A.21 on a Viessmann One Base: System pressure has exceeded maximum pressure

What it means

The manual says: System pressure has exceeded maximum pressure.

Part involved: System pressure has exceeded maximum pressure.

When it shows: System characteristics: The safety valve in the indoor unit has opened.

Likely causes:

• Hydraulic system pressure too high.

Is it safe?

The manual does not say whether the heat pump stops.

Resetting itself: Not stated per code in Viessmann document 6199893 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 system pressure shown on the HMI "System overview" (6249162 p.35) or pressure gauge and report a low reading; make sure the filling loop is closed.

What the engineer will check

• Check the expansion vessel.

Who to ask for: Viessmann-trained heat pump installer/service engineer.

What you might notice

• Pressure keeps dropping

Sources

• Service instructions for contractors: Heat pumps in monoblock design incl. hybrid – System configuration and diagnostics (Vitocal 150-A/151-A/250-A/252-A, IDU-A, AWMOF-151/251, AquaSnap 30AWH, BI-BLOC R290) (6199893 GB 01/2026) ↗ · 6199893 GB 01/2026 (VSA00009) · page 202-203 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026