Applicable Systems
Lochinvar Knight Boiler (SMART SYSTEM Control)
Symptoms
The Lochinvar Knight boiler SMART SYSTEM control displays fault message E02, indicating a flame signal fault. The flame sense circuit is detecting a flame signal when no flame should be present (flame sensed with gas valve closed), or the flame signal is too weak or unstable during firing.
Possible Causes
- Flame sense electrode shorted to ground through moisture or carbon buildup
- Flame sense wiring is grounded to the boiler chassis
- Control board flame sense input circuit is faulty, giving false readings
- Leaky gas valve allowing a small standing pilot flame when the valve should be closed
- External flame or heat source affecting the flame sensor
Troubleshooting Steps
- Step 1: Check the flame signal reading on the SMART SYSTEM display with the gas valve closed
Flame signal reads 0 microamps with gas valve closed (no flame present) - Step 2: Inspect the flame sense electrode wiring for any grounding to the chassis
Wiring is insulated and not touching any metal surfaces - Step 3: Remove and clean the flame sense electrode with emery cloth
Electrode surface is clean and free of carbon or moisture - Step 4: Verify the flame sense electrode is properly positioned and gapped
Electrode is in the correct position as shown in the service manual - Step 5: If false flame signal persists, the gas valve may be leaking and should be tested for tightness
Gas valve passes the tightness test with zero leakage
How to Reset
To reset: 1) Fix the flame sense wiring or electrode issue. 2) If gas valve is leaking, replace it immediately. 3) Press the RESET button on the SMART SYSTEM control. Caution: A flame signal with the gas valve closed is extremely dangerous as it may indicate a leaking gas valve. Do not reset until the gas valve tightness is verified.
Common Replacement Parts
- 100265: Lochinvar flame sense electrode (Replace if the ceramic insulator is cracked, allowing internal shorting)
Frequently Asked Questions
- Q: Why would the control detect flame when there is no flame?
The flame sense circuit works by measuring a small DC current that flows through the flame (flame rectification). If the electrode or its wiring is shorted to ground, the control will read a false flame signal. This is dangerous because the control thinks flame is present when it is not, potentially allowing gas to flow without ignition.
- Q: What is flame rectification?
In a flame rectification system, the flame acts as a electrical rectifier (one-way current path) between the electrode and the burner ground. The control applies AC voltage to the electrode and measures the DC component of the current. A valid flame produces a DC signal of several microamps. A short to ground produces a different signal pattern that the control can detect as a fault.
Source: Lochinvar Knight Boiler Service Manual KB-SER-08, Troubleshooting Table
Category: Boiler