F.793 on a Viessmann One Base: Failure, instantaneous heating water heater, phase 3
Short answer
On a Viessmann heat pump, F.793 means: Failure, instantaneous heating water heater, phase 3. 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: Failure, instantaneous heating water heater, phase 3.
Part involved: Failure, instantaneous heating water heater, phase 3.
When it shows: System characteristics: Phase 3 of the instantaneous heating water heater not available.
Likely causes:
- No power supply to phase 3 of the instantaneous heating water heater.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| 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: 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
- Turn off the ON/OFF switch on the indoor unit. Wait at least 2 min. Turn on the ON/OFF switch.
- Check the following connections:.
- Wall mounted indoor unit: 230 V~ junction box, terminals 136.1 to 136.N3 Floorstanding indoor unit: Terminal area 230 V~/400 V~, terminals 136.L1 to 136.N3.
- Relay K3 on EHCU electronics module.
- High limit safety cut-out.
- Instantaneous heating water heater.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
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
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 144 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026