F.817 on a Vaillant aroTHERM plus: Freq. converter fault: Compressor
Short answer
On a Vaillant heat pump, F.817 means: Freq. converter fault: Compressor. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Freq. converter fault: Compressor.
Part involved: Freq. converter fault: Compressor.
Likely causes:
- Defect in the compressor (e.g. short circuit); Defect in the frequency converter; Connection cable to the compressor is defective or loose.
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. The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant.
What the engineer will check
- Measure the winding resistance in the compressor; Measure the frequency converter output between the three phases, (it must be > 1 kΩ); Check the wiring harness and plug connections.
Who to ask for: Heat pump engineer with electrical competence (qualified electrician for supply/phase faults).
Certification: Competent electrician (BS 7671 / Part P) for supply-side work.
What you might notice
What owners and engineers report
15 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
- Heat pump will not start on the aroTHERM plus (VWL ../6 A): 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.37; uniTOWER VIH QW 190/6 E 0020291525_05 p.62; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.60 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026