
  • Failure Signs: Inconsistent heating, unresponsive display.

2. Gas Valve

  • Function: Regulates gas flow from the supply line to the burner.
  • Types:
    • Standing Pilot: Constant small flame.
    • Electronic Ignition: Sparks only when heating.
  • Failure Signs: Gas leaks (rotten egg smell), no heat.

3. Burners

  • Function: Mix gas with air for combustion; generate heat.
  • Design: Stainless steel tubes with ports for flame distribution.
  • Issues: Clogged ports (reduced flame), carbon buildup.

4. Heat Exchanger

  • Function: Transfers heat from combustion gases to indoor air without mixing.
  • Material: Steel or aluminized steel.
  • Critical Failure: Cracks (leak CO into home; shut off immediately).

5. Blower Motor

  • Function: Circulates heated air through ductwork.
  • Types:
    • PSC (Permanent Split Capacitor): Single-speed.
    • ECM (Electronically Commutated Motor): Variable-speed, energy-efficient.
  • Failure Signs: Weak airflow, unusual noises.

6. Ignition System

  • Types:
    • Hot Surface Igniter (HSI): Glowing ceramic element (most common).
    • Intermittent Pilot: Spark ignites pilot flame.
  • Failure Signs: Furnace won’t start; igniter doesn’t glow.

7. Flame Sensor

  • Function: Detects burner flame; shuts off gas if no flame (safety feature).
  • Issue: Dirt/corrosion causes false shutdowns (clean annually).

8. Draft Inducer Motor

  • Function:
    • Pulls combustion gases out through flue.
    • Creates negative pressure for safe venting.
  • Failure Signs: Loud grinding, furnace locks out.

9. Pressure Switch

  • Function: Confirms draft inducer is operating before allowing gas flow.
  • Failure: Stuck open/closed; causes system to abort startup.

10. Control Board

  • Function: “Brain” of furnace; sequences operations (ignition, blower, gas valve).
  • Failure Signs: No power, erratic cycling, error codes.

11. Air Filter

  • Function: Traps dust before air enters furnace.
  • Maintenance: Replace every 1–3 months (prevents blower/heat exchanger strain).

12. Flue Vent

  • Function: Exhausts combustion gases outdoors.
  • Materials:
    • Metal (B-Vent): For natural draft furnaces.
    • PVC: For high-efficiency condensing furnaces.
  • Blockage Risks: Bird nests, ice, debris.

How a Gas Furnace Works

  1. Thermostat calls for heat.
  2. Draft inducer starts → pressure switch validates.
  3. Igniter activates → gas valve opens → burners light.
  4. Heat exchanger warms air.
  5. Blower circulates heated air.
  6. Flue vent exhausts combustion byproducts.

Efficiency Ratings

  • AFUE (Annual Fuel Utilization Efficiency):
    • Standard: 80% AFUE (20% heat lost).
    • High-Efficiency: 90–98% AFUE (condenses flue gases to capture extra heat).

Troubleshooting Common Issues

Symptom Likely Culprit Quick Check
No heat Igniter, gas valve Verify gas supply; inspect igniter.
Short cycling Dirty filter, airflow Replace filter; check vents.
Blower won’t stop Faulty thermostat Test thermostat settings.
CO alarm sounds Cracked heat exchanger EVACUATE; call professional.

Safety Notes

  • Carbon Monoxide (CO): Install detectors near bedrooms.
  • Annual Inspection: Essential to check heat exchanger, flue, and burners.
  • Gas Leaks: Smell rotten eggs? Shut off gas, evacuate, call emergency services.

Lifespan & Replacement

  • Typical Lifespan: 15–20 years.
  • Upgrade Tip: Replace with ≥90% AFUE unit for 30% lower fuel costs.

Find out more about Heating Repair

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