A.66 on a Viessmann One Base: Fault, feedback signal, secondary pump/heating circuit pump for heating/cooling circuit 1
Short answer
On a Viessmann heat pump, A.66 means: Fault, feedback signal, secondary pump/heating circuit pump for heating/cooling circuit 1. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Fault, feedback signal, secondary pump/heating circuit pump for heating/cooling circuit 1.
Part involved: Fault, feedback signal, secondary pump/heating circuit pump for heating/cooling circuit 1.
When it shows: System characteristics: Secondary pump/heating circuit pump, heating/cooling circuit 1 not running. Limited control of heating circuit pump, heating/cooling circuit 1.
Likely causes:
- Incorrect circulation pump installed (other manufacturer) Incorrect circulation pump installed (pump type).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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 heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).
- 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.
- Check the operating program, time program and room temperature settings on the HMI.
What the engineer will check
- Start actuator test and function check via HMI programming unit or ViGuide. Check function of the circulation pump.
- Replace the circulation pump: Only use original parts from the manufacturer or spare parts approved by the manufacturer. A.67 Manufacturer identification, secondary pump/heating circuit pump ….
- Replace the circulation pump: Only use original parts from the manufacturer or spare parts approved by the manufacturer.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
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 204 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026