DEFROST on a Stiebel Eltron WPM 3 heat pump manager: During defrost the flow, return or frost-protection temperature fell below 10 °C (or flow rate too low, e.g. <10 l/min WPL 15, <15 l/min WPL 25)
Short answer
On a Stiebel Eltron heat pump, DEFROST means: During defrost the flow, return or frost-protection temperature fell below 10 °C (or flow rate too low, e.g. <10 l/min WPL 15, <15 l/min WPL 25). 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: During defrost the flow, return or frost-protection temperature fell below 10 °C (or flow rate too low, e.g. <10 l/min WPL 15, <15 l/min WPL 25).
Part involved: water flow / pump / booster heater.
When it shows: During defrost the flow, return or frost-protection temperature fell below 10 °C (or flow rate too low, e.g. <10 l/min WPL 15, <15 l/min WPL 25).
Likely causes:
- Water flow rate too low, water temperature too low.
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.
- Check the heating system pressure gauge reading is in the normal range stated by your installer.
- Check the outdoor unit air intake/discharge is not blocked by leaves, snow or ice (air source units).
What the engineer will check
- Water flow rate too low, water temperature too low.
Who to ask for: Heating / heat pump engineer.
What you might notice
Sources
- OPERATION AND INSTALLATION Heat pump manager WPM 3 (DM0000035521) ↗ · A 316199-43934-9674 · page 46, 47, 50 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026