INV-H-ARITHMETIC on a Stiebel Eltron WPM 3 heat pump manager: Major inverter fault: Arithmetic fault in the measuring and analysis process
Short answer
On a Stiebel Eltron heat pump, INV-H-ARITHMETIC means: Major inverter fault: Arithmetic fault in the measuring and analysis process. 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: Major inverter fault: Arithmetic fault in the measuring and analysis process.
Part involved: compressor or fan inverter.
When it shows: Major inverter fault: Arithmetic fault in the measuring and analysis process.
Likely causes:
- Inverter fault.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes - restarts after IWS fault time/idle time.
Manual reset: after 5 faults in 2 h run: IWS reset.
What you can check
- Read and note the full message code and time from the WPM display (DIAGNOSIS / NOTIFICATIONLIST if accessible without a code).
- Check the heat pump and controller circuit breakers/isolators at the consumer unit are on.
What the engineer will check
- Inverter fault.
Who to ask for: Heat pump service engineer (electrical/controls).
Certification: Qualified electrician competence for live-work on appliance electrics.
What you might notice
Sources
- OPERATION AND INSTALLATION Heat pump manager WPM 3 (DM0000035521) ↗ · A 316199-43934-9674 · page 48, 49, 52 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026