Mastering the CI2: Lessons from the Field on O2 Levels and Vent Calibration

In the world of high-efficiency commercial boilers, the Viessmann CI2 is a sophisticated piece of equipment. However, as with any advanced technology, real-world applications sometimes reveal nuances that aren’t fully captured in the initial manuals. Recent troubleshooting on a problematic installation has provided critical insights into intermittent faults (specifically F.348 gas valve errors) and unexpected shutdowns.

If you are installing or servicing CI2 units, the following takeaways will help you ensure stable, long-term operation.

1. The Critical Nature of O2 Calibration

While the installation manual provides a starting point for vent lengths based on total piping feet, this is often insufficient for trouble-free operation. We have found that the "sweet spot" for these units is very narrow.

The Solution:

Do not rely solely on the physical measurement of the venting. Instead, use the boiler's feedback to dial in the settings:

• The Procedure: Put the boiler into high fire and allow the burner to stabilize for at least three minutes.

• The Target: Monitor the O2% on the HMI. Your goal is just under 6% O2.

• The Adjustment: Adjust the vent length setting in the software to influence fan speed. This adjustment directly affects your $CO_{2}$ and $O_{2}$ percentages.

• The Sweet Spot: Technical feedback suggests a range between 5.5% and 5.9% O2. For reference, a stable unit recently resolved its issues by being locked in at 5.95%.

Pro-Tip: If a CI2 boiler is failing to ramp up to its setpoint and remains stuck at a reduced output, your first move should be checking the O2 levels. This symptom is frequently caused by improper gas pressure, restricted combustion air, or incorrect vent length settings.

2. Control Module Compatibility (BCU/HMU)

In a pinch, it is common practice to "borrow" a part from a stock unit to get a customer back online. However, with the CI2 series, this will not work.

• BCUs and HMUs cannot be swapped between boilers, even if the units are identical in size and model.

• Attempting to install a control module from a different unit will likely result in communication failures or persistent faults. Always order parts specific to the serial number of the unit being serviced.

3. Component Updates and Emerging Faults

When dealing with persistent intermittent faults, ensure that the HMU (Heat Management Unit) and BCU (Burner Control Unit) are flashed with the latest firmware updates. In some cases, replacing the Fan Assembly, E-box, or BCU may be necessary, but these hardware changes are often secondary to the software and calibration steps mentioned above.

A Note on F.889 Faults:

Viessmann is currently tracking F.889 faults closely. If you encounter this specific error code, please document the conditions of the boiler and report it. The engineering team in Germany is actively working to isolate the root cause of this specific fault.

Summary Checklist for Start-ups:

1. Check O2%: Always verify O2 on high fire after 3+ minutes of stabilization.

2. Calibrate Vent Length: Adjust software vent length to hit the $5.5\% - 5.9\%$ O2 target.

3. No Swapping: Never attempt to swap BCU/HMU modules between units.

4. Update Firmware: Ensure all electronic components are running the latest version from Viessmann.

By following these field-tested steps, you can significantly reduce "nuisance" callbacks and ensure these high-performance boilers run reliably from day one.