How to Read What a Flame Is Telling You

Why this matters

A flame is the fastest diagnostic instrument on the truck and the only one that costs nothing to use. Read properly it tells you where in a burner rack a mixture problem lives, whether the fault is in the appliance's air supply or in one specific port, and whether the thing in front of you is about to do something violent. What it cannot tell you is what left the appliance, and the most dangerous property of a flame is that a textbook blue one is entirely compatible with hazardous carbon monoxide. This article is about extracting everything the flame legitimately reports and stopping there.

The steps below are ordered by what you lose if you skip them, worst loss first, not by convenience.

Before you look at anything

Have a carbon monoxide monitor running in the space before the appliance fires. Carbon monoxide is odourless and colourless at every concentration that will hurt you. If it climbs toward the OSHA general industry permissible exposure limit of 50 ppm as an 8-hour time-weighted average at 29 CFR 1910.1000 Table Z-1, or if anyone reports headache, nausea or confusion, everyone leaves the building immediately, nobody operates a switch on the way out, ventilate from outside, and nobody re-enters until an instrument reads it clear. If you smell gas, that is the other instruction: everyone out immediately, no switches, no lights, no phone used inside, call from outside.

Two failures you may be about to observe carry immediate injury risk, so decide now what you will do rather than in the moment.

Flame rollout, meaning flame outside the burner compartment: shut the appliance off at its own gas shutoff immediately, do not stand in front of the opening, move occupants clear, and leave the appliance out of service. Rollout usually means a blocked flue or heat exchanger and it ignites what is next to it.

Delayed ignition, the audible thump when accumulated gas lights all at once: shut it off at the gas shutoff. Do not cycle the appliance repeatedly to reproduce it with your face at the sight port. That is a small deflagration and it can open the burner box toward you. Observe from the side of the opening, with eye protection, using a mirror or a phone camera rather than your head.

Before you reach into a burner compartment for any reason, close the appliance gas shutoff, open and lock the electrical disconnect under 29 CFR 1910.333(b)(2), and let the assembly cool or handle it with gloves rated for contact heat.

Step 1: Observe the flame in the appliance's normal closed configuration

Look through the sight port with the burner compartment closed and every panel in place, before you open anything.

What you lose by skipping it. The flame you came to read. Removing a panel changes the air available to the burner and often changes the pressure in the compartment, so the flame you see afterwards is a different flame burning in a different environment. This loss is first on the list because it is the only one you cannot recover in the same visit without closing everything up and running a fresh cycle, and because a fault that disappears when the door opens is itself one of the most useful findings you can get.

Step 2: Watch the whole ignition sequence, not the settled flame

Be looking before the igniter energises, and keep watching through main burner establishment and cross-lighting across the rack.

What you lose by skipping it. The entire class of faults that only exist during light-off, and those are the ones with the highest injury potential: delayed ignition, rollout, a burner that lights late and lights from its neighbour, flame lifting off during the first seconds. A settled flame two minutes in has already thrown that information away. Note how long cross-lighting takes and whether it is even; a rack that takes noticeably longer to carry across is telling you about port condition before you have looked at a single port.

Step 3: Read the flame's attachment to the port before you read anything else

Look at the base of the flame where it meets the port.

What you lose by skipping it. Velocity faults, which colour will never report. There are two and they are opposite:

• Lifting, where the flame base sits detached above the port, means the mixture is leaving the port faster than the flame can burn back into it: overpressure at the manifold, a wrong or enlarged orifice, or too much primary air.
• Flashback, where the flame burns back inside the mixing tube with a roaring or popping sound, means the flame speed exceeds the mixture velocity: underfiring, a damaged port, or the wrong fuel for the orifices fitted.

Flashback is a shutdown, not an observation to note and continue past. Close the appliance gas shutoff.

Step 4: Compare across the rack before you judge any single flame

Look at all the burners together, not the one nearest the port.

What you lose by skipping it. The localisation, which is the main thing flame reading is actually good for. A fault affecting every burner in the rack identically points at something upstream of all of them: manifold pressure, the primary air path serving the whole assembly, or the air in the compartment. A fault on two burners out of five points at those two ports. Those are different jobs with different parts, and a single-burner observation cannot distinguish them at all.

Step 5: Read colour last, and read it against a known light

Colour is the observation everyone starts with and it is the most contaminated one. Ambient light, a tinted sight glass, dust in the air and a phone camera's white balance all shift it.

What you lose by skipping it. Less than you lose anywhere else on this list, which is exactly why it goes last. Yellow tipping does tell you a port is short of primary air or partly obstructed. It does not tell you how much carbon monoxide is being produced, and the relationship between the two is not proportional, because serious carbon monoxide appears at levels well below anything that changes what you can see. That ordering of products has its own article.

Step 6: Measure the flue gas, because the flame does not know what left the appliance

What you lose by skipping it. The answer. Every step above narrows where to look. None of them quantifies anything, and none of them sees past the port.

Three appliances, two of which look the same

Appliance A. Yellow tipping on every burner in the rack, uniform across all of them, appearing within about 30 seconds of light-off. Open the burner compartment door and the tipping clears within seconds. Air-free carbon monoxide at steady state measures 210 ppm.

Uniform across the rack plus responsive to cabinet air says the fault is upstream of every burner and is about the air reaching the compartment. Look at the compartment air path, a blocked primary air opening serving the assembly, or a return air leak drawing on the burner area. Nothing here is a port problem, and cleaning ports would consume an hour and change nothing.

Appliance B. Yellow tipping on two of the five burners, positions three and four, present from cold start through steady state, and completely unaffected by opening the compartment door. Air-free carbon monoxide at steady state measures 185 ppm.

To the eye these two look like the same complaint, and the two carbon monoxide figures are close enough that a single flue reading would not separate them either. The flame observation does: localised plus unresponsive to cabinet air says those two ports, not the appliance's air supply. Look for obstruction, a shifted burner, a distorted port, or damage from a previous cleaning.

Appliance C, the one that matters. Sharp blue flames with distinct inner cones, well attached at every port, uniform across the rack, stable through light-off. By every observation in steps 1 through 5 this appliance is in good order. Air-free carbon monoxide at steady state measures 260 ppm, higher than either of the others.

There is no contradiction. The flame you can see is the mixture at the port. A heat exchanger defect that quenches the flame on a cool surface behind the visible envelope, an over-fired condition, or a vent problem changing residence time all interrupt the reaction downstream of anything the sight port shows you. The quenching mechanism behind that is covered in the combustion fundamentals article. The practical rule is short: a good-looking flame is not a pass, and it never was.

All three of these sit under the 400 ppm air-free ceiling that the ANSI Z21 and CSA listing standards set for products of combustion, and all three sit above a 100 ppm air-free shop acceptance. Reading them against 400 alone would have signed off all three.

What each observation is and is not allowed to tell you

How to verify you got this right

Verify a flame-driven repair the same way you diagnosed it, then verify it the way the flame cannot.

Close the appliance up completely, run it from a genuine cold start, and repeat steps 1 through 5 in the same order through the same sight port under the same lighting. Appearance should now be uniform across the whole rack, attached at every port, and stable through cross-lighting. If tipping is gone on the burners you worked on and has appeared on a neighbour, you moved the air rather than fixing the obstruction.

Then take a full flue gas run, sampled properly and converted to an air-free basis, at more than one point in the cycle. That is the measurement that closes the ticket, and the flame observation is what let you get there in one visit instead of three. Record both: what the flames looked like, on which burner positions, and what the analyser read at which times. The next tech reading "flame looked good" has nothing. The next tech reading "uniform blue, attached, cross-lit in under 2 seconds, 38 ppm air-free at steady state" has a baseline.

References

• ANSI Z21 and CSA appliance standards, which limit carbon monoxide in the products of combustion to 400 ppm air-free for listed gas appliances
• 29 CFR 1910.1000 Table Z-1, general industry permissible exposure limit for carbon monoxide
• 29 CFR 1910.333(b)(2), electrical lockout requirements for general industry
• Appliance manufacturer documentation for manifold pressure, orifice sizing and expected burner appearance
• See related: What Incomplete Combustion Produces and Why It Matters; How to Read a Flue Gas Result