When this error occurs, you will likely notice a complete lack of central heating and domestic hot water. You might hear the boiler attempting to fire up—characterized by the internal fan spinning and the rhythmic clicking of the ignition sparks—only for it to fall silent and display the “EE” code. Do not panic; while this indicates a critical fault in the combustion sequence, it is a common diagnostic event that can typically be resolved through systematic cleaning or component replacement.
Symptoms
The most obvious sign of an Error EE is the alphanumeric code flashing on the Vitotronic control interface. However, the physical behavior of the unit provides additional context for the engineer. You may observe the “Ignition Recycling” phenomenon, where the boiler tries to light three to five times before hard-locking. During these attempts, the burner might roar to life for a split second before dying out.
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In some instances, you might experience “intermittent hot water,” where the boiler works during low-demand periods but fails during high-demand periods when the gas-to-air ratio is under more stress. Additionally, if the unit does manage to stay lit briefly, you might hear a “booming” or “huffing” sound, indicating that the flame is lifting off the burner surface, preventing the ionization rod from maintaining a circuit.
The Complete Solution
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Safety First and Housing Removal:
Before touching any internal components, isolate the electrical supply to the boiler at the fused spur. Close the gas isolation valve. Using your Phillips or Torx driver, remove the front casing of the Viessmann unit. Ensure you are grounded to avoid static discharge to the PCB. -
Inspection of the Ionization Electrode:
Locate the electrode assembly on the burner door. There are typically two sets of wires: the ignition cables (thick) and the ionization cable (thinner). Inspect the ceramic insulator for cracks. Warning: A cracked ceramic will allow the signal to short-circuit, requiring a full replacement of the part. If the ceramic is intact, remove the electrode and gently scrub the metal rod with a non-metallic abrasive pad until the bare metal is shiny. Do not use heavy sandpaper, as deep scratches promote faster carbon buildup in the future. -
Testing the Ionization Current:
Reinstall the electrode and restore power. Set your multimeter to the DC Microamp (µA) scale. You must connect the meter in *series* with the ionization lead. When the boiler attempts to fire, watch the reading. A healthy Viessmann Vitodens typically requires a signal between 5µA and 25µA. If the reading is below 3µA, the flame is too weak or the rod is poorly positioned. -
Checking Gas Supply and Valve:
Attach a manometer to the inlet pressure test point on the gas valve. Ensure the static pressure meets the manufacturer’s specification (usually 20mbar for Natural Gas). If the pressure drops significantly when the valve clicks open, the issue lies with the gas supply, not the boiler. If pressure is stable but the flame is weak, the gas valve’s offset may need adjustment by a certified professional using a flue gas analyzer to check CO2 levels. -
Clearing the Lockout:
Once the component is cleaned or replaced, press the “Reset” button (usually marked with a lightning bolt or “R”) on the control panel. Hold it for 2 to 3 seconds. The boiler will initiate a pre-purge cycle. Monitor the sight glass to ensure the flame is stable and encompassing the ionization rod.
- Technical Difficulty: Moderate to High (Requires internal access and electrical testing)
- Estimated Time: 45 – 90 Minutes
- Tools Needed:
- Digital Multimeter (capable of measuring Microamps µA)
- Torx T20 and T25 Screwdriver set
- Non-metallic abrasive pad (e.g., Scotch-Brite) or fine-grit emery cloth
- U-tube or Digital Manometer (for gas pressure verification)
- Estimated Repair Cost: $0 (if cleaning) to $450 (if gas valve or PCB replacement is required)
Why is my Viessmann showing Error EE?
As a senior engineer, I categorize the causes of Error EE into four primary failure modes. Understanding these is vital for an accurate fix:
- Fouled or Carbonized Ionization Electrode: This is the most frequent culprit. The ionization rod sits directly in the flame. Over time, it develops a coating of carbon or metallic oxidation. Because the boiler uses the “Flame Rectification” principle—where the flame acts as a conductor for a small DC current—this debris acts as an insulator, artificially lowering the microamp signal sent back to the PCB.
- Incorrect Gas-to-Air Ratio: If the gas valve is out of calibration or the air intake is partially restricted, the resulting flame may be too “lean” or “lazy.” A lean flame often lifts away from the burner, meaning it doesn’t make physical contact with the electrode. If the electrode isn’t engulfed in the blue core of the flame, the circuit remains open, triggering the EE code.
- Degraded Ignition Lead or Transformer: Even if the rod is clean, the high-tension lead transporting the signal back to the control board can suffer from heat-induced brittleness or “tracking” (leaking current to the chassis). If the signal leaks to the ground before reaching the processor, the boiler assumes no flame exists.
- Insufficient Gas Inlet Pressure: If the building’s gas regulator is failing or the pipework is undersized, the burner may starve during the high-fire ignition phase. A momentary drop in pressure causes the flame to flicker just enough to break the ionization threshold, resulting in a safety shutdown.
How to Prevent Error EE
Annual Combustion Analysis: The most effective prevention is a yearly service by a technician who uses a calibrated Flue Gas Analyzer. By adjusting the O2 and CO2 levels to the narrow “sweet spot” defined in the Viessmann technical manual, you ensure the flame stays rooted to the burner, providing a strong, consistent ionization signal.
Proactive Electrode Replacement: In my professional experience, I recommend replacing the ignition and ionization electrodes every 2 to 3 years, regardless of their current state. These are “wear parts” exposed to extreme thermal stress; waiting for them to fail inevitably leads to a lockout at the most inconvenient time (usually the first cold snap of winter).
Clean Air Supply: Ensure the external flue terminal is free from obstructions like bird nests or overgrown vegetation. A restricted air supply creates an oxygen-starved flame that is electrically “noisy” and difficult for the PCB to sense accurately.
Frequently Asked Questions
Q: Can I just keep resetting the boiler to make it work?
A: While a reset might occasionally clear a “nuisance” trip caused by a momentary dip in gas pressure, repeatedly resetting an EE error is ill-advised. If the flame signal is weak due to carbon buildup, the condition will only worsen, potentially leading to “explosive ignition” where gas builds up in the chamber before finally catching a spark. Solve the root cause immediately.
Q: Does Error EE mean I need a new boiler?
A: Absolutely not. In over 90% of cases, Error EE is a maintenance issue involving the cleaning or replacement of electrodes—a relatively low-cost repair. Even a faulty gas valve or PCB, while more expensive, is far cheaper than a full boiler replacement.
Q: Is Error EE dangerous?
A: The error itself is actually a safety feature doing its job. The boiler has detected that it cannot safely verify a flame is present, so it shuts off the gas to prevent unburned fuel from accumulating. However, the underlying cause (like a failing gas valve) should be handled by a qualified professional to ensure the unit remains gas-tight and safe.