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Gas Furnace & Combustion Safety Study Guide

Gas furnace sequence of operation, ignition, flame sensing, gas pressure, venting and combustion safety.

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A refresher for people already in the trade, not a training course. Always follow the equipment manufacturer’s instructions and current codes.

Lesson 1 of 8

Gas Furnace Operation

The Gas Heating Sequence of Operation

  1. Thermostat calls for heat
  2. Inducer draft motor starts, proving draft via a pressure switch
  3. Ignition system lights the burners
  4. Flame sensor confirms ignition; gas valve stays open
  5. Blower starts after a short heat-up delay
  6. Thermostat is satisfied; burners shut off, blower runs briefly to extract remaining heat
Check yourself: What proves that the inducer draft motor is running before ignition is allowed?
  1. The thermostat
  2. The pressure switch
  3. The flame sensor
  4. The blower relay

Answer: B. The pressure switch

Lesson 2 of 8

Ignition Systems

Types of Furnace Ignition

  • Standing pilot: a small gas flame burns continuously; a thermocouple proves the pilot is lit.
  • Intermittent pilot (spark ignition): a spark ignites a pilot only when there is a call for heat.
  • Hot surface ignition (HSI): an electrically heated element glows and ignites the burners directly — the most common in modern furnaces.
Check yourself: Which ignition type uses an electrically heated element to light the burners directly?
  1. Standing pilot
  2. Intermittent spark pilot
  3. Hot surface ignition
  4. Manual ignition

Answer: C. Hot surface ignition

Lesson 3 of 8

Gas Valves & Manifold Pressure

Setting and Verifying Gas Pressure

  • Manifold pressure is measured at the gas valve outlet with a manometer, and must match the manufacturer's specification for the gas type (natural gas vs propane).
  • Typical natural gas manifold pressure is around 3.5" WC; propane is typically higher, around 10" WC — always confirm on the rating plate.
  • Incorrect manifold pressure causes poor combustion, sooting, or insufficient heat output.
Check yourself: What instrument is used to measure manifold gas pressure?
  1. Multimeter
  2. Manometer
  3. Micron gauge
  4. Anemometer

Answer: B. Manometer

Lesson 4 of 8

Venting & Combustion Air

Getting Exhaust Out and Air In Safely

  • Category I furnaces vent using natural draft up a chimney or vertical vent; Category IV (high-efficiency, condensing) furnaces use a sealed PVC vent with a powered inducer.
  • Adequate combustion air must be supplied to the equipment room — a starved system can pull exhaust gases back into the space (backdrafting).
  • Condensing furnaces produce acidic condensate that must drain properly, or it can damage the heat exchanger.
Check yourself: What is a risk when a furnace does not get adequate combustion air?
  1. Higher efficiency
  2. Backdrafting exhaust gases into the space
  3. Lower gas pressure
  4. Faster ignition

Answer: B. Backdrafting exhaust gases into the space

Lesson 5 of 8

Safety Switches & Limit Controls

Protective Controls on Every Furnace

  • High-limit switch: shuts off burners if the heat exchanger overheats, usually from restricted airflow.
  • Flame rollout switch: trips if flame escapes the burner compartment — often due to a blocked heat exchanger or vent.
  • Pressure switch: confirms proper draft before allowing ignition.
  • Never bypass a safety switch to "get the customer's heat working" — it is there to prevent fire or carbon monoxide hazards.
Check yourself: What is the correct action if a safety switch keeps tripping?
  1. Permanently jumper the switch
  2. Diagnose and fix the underlying cause
  3. Replace it with a lower-rated part
  4. Ignore it if the customer is in a hurry

Answer: B. Diagnose and fix the underlying cause

Lesson 6 of 8

Combustion Analysis Basics

Verifying Clean, Safe Combustion

  • A combustion analyzer measures flue gases including CO (carbon monoxide), O2, and CO2 to confirm the burner is tuned correctly.
  • High CO readings indicate incomplete combustion — a serious safety issue that must be corrected immediately.
  • Combustion analysis should be part of every gas furnace maintenance visit, not just a complaint-driven check.
Check yourself: A high carbon monoxide (CO) reading during combustion analysis indicates what?
  1. Perfect combustion
  2. Incomplete combustion — a safety issue
  3. Excess airflow
  4. Correct gas pressure

Answer: B. Incomplete combustion — a safety issue

Lesson 7 of 8

Heat Exchangers & Cracked Exchanger Hazards

The Most Safety-Critical Furnace Component

  • The heat exchanger separates the combustion gases from the breathable air the blower pushes into the home.
  • A cracked heat exchanger can leak carbon monoxide into the living space — this is one of the most serious hazards a technician can find.
  • Signs of a possible crack include soot patterns, a flame that flickers with the blower cycling, or a rattling noise from the heat exchanger.
  • Any furnace with a confirmed cracked heat exchanger must be shut down and tagged out immediately, not just noted for a future repair.
⚠️ Never restart a furnace with a confirmed cracked heat exchanger, even temporarily.
Check yourself: What is the main danger of a cracked heat exchanger?
  1. Reduced airflow only
  2. It can leak carbon monoxide into the living space
  3. It lowers the AFUE rating
  4. It causes the pilot light to go out

Answer: B. It can leak carbon monoxide into the living space

Lesson 8 of 8

AFUE & Furnace Efficiency

Understanding AFUE Ratings

  • AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel energy converted into usable heat over a typical year.
  • An 80% AFUE furnace vents combustion byproducts at a higher temperature through a standard metal flue.
  • A 95%+ AFUE "high-efficiency" furnace condenses moisture out of the exhaust, requiring PVC venting and a condensate drain.
  • Never assume flue material — high-efficiency furnaces cannot be vented like standard-efficiency units, and mixing the two approaches is a code violation and safety hazard.
Check yourself: Why do high-efficiency (95%+ AFUE) furnaces require PVC venting instead of metal flue?
  1. PVC is cheaper to install
  2. The exhaust condenses moisture and runs cooler, which metal flue is not rated for
  3. PVC improves gas pressure
  4. It is only a cosmetic preference

Answer: B. The exhaust condenses moisture and runs cooler, which metal flue is not rated for

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