A.116 on a Viessmann One Base: Unstable mains voltage (power supply utility)

What it means

The manual says: Unstable mains voltage (power supply utility).

Part involved: Unstable mains voltage (power supply utility).

When it shows: System characteristics: Refrigerant circuit temporarily off.

Likely causes:

• Unstable mains voltage (power supply utility).

Is it safe?

The heat pump keeps running with some functions stopped.

Resetting itself: Not stated per code in Viessmann document 6199961 GB 01/2026 (VSA00009).

Manual reset: User may acknowledge via HMI after noting the code (6249162 p.57); the message returns if the cause persists.

What you can check

• On the HMI, tap the message and "?" to read the system characteristics and Viessmann's user tips; note the code (e.g. F.xxx) and text for your contractor (operating instructions 6249162 p.57).
• Acknowledge the message once via the HMI; do not acknowledge fault messages several times in quick succession (6249162 p.58) - notify your contractor.
• Check the heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).

What the engineer will check

• Check power supply (power supply utility).

Who to ask for: Viessmann-trained heat pump engineer / qualified electrician.

Certification: Qualified electrician for work on electrical equipment (Viessmann safety instructions).

What you might notice

• Keeps stopping / cutting out
• Intermittent fault
• Heat pump will not start

Sources

• Service instructions: Heat pumps in split design incl. hybrid – System configuration and diagnostics (Vitocal 200-S/222-S One Base) (6199961 GB 01/2026) ↗ · 6199961 VSA00009 · page 187 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026