Intermittent Flame-Signal Lockout on a Condensing Boiler
A condensing boiler in a basement mechanical room locks out intermittently, mainly on damp days. The sequence is: call for heat → circulator → draft inducer → spark → gas valve opens → flame establishes for 2–3 seconds → flame signal drops from 0.9 µA to 0 → lockout after three tries. The flame sensor is clean; its wire is routed within 1 in of the inducer motor; the boiler is bonded to the water piping. Supply voltage is 120 V, transformer output 26 VAC, and the gas valve receives 24 VAC during trial. The technician has already replaced the ignition control and igniter. A condensate trap is full but draining. The vent terminates 12 in above a window, and recirculation is possible. Outdoor humidity is 95%.
1. Explain why replacing the ignition control and igniter did not solve the problem. Use the sequence of operation to narrow the fault.
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The sequence reaches flame and then loses it, so the spark circuit and gas valve are functioning at least initially. Replacing the board and igniter addressed parts that are not the likely cause. A flame that lights for 2–3 seconds and then drops points to flame proving: the control no longer sees sufficient flame current. The measured 0.9 µA may be below the manufacturer's minimum, or the signal may be intermittent.
Moisture, a poor ground, a cracked electrode insulator, a wire routed near the inducer, or a flame that lifts off the burner can all cause this. Recirculation could disturb the flame. The correct approach is to test the flame-proving circuit, not replace more parts.
2. List the measurements and inspections you would perform next, and explain what each tells you.
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Measure flame current in series with the sensor lead using a microammeter; compare it to the manufacturer's minimum. Inspect the sensor electrode gap, position, and insulator for cracks or moisture. Check the burner ground path and the control ground; measure resistance from the burner to the control ground. Verify polarity at the outlet and at the control. Wiggle-test the sensor wire while watching flame current; check routing away from the inducer. Observe flame shape and stability during the trial. Check vent termination and combustion-air supply for recirculation. Measure gas pressure during the trial. These tests separate a flame-signal problem from a gas or air problem.
3. State the repair and verification steps you would take, and what you would document.
Read the worked answer
Repair the specific defect: replace a cracked sensor or electrode, correct the ground or polarity, reroute or repair the sensor wire, seal moisture entry, or correct the vent termination if recirculation is confirmed. Do not just increase trial time or replace the board. After repair, run several complete cycles, including a cold start and a damp-condition simulation if possible.
Record flame current at stable run, gas pressure, vent readings, and any parts replaced. Document the original lockout pattern, measurements, repair, and final verification. If the vent is noncompliant, correct it before signing off.