Sequence of operations: diagnosing a burner

A burner doesn’t fail “at random”

A commercial burner that won’t start on a winter morning, or that locks out mid-cycle, feels like a temperamental black box. It isn’t. Every automatic gas burner runs a sequence of operations: a fixed series of steps, always in the same order, orchestrated by the flame safeguard. Diagnosing a burner isn’t about guessing which part to swap — it’s about knowing which step the sequence stopped at, then checking what that step requires.

The good news is that this sequence is nearly universal. Whether it’s a Honeywell 7800-series control, a Fireye Flame-Monitor or a Siemens LMV, the underlying logic is the same. What changes is the timing, the fault codes and how finely flame is detected. Here is the sequence the way we follow it in the field, step by step, with the diagnostic reasoning that goes with it. It’s exactly the kind of method our technicians apply in gas heating and combustion work.

🧰 Tools required The flame safeguard manual (it gives the timing and codes specific to the unit), a digital multimeter able to read microamps DC, a manometer for gas and air pressures, and the unit’s wiring diagram. Without the control’s manual, you’re working blind on the exact values.

⚠ Safety Never bypass a safety to “see if it’ll light.” Gas pressure switches, low-water cutoff, temperature limits and pre-purge exist to prevent a dangerous ignition. Any work on a gas appliance falls under the CSA B149.1 installation code and belongs to qualified personnel.

1. The call and safety interlocks

It all starts with a heat call — from an aquastat, a thermostat or a building management system. But the call alone isn’t enough: the flame safeguard only starts the sequence if the entire interlock chain is closed. That typically includes temperature limits (high limit), high- and low-gas-pressure switches, and, on a boiler, the low-water cutoff.

On the diagnostic side, this is where you start: if nothing happens on the call, first confirm a call actually reaches the control, then walk the interlock chain contact by contact. A single open contact — a low-water cutoff, say — is enough to freeze the sequence before pre-purge even begins, with no flame code at all.

2. Pre-purge and air proving

Once the chain is closed, the control starts the blower and runs the pre-purge: a forced ventilation of the combustion chamber and flue, meant to clear out any unburned gas before the first spark. It’s an anti-explosion protection, not a formality — and it can’t be shortened.

Two conditions frame this step:

• Airflow proving: an air-proving switch must close to confirm the blower is actually moving air. If it doesn’t close, the control stays stalled and never reaches ignition.
• The pre-purge delay: a minimum time imposed by the control, often a function of chamber volume.

In diagnostics, a burner that “runs the blower but never lights” very often points here: a failed air switch, a plugged pressure tap, or a damper that doesn’t prove its high-fire position on larger-capacity burners.

3. The trial for ignition

With pre-purge done, the control enters the trial for ignition: it energizes the ignition source — high-voltage spark or igniter — then opens the gas valve, pilot first on a pilot system, or main on direct spark ignition. This window is timed: it’s the ignition-proving period, during which a flame must not only appear but be proven by the detection system.

This is the most diagnostically dense step, because three things must converge: gas at the right pressure, a live ignition source, and working flame detection. If the burner makes one attempt and locks out immediately, you isolate which of the three is missing — and the flame signal is what settles it.

4. Flame proving: the measurement that settles it

The control doesn’t “see” flame — it measures it. Two technologies dominate:

• The rectification rod (flame rod), which uses the flame’s conductivity to pass a small DC current. You measure it in microamps. The common order of magnitude is a few µA — often around 2 to 7 µA depending on the system — with a minimum threshold below which the control declares loss of flame. The exact reference value and threshold are specific to the control: read them from the manufacturer’s data sheet, never from memory.
• The UV or infrared scanner, which detects the flame’s radiation, typical of higher-output burners.

The key move: measure the flame signal during the trial for ignition. A zero signal points to an ignition that didn’t happen (no gas, no spark, or flame not reaching the rod). A weak but present signal — below the control’s threshold, for instance — points to failing detection: a fouled or misaligned rod, a poor ground, or reversed polarity. That distinction steers the rest of the troubleshooting and keeps you from replacing a gas valve when the real culprit is a $15 rod. To go deeper on the component itself, see the gas burner glossary entry.

5. Run and modulation

With flame proven inside the allotted window, the control goes to run: the main valve stays open and the burner fires. On a modulating burner, the controller then trims gas and air between low and high fire to match demand, keeping the air-fuel ratio within range. Throughout the run, the control continuously watches the flame signal: it must stay stable and above the threshold.

In diagnostics, a burner that starts then locks out after a few seconds or minutes tells a different story than one that never lights: you’re looking for a loss of flame during run (signal dropping, unstable flame, fouling) or a loss of airflow. The fault code the control displays often pinpoints the step where the loss occurred.

6. Post-purge and lockout

When the call ends, the control closes the gas valve and keeps the blower running for a post-purge to clear residual combustion products, then shuts the cycle down cleanly. Conversely, if a safety condition drops along the way, the control triggers a lockout and freezes the system until a manual reset — a deliberate fault memory, to force human verification before restarting.

The field reflex: read the fault code and note which step the lockout occurred at before any reset. Resetting without diagnosing erases the most useful information and risks restarting the unit on a real fault. A recurring lockout at ignition and a lockout mid-run don’t call for the same intervention.

From sequence to structured diagnosis

The value of knowing the sequence cold is that it turns a vague symptom into a precise question: where does it stop? Blower won’t start → look at the call and interlocks. Blower runs but nothing lights → pre-purge and air proving. One attempt then lockout → trial for ignition and flame signal. Starts then cuts out → loss of flame or air during run. Each answer points to a subset of components, instead of a swap-by-elimination.

That discipline is also what separates a lasting restart from a repair that comes back the following week. A burner that locks out in full production calls for an emergency repair carried out methodically, not a repeated reset that masks the real fault.

That’s the level of rigour the Montréal Combustion team brings to every job.