81 on a Hitachi heat pump: Momentary power interruption or low voltage detected (retry code P81)
Short answer
On a Hitachi heat pump, 81 means: Momentary power interruption or low voltage detected (retry code P81). 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: Momentary power interruption or low voltage detected (retry code P81).
Part involved: power supply.
When it shows: Momentary power interruption (<2 s) or low voltage during compressor operation; abnormal >60 s recorded as alarm 81.
Likely causes:
- Brief power interruption or low voltage during compressor operation.
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.
No need to switch off — call an engineer if it comes back.
Resetting itself: yes - released when power normal for >10 s.
Manual reset: not required.
What you can check
- Read and note the alarm code and any retry code (e.g. P70) shown on the unit controller.
- Check the heat pump supply breaker/isolator at the consumer unit is ON.
- Restart once via the unit controller (or switch off/on at the isolator) - the manual states the unit must be stopped to release most alarms; if the code returns, call an engineer.
What the engineer will check
- Follow SMGB0146 section 6.7 troubleshooting flowchart for alarm 81.
- Measure supply voltage and phase sequence/imbalance at the terminal board.
- Check supply cable size and connections per manual.
Who to ask for: Electrician / heat pump service engineer.
Certification: BS 7671 competent electrician for supply issues.
What you might notice
Sources
- Service Manual - YUTAKI R32 / R410A series (Split, Monobloc and Hydrosplit systems) ↗ · SMGB0146 rev.0 - 12/2022 · page table printed p.656 (viewer p.682); troubleshooting flowchart printed p.732 (viewer p.758) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026