F.864 on a Viessmann One Base: Defrost not completed successfully
Short answer
On a Viessmann heat pump, F.864 means: Defrost not completed successfully. 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: Defrost not completed successfully.
Part involved: Defrost not completed successfully.
When it shows: System characteristics:; No defrost; Compressor cannot be switched on.; Further messages may be active: F.75, F.866, F.867, F.1080, A.16, A.66, A.67, A.68, A.69.
Likely causes:
- Defrost process aborted 10 times Further message: A.143.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| 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: Measures include resetting/unlocking the refrigerant circuit via HMI service menu "Reset outdoor unit" or ViGuide (engineer, password-protected; 6199893 p.118).
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 outdoor unit is not blocked by leaves, debris or snow.
What the engineer will check
- Acknowledge messages.
- If necessary, isolate system from power supply. Wait at least 2 min. Switch on the system.
- Check minimum flow rate of secondary circuit.
- Flush the system if necessary. Vent via the venting program.
- Check all installed heating filters on the secondary side. Clean the following filters if necessary:.
- Ball valve with filter in the outdoor unit.
- Heating filter with magnetite separation, e.g. in the return lines on the secondary side before the inlet to the outdoor unit and before the inlet ….
- Heating filter in heat meter.
Who to ask for: Viessmann-trained heat pump engineer with F-gas certification and R290 (A3 flammable refrigerant) competence.
Certification: F-gas certificate (UK F-gas Regulations; EU 2015/2067 Cat. I cited by Viessmann) plus A3/R290 competence per EN 378-4 or IEC 60335-2-40 Annex HH (6202263 p.2).
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 153 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026