20240 on a Stiebel Eltron WPM heat pump manager / WPMsystem: The minimum overheating value of the oil sump compared to the condensation temperature for the monitoring time was permanently undershot
Short answer
On a Stiebel Eltron heat pump, 20240 means: The minimum overheating value of the oil sump compared to the condensation temperature for the monitoring time was permanently undershot. 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: The minimum overheating value of the oil sump compared to the condensation temperature for the monitoring time was permanently undershot.
Part involved: compressor / expansion valve / pressure switches.
When it shows: The minimum overheating value of the oil sump compared to the condensation temperature for the monitoring time was permanently undershot.
Likely causes:
- Sensor fault in oil sump temperature sensor, oil sump temperature sensor insufficiently thermally coupled to the oil sump of the compressor, expansion valve faulty (not closing sufficiently to increase superheating).
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 - heat pump restarts after idle time (~10-12 min); locks out after 5 faults in 2 h run.
Manual reset: after 5 faults in 2 h: IWS/WPM 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.
- Check the outdoor unit air intake/discharge is not blocked by leaves, snow or ice (air source units).
What the engineer will check
- Sensor fault in oil sump temperature sensor, oil sump temperature sensor insufficiently thermally coupled to the oil sump of the compressor, expansion valve faulty (not closing sufficiently to increase superheating).
Who to ask for: F-gas/refrigeration-qualified heat pump engineer (R290/A3 flammable-refrigerant competence for R290 models).
Certification: F-gas certification for fluorinated refrigerants (R410A/R454C); flammable refrigerant (R290) handling training for R290 models.
What you might notice
Sources
- WPM notification list (WPM V2.9) - multilingual; English section pp.15-26 (DOC-00081866) ↗ · WPM V2.9 · page 18 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026