F.1079 on a Viessmann One Base: Flow rate repeatedly too low during defrost process
Short answer
On a Viessmann heat pump, F.1079 means: Flow rate repeatedly too low during defrost process. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Flow rate repeatedly too low during defrost process.
Part involved: Flow rate repeatedly too low during defrost process.
When it shows: System characteristics: Refrigerant circuit locked.
Likely causes:
- Defrost fault.
- Minimum flow rate in secondary circuit not reached.
- Further message: A.16 and A.143 10 times.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
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: 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 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 outdoor unit is not blocked by leaves, debris or snow.
What the engineer will check
- Open shut-off valves in the direction of flow. Remove any blockages.
- Clean the heating water filter. Clean the dirt separator if necessary.
- Check the integral secondary pump/heating circuit pump in heating/cooling circuit.
- Check the impeller. Remove the rotor if necessary. Remove dirt. Flush the system if necessary.
- Check connection of flow sensor (if installed) at X19.1 to X19.4 on the EHCU electronics module:.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
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 183 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026