pU-flashing on a Hitachi Yutaki M: Excessively high water temperature (compressor stop) - pU flickering
Short answer
On a Hitachi heat pump, pU-flashing means: Excessively high water temperature (compressor stop) - pU flickering. 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: Excessively high water temperature (compressor stop) - pU flickering.
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.
No need to switch off — call an engineer if it comes back.
Resetting itself: unknown.
Manual reset: Power OFF/ON to remove alarms in Yutaki M and system controller (TCGB0090 p.141 for F16/F17).
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.
- Check the system pressure gauge on the indoor unit (Yutaki S / S Combi have a factory manometer; Yutaki M uses a field-fitted gauge) - Hitachi gives about 1.5-2.0 bar as normal and it must stay above 1 bar and below 3 bar.
- Check the filling loop is closed.
What the engineer will check
- Follow SMGB0146 section 6.7 troubleshooting flowchart for alarm pU-flashing.
- Check water pump operation and fuse, pump feedback.
- Check all circuit valves open, clean strainers/filters (Filter+ valve).
- Check water pressure > 1 bar, bleed air, check flow and pressure switch/sensor continuity.
- Check heater contactors, water thermistors and minimum system water volume.
Who to ask for: Heating engineer / MCS heat pump installer.
Certification: G3 unvented qualification where a DHW cylinder is involved.
What you might notice
Sources
- YUTAKI M Series RHUE-3AVHN1 / RHUE-(3-6)A(V)HN-HM Technical Catalogue ↗ · TCGB0090 rev.0 - 12/2013 · page p.139 (PDF p.147) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026