Gas Furnace Service and Combustion Basics

Every winter, a first-year HVAC apprentice will ride shotgun on dozens of furnace no-heat calls. A gas furnace is a controlled explosion repeated thousands of times an hour, every component engineered to make that combustion safe, efficient, and reliable. Understanding combustion, the furnace's sequence of operation, and the symptoms of each failure mode separates a tech who swaps parts until it works from one who diagnoses the failure, fixes it right the first time, and keeps the customer alive. This guide covers furnace types, combustion science, the analyzer readings you need to know, every major component, the sequence of operation, the common failures by age and symptom, and the safety absolutes you never bend.

Furnace Types (By AFUE)

AFUE (Annual Fuel Utilization Efficiency) is the seasonal efficiency of a furnace - how much of the heat from the fuel actually ends up in the conditioned space.

| Type                     | Typical AFUE | Venting         | Key Feature                              |
|--------------------------|--------------|-----------------|------------------------------------------|
| Atmospheric (natural draft) | 60-70%       | Masonry/Type B  | Standing pilot, dilution air, legacy only |
| Induced draft (mid-eff)  | 80-83%       | Metal vent      | Induced-draft fan, electronic ignition   |
| Condensing (high-eff)    | 90-98%       | PVC             | Secondary HX, condensate drain            |
| Modulating condensing    | 95-98%+      | PVC             | Variable gas valve, continuous fit       |

Natural draft atmospheric furnaces are largely gone from new install (banned or tightly restricted in many states for new construction) but still common in 30+ year-old homes. Induced-draft 80% AFUE is common in retrofit where PVC venting is impractical. Condensing 90%+ furnaces are the new-install standard in most climate zones, and modulating-gas-valve versions are premium high-comfort equipment.

Combustion Science - The Chemistry

Natural gas (methane, CH4) combusts as:

CH4 + 2 O2 -> CO2 + 2 H2O + heat

One molecule of methane plus two molecules of oxygen produces one of carbon dioxide, two of water, and heat. Propane (C3H8) is similar with more carbon:

C3H8 + 5 O2 -> 3 CO2 + 4 H2O + heat

Perfect (stoichiometric) combustion takes exactly the right amount of air. Too little and you get carbon monoxide (CO), soot, and incomplete combustion. Too much and you get wasted heat up the flue.

Real furnaces run with 10-50% excess air above stoichiometric. Excess air makes combustion safer (ensures complete burn) at a small efficiency cost. The combustion analyzer reads this excess as the O2 reading - more excess = more O2 in the flue.

Combustion Analyzer Readings

A combustion analyzer (Bacharach Fyrite InTech, Testo 310, UEI EAGLE 2) is the single most important service tool on a furnace. It probes the flue and reads oxygen, carbon monoxide, flue temperature, and calculates CO2 and efficiency. Target readings for natural gas:

| Reading       | Target (Natural Gas, Steady-State)   |
|---------------|--------------------------------------|
| O2            | 5-10% (lower = more efficient)       |
| CO (air-free) | <100 ppm good; <50 ppm ideal; >400 ppm stop and investigate |
| CO2           | 7-10%                                 |
| Stack temp    | Matches rating plate (often 300-500 deg F for 80% AFUE) |
| Excess air    | Typically 30-50%                      |

LP (propane) runs similar with slightly different CO2 target (9-11%).

CO that is "air-free" (the analyzer compensates for the excess air dilution) is the number to trust. A raw CO of 50 ppm at high excess air can be "air-free CO" of 200+ ppm - dangerous.

If CO is above 400 ppm air-free at steady state, stop the furnace and investigate. Common causes: flame impingement on dirty heat exchanger, combustion air blocked, burner misalignment, cracked heat exchanger.

Key Components - Walkthrough

Draft Inducer (Induced-Draft Motor)

A small motor-driven fan in the flue path. Pulls combustion gases through the heat exchanger and pushes them out the vent. Runs on the 120 VAC line. Failures: bearings dry out (loud squeal), windings fail (no spin, or hums but no spin). Replace as assembly.

Pressure Switch

A diaphragm-operated normally-open switch. The running inducer creates a small negative pressure in its sampling tube (typically 0.3-1.5 in WC vacuum); that pressure closes the switch. The furnace control board will not proceed to ignition until the pressure switch proves draft. Failures: blocked hose (condensate in a condensing furnace), cracked diaphragm (fails open), or restricted flue (never closes because draft is weak).

Hot Surface Ignitor (HSI)

A silicon carbide or silicon nitride element that glows red-hot (2,000+ deg F) to ignite the gas. Draws 4-5 amps, 80-120 VAC. Resistance when cold is 40-90 ohms (silicon carbide) or 11-18 ohms (silicon nitride - modern). Silicon carbide ignitors crack from thermal shock - never touch with fingers (skin oils accelerate failure). Silicon nitride ignitors are more robust but still fragile.

Standing Pilot (Older Furnaces)

Older atmospheric furnaces use a continuously-burning pilot flame instead of HSI. A thermocouple (not a flame sensor) proves the pilot is lit, generating a tiny DC millivolt signal that keeps the main gas valve open.

Flame Sensor (Rectification)

A ceramic-coated metal rod in the flame. When flame is present, ionized gas in the flame rectifies AC current from the control board into a DC micro-amp signal (typical 1-5 uA, some boards want 1-10 uA). The board watches this signal and proves flame is present. Failures: dirty flame rod - oxide film on the rod blocks the signal. Clean with emery cloth or #000 steel wool (never sandpaper - too abrasive, changes rod dimension).

Flame Rollout Switch

A bimetal high-temperature switch mounted at the burner compartment. If flame rolls out of the burner chamber (due to blocked flue or cracked HX), the rollout switch trips and locks out the gas valve. If the rollout switch trips, do not simply reset - find the cause. Most commonly this is a cracked heat exchanger or a severely blocked flue.

High-Limit Switch

A bimetal switch on the supply-air plenum. If supply-air temperature exceeds a set threshold (160-200 deg F typical), the high limit opens and shuts off the gas valve. Common causes of high-limit trip: dirty filter (low airflow), closed supply register, failed blower motor, failed blower capacitor.

Gas Valve

The master valve that controls gas flow to the burners. Three common types:

• Single-stage - Full on or full off. Most older furnaces.
• Two-stage - Low fire and high fire. Modulates based on load.
• Modulating - Variable from 30% to 100% of rated input. Premium equipment, smoother temperature control.

Manifold pressure (the regulated pressure at the gas valve outlet to the burners) is 3.5 in WC for natural gas, 11 in WC for LP. Always verify manifold pressure on a commissioning or tune-up - a low manifold pressure means poor combustion, a high one means overfiring and hot heat exchanger.

Blower Motor and Capacitor

Moves conditioned air through the home. ECM (electronically commutated motor) or PSC (permanent split capacitor). PSC blowers rely on a run capacitor - the #1 no-heat symptom on a cold morning is a failed blower capacitor; the burners fire, high-limit trips, furnace lockouts.

Heat Exchanger

The sealed metal chamber that separates combustion gases from the conditioned air. Combustion inside, home air outside. A crack in the HX sends CO into the home. A cracked heat exchanger is the most dangerous failure in the HVAC trade. Detection: visual inspection with mirror and flashlight, borescope, combustion analyzer testing supply air for CO, flame pattern observation (a flickering flame when the blower starts often means combustion is being disturbed by air coming through a crack).

Sequence of Operation - A Standard 90% Condensing Furnace

When the thermostat calls for heat (W1 closes), the control board runs this sequence:

1. Call for heat - W1 energizes.
2. Inducer motor starts - Pulls air through the heat exchanger and creates draft.
3. Pressure switch closes - Confirms inducer is creating draft. If it does not close in ~30 seconds, the board locks out with a pressure switch error.
4. Hot surface ignitor energizes - Board applies 120 VAC to HSI; it glows red (30-60 seconds).
5. Gas valve opens (trial for ignition) - Board energizes the gas valve, HSI is still glowing, gas ignites at the burners.
6. Flame sensor proves flame - Within 2-4 seconds of gas valve opening, rectification signal must rise above ~1 uA. If not, board closes gas valve, waits, and retries (typically up to 3 attempts before lockout).
7. HSI de-energizes - Gas is lit, ignitor no longer needed.
8. Blower delay - Board waits 30-60 seconds for the heat exchanger to warm up, then energizes the blower on heat speed.
9. Run - Burners run, blower runs, until thermostat is satisfied (W1 de-energizes).
10. Shutdown - Gas valve closes, inducer runs a post-purge (30-60 seconds) to clear residual combustion gases, blower runs a cool-down (60-120 seconds) to remove residual heat from the HX, then everything shuts off.

Each step has a failure mode. Knowing the sequence is how you diagnose a no-heat call in 5 minutes instead of 45.

Common Failures By Age and Symptom

"Furnace clicks but does not fire" (Inducer runs, no ignition)

Usually pressure switch not closing (blocked condensate line or cracked hose on a 90%), or HSI cracked/open, or gas valve stuck closed.

"Furnace fires for 2-4 seconds then shuts down"

Classic dirty flame sensor. Board lights gas, fails to prove flame, tries again, locks out after 3 attempts. Clean the flame rod with emery cloth; if that does not restore signal, replace.

"No heat, 20-year-old furnace, CO detector tripping in the house"

Suspect a cracked heat exchanger. Shut the furnace down, turn off the gas, get the customer and pets out of the house, ventilate, and diagnose. Do not re-fire the furnace until HX is confirmed sound.

"Furnace heats briefly, then shuts off on high limit"

Airflow. Dirty filter, closed registers, failed blower capacitor, failed blower motor.

"No draft" / "Inducer not running"

Inducer motor bearings seized (often a rising noise signal for weeks prior) or inducer motor winding failed. Replace.

"No heat, pilot out (old standing-pilot atmospheric furnace)"

Thermocouple failed, or pilot orifice clogged. Relight pilot per nameplate instructions; if it will not stay lit after 60 seconds, replace thermocouple.

"Furnace runs but house is cold"

Could be airflow issue, undersized furnace, duct leaks, or gas input too low (manifold pressure low). Check filter, check manifold, check static pressure.

Annual Tune-Up Steps

Fall is the season. Every tune-up, in order:

1. Talk to the homeowner - Any complaints? CO detector ever trip? Smell gas ever?
2. Turn off power and gas at the furnace disconnect before opening.
3. Remove burner compartment cover, inspect heat exchanger with a flashlight and mirror. Look for cracks, rust scale, carbon buildup.
4. Vacuum the burner compartment - Dust, pet hair, lint.
5. Clean the flame sensor - Pull, wipe with emery cloth or #000 steel wool, reinstall. Torque specs: hand-tight plus 1/4 turn - the rod is ceramic and brittle.
6. Check HSI resistance - Out-of-circuit ohm reading. Silicon carbide 40-90 ohms, silicon nitride 11-18 ohms. Replace if open or out of range.
7. Inspect pressure switch hose - Clear of condensate, no cracks, tightly seated on both ends.
8. Verify gas manifold pressure - Connect manometer to the outlet pressure tap of the gas valve, run the furnace, confirm 3.5 in WC NG or 11 in WC LP. Adjust per manufacturer if out of spec.
9. Measure static pressure - Supply plenum and return plenum. High static = airflow problem.
10. Run combustion analysis - Probe flue, let furnace stabilize 5 minutes, record O2, CO air-free, stack temp. Adjust manifold if out of spec.
11. Change filter - Or confirm it was just changed.
12. Check flue for corrosion, blockage, backdraft - Especially atmospheric and 80% induced. Spill test the draft hood (atmospheric only).
13. Test CO detector in the home - If there is no working CO detector, refuse to re-fire the furnace and explain why to the homeowner.
14. Cycle the furnace through a full sequence of operation and confirm every step.
15. Document readings - Paperwork to homeowner and shop.

Safety Absolutes

• Never operate a furnace with a suspected cracked heat exchanger. Shut it down, red-tag it, write a condemnation letter, inform the homeowner. Customer dies, you are on the witness stand.
• Always use a combustion analyzer on tune-ups. Eyeballing flame color is not a substitute. A yellow-tipped flame can still have 600 ppm CO; a blue flame can still have 200 ppm.
• Verify a working CO detector in the home. If there is not one, install one or refuse to fire the furnace.
• Shut off gas AND power at the disconnect before opening any burner compartment or control panel.
• Never bypass a safety - pressure switch, rollout, high limit, flame sensor. If a safety is tripping, find the cause. Jumping a safety to "get the customer through the night" is malpractice and gets techs fired.
• Venturi suction test if ever in doubt on an atmospheric furnace - light an incense stick near the draft hood; smoke should be pulled up into the flue, not spilled into the room. Spillage = blocked flue or inadequate combustion air.
• Carbon monoxide is odorless - trust the detector, not your nose. Symptoms (headache, nausea, drowsiness, confusion) are late signs.
• If you smell gas on a call - shut off gas at the meter or LP tank. Do NOT flip any electrical switches, do NOT use cell phones or two-way radios inside the structure, do NOT light anything. Evacuate all occupants and pets, prop doors open for ventilation, and call the gas utility (and 911 if the leak is large) from outside and well away from the building. A spark or an arcing relay in a gas-filled space is how techs level houses and end careers.

Day 1 Checklist (Going On A Furnace Call)

• Combustion analyzer (calibrated, pump and sensor healthy)
• Manometer for gas pressure
• Multimeter for voltage, continuity, capacitance, flame micro-amps
• Inspection mirror and flashlight
• Emery cloth or #000 steel wool
• Replacement flame sensor, igniter, pressure switch (truck stock)
• CO meter (separate from the combustion analyzer)
• Thermometer for supply/return temp rise
• Pipe dope or Teflon tape (gas-rated) for gas-pipe work
• Red-tag forms for condemnation

Expert Tips

• "Short cycle on flame failure = dirty flame sensor." Four seconds in, four seconds out. Clean the rod first.
• "No CO detector, no fire." Install one or leave the furnace off.
• "Manifold pressure is the money pressure." 3.5 NG, 11 LP. If it is not right, nothing else is.
• "Cracked HX is a death sentence - for the furnace." Condemn it in writing.
• "Watch the sequence of operation." Every no-heat has a sequence failure - find where it stops.
• "Vacuum the burner compartment every tune-up." Dust in the burners clogs the air ports and causes CO to climb.