Common Burner + Flame Diagnosis Reference

Why this matters

Gas burners - in furnaces, water heaters, boilers, ovens, fireplaces - share diagnostic logic. Reading the flame tells you a lot about combustion. The tech who can interpret flames + diagnose common burner issues handles many service calls effectively. This is the field card.

Normal flame characteristics

Natural gas / propane:

• Color: blue (with possibly small yellow tips)
• Shape: uniform, stable
• Sound: soft whoosh / steady
• Behavior: doesn't wave, doesn't extinguish

LP (propane):

• Slightly more yellow tendency
• Sometimes orange tips normal
• Otherwise blue

Abnormal flame indicators

All-yellow flame:

• Insufficient combustion air
• Soot deposits forming
• Carbon monoxide possible
• Cause: blocked air intake, clogged orifices, OR primary air adjustment

Yellow tips on blue base:

• Slight imbalance
• Slight CO formation
• Minor adjustment needed

Wavering / unstable flame:

• Air movement in burner area
• Excessive primary air
• Burner positioning wrong
• Draft issues

Lifting / blowing flame:

• Too much primary air
• Gas pressure too high
• Burner orifice damaged

Flame outside burner ports:

• Air OR gas restriction in burner
• Soot buildup
• Cracked burner

Floating flame:

• Detached from burner
• Pressure issues
• Possible flame failure

No flame:

• Failure to ignite
• Ignition system fault
• Gas valve closed
• Pilot out (older systems)

Combustion air

Required for proper combustion:

• Natural gas: ~10 cubic feet air per cubic foot gas
• Propane: ~24 cubic feet air per cubic foot gas

Provided through:

• Natural draft (older equipment) - through cabinet
• Sealed combustion (newer) - outside air directly
• Make-up air ventilation in tight homes

Insufficient air:

• Incomplete combustion
• Yellow flame
• CO formation
• Sooting on equipment

Common burner types

Atmospheric burner (older):

• Open to room air
• Natural draft
• Common older furnaces + water heaters

Sealed combustion:

• Intake + exhaust direct to outside
• 90+% AFUE furnaces
• More efficient + safer
• Newer construction

Power burner (oil OR commercial gas):

• Forced air via fan
• More precise combustion
• Often industrial

In-shot burner:

• Common modern furnaces
• Flame projects into heat exchanger tube
• 80% AFUE furnaces typically

Tubular burner:

• Long horizontal tube
• Drilled ports
• Older atmospheric

Ignition systems

Standing pilot (older):

• Small flame always burning
• Thermocouple senses pilot
• If pilot out: gas valve closes
• Drawback: gas consumption + reliability

Intermittent pilot (electronic ignition):

• Pilot ignites on call for heat
• Then main burner
• More efficient

Hot surface igniter (HSI):

• Glows hot when energized
• Lights gas directly (no pilot)
• 80%+ furnaces
• Common failure: HSI cracks

Direct spark ignition:

• Spark electrode directly to flame area
• Boilers + some furnaces
• Reliable when working

Flame sensor:

• Verifies flame after ignition
• Microamp signal (1-10 typically)
• Sometimes "rod" sensor
• Common failure: dirty / oxidized

Common ignition failures

HSI doesn't glow:

• Burned out (open circuit)
• Wire connection loose
• Voltage low
• Replace

HSI glows but no ignition:

• Gas valve not opening
• Gas pressure issue
• Wrong location relative to burner

Pilot won't stay lit:

• Thermocouple failed (most common)
• Pilot orifice dirty
• Gas pressure low
• Replace thermocouple (cheap)

Spark + gas but no ignition:

• Spark in wrong location
• Insufficient gas
• Cracked igniter

Flame established but fails (lockout):

• Flame sensor dirty
• Wrong sensor position
• Sensor failed
• Clean OR replace sensor

Diagnostic sequence

For "burner not igniting":

1. Verify call for heat:

• Thermostat calling?
• 24V to gas valve?

1. Verify ignition:

• HSI glowing?
• Spark visible?
• Pilot lit (older)?

1. Verify gas:

• Gas valve opening?
• Gas pressure correct?
• Manifold pressure proper?

1. Verify flame:

• Flame established?
• Sensed within timeout?
• Flame sustained?

1. Verify safety:

• Pressure switch made?
• Limit switch closed?
• Inducer running?

Gas pressure

Natural gas residential:

• Supply: 5-7 inches water column (W.C.)
• Manifold (post-valve): 3.5 inch W.C. typical
• Each appliance specific

Propane residential:

• Supply: 10-11 inch W.C.
• Manifold: 10-11 inch W.C. (usually)

Test points:

• Pressure tap on gas valve
• Manometer measurement
• Adjustments rare on residential (factory set)

Pressure issues:

• Too low: weak flame, fails to light, slow ignition
• Too high: large flame, soot, possible damage

Flame sensor cleaning

Frequent maintenance item:

1. Power OFF (LOTO)
2. Remove sensor (one screw usually)
3. Inspect for buildup (white oxide common)
4. Lightly sand OR use scotch-bright (don't gouge)
5. Wipe clean
6. Reinstall in proper position
7. Test ignition + flame

Heavy buildup = sensor needs replacement.

Burner cleaning

If burners visibly dirty:

1. Power + gas OFF
2. Remove burner assembly (per manual)
3. Inspect for:

• Soot deposits
• Rust
• Debris (insect nests, spider webs in cool weather)
• Cracks

1. Clean with vacuum / soft brush
2. Don't sand OR damage ports
3. Reinstall properly
4. Test ignition + flame

Heavy rust OR cracks: replace burner.

Heat exchanger inspection

Cracked heat exchanger (gas furnace):

• Most-dangerous failure
• Combustion gas (with CO) leaks into supply air
• Distribute through ductwork
• Customer + family at risk

Inspection:

• Visual (with mirror, camera scope)
• Pressure decay test (sometimes)
• Flame distortion when blower starts (sign of crack)
• CO testing (combustion analyzer)

If cracked: shutdown + replace OR replace furnace.

Combustion testing

For verified safe combustion:

Combustion analyzer reads:

• O2 (oxygen)
• CO (carbon monoxide)
• CO2 (carbon dioxide)
• Stack temp
• Efficiency calculation

Typical good readings:

• O2: 5-10%
• CO: under 100 ppm air-free
• CO2: 7-9% (per appliance)
• Stack temp: per equipment spec

Bad readings:

• Low O2: rich (too much fuel)
• High O2: lean (too much air; efficient but cool flame)
• High CO: incomplete combustion (danger)

Adjust if possible per manufacturer; replace if can't be corrected.

Specific scenarios

Furnace won't ignite at all:

• Power, control, gas, ignition diagnosis
• Often capacitor / control board / igniter

Furnace ignites then fails ("lockout"):

• Flame sensing issue
• Clean sensor first
• Verify sensor placement
• Replace if needed

Furnace ignites slowly (delayed):

• Gas pressure low
• HSI weak (replacing soon)
• Dirty burner

Furnace rumbles / vibrates during operation:

• Combustion issue
• Cracked heat exchanger possible
• Stop + diagnose

References

• NFPA 54 (National Fuel Gas Code)
• ANSI Z21 standards (gas appliances)
• ACCA combustion testing standards
• Manufacturer service manuals
• Manuall internal: Combustion Fundamentals, Carbon Monoxide Testing + Response