Intermittent DSI Lockout on a Commercial Convection Oven
A commercial kitchen has a 120,000 BTU/h direct-spark-ignition convection oven. The oven locks out intermittently after 3 to 5 minutes of operation, more often when the kitchen hood exhaust is at high speed. The technician measures flame signal at the ignition control: 1.0 µA DC, while the manufacturer's minimum is 2.0 µA. The spark electrode also serves as the flame sensor. The ceramic insulator has a hairline crack, and the electrode gap is 1/8 in; the spec is 3/16 in. The burner has a new ground wire, but resistance from the burner to the panel ground is 0.8 Ω. Supply voltage is 120 V, polarity is correct. The gas manifold pressure is 3.5 in w.c., matching the nameplate. The ignition control locks out on three trials and then must be manually reset. The hood exhaust is 1,200 CFM; the kitchen has a 400 CFM make-up air unit.
1. Explain how flame rectification works and why a 1.0 µA signal can cause lockout even though a flame is present.
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Flame rectification uses the flame as a conductor. The ignition control applies an AC voltage to the electrode; the flame conducts current in one direction better than the other, producing a small DC current. The control interprets that DC current as proof of flame. A cracked ceramic insulator, contaminated electrode, wrong gap, moisture, or a poor burner ground can shunt or reduce the signal. The 1.0 µA reading is below the 2.0 µA minimum, so the control may drop out after the trial-for-ignition period or during steady operation. The crack can open as the oven heats, which explains the 3 to 5 minute lockout. The 0.8 Ω burner-to-panel ground is high enough to reduce flame current; the ground path must be solid. A flame can be visible and still fail rectification.
2. List the tests in the order you would perform them to isolate the cause, and state the expected result for each.
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- Visually inspect the electrode, ceramic, burner, and wiring. 2) Clean or replace the electrode and set the gap to 3/16 in. 3) Measure flame current with a microammeter in series at the control, both at startup and after 5 minutes. 4) Measure burner-to-ground resistance; clean mating surfaces and repair the ground. 5) Verify polarity and earth ground at the receptacle. 6) Check burner alignment and gas pressure. 7) Turn the hood exhaust off and repeat the flame-current test; if the signal rises, investigate draft, make-up air, and flue recirculation. Expected results: stable flame current above 2.0 µA, ground near 0 Ω, and no lockout through several cycles.
3. Recommend the repair and explain why adjusting the gap alone is not enough.
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Replace the cracked electrode, set the correct gap, clean the burner and electrode, and repair the ground path so resistance is as close to 0 Ω as the manufacturer requires. Then verify a stable flame signal above 2.0 µA for at least 5 minutes and through several ignition cycles. Adjusting the gap alone will not fix a cracked insulator or a high-resistance ground; the crack can leak current to ground and the ground path limits the return current. If the signal is still marginal, check the ignition control, burner position, and draft. Also verify the make-up air and hood interlock, because the lockouts worsen when the hood runs.