73 on a Hitachi heat pump: Mixing over-temperature limit protection for mixed circuit (circuit 2)
Short answer
On a Hitachi heat pump, 73 means: Mixing over-temperature limit protection for mixed circuit (circuit 2). 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: Mixing over-temperature limit protection for mixed circuit (circuit 2).
Part involved: mixing circuit 2.
When it shows: Circuit 2 supply temperature > target temperature + offset.
Likely causes:
- Circuit 2 supply temperature > target temperature + offset.
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: no.
Manual reset: Stop the unit (via unit controller / power off-on) after the cause is fixed.
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 73.
- 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
- Service Manual - YUTAKI R32 / R410A series (Split, Monobloc and Hydrosplit systems) ↗ · SMGB0146 rev.0 - 12/2022 · page table printed p.655 (viewer p.681); troubleshooting flowchart printed p.726 (viewer p.752) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026