F.685 on a Vaillant heat pump: Communication fault: eBUS
Short answer
On a Vaillant heat pump, F.685 means: Communication fault: eBUS. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the aroTHERM
The manual says: Communication fault: eBUS.
Part involved: Communication fault: eBUS.
Likely causes:
- Product not connected to the control; eBUS polarity inverted.
On the flexoTHERM exclusive (VWF ../4)
The manual says: Connection error: Contr. not recognised.
Part involved: Contr. not recognised.
Likely causes:
- System control was previously detected but the connection is broken.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Connection error: Control not detected.
Part involved: Control not detected.
Likely causes:
- The VRC 700 control was previously detected but the connection is broken.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure).
What the engineer will check
On the aroTHERM
- Check eBUS connection and polarity between heat pump and control (VRC 470/VWZ AI).
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
On the flexoTHERM exclusive (VWF ../4)
- Check the eBUS connection to the system control.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Check the eBUS connection to the VRC 700 control.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
What owners and engineers report
2 owner and engineer reports mention this.
- No heating on the aroTHERM: 2 reports
Sources
- Installation instructions aroTHERM VWL 55/3 A 230 V, VWL 85/3 A 230 V (GB) ↗ · 0020217827_03 · page VWL ../3 A 0020217827_03 p.33; VWL ../2 A 0020147164_09 p.35 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.51; flexoTHERM 230 V 0020213395_04 p.46 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.37 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026