84 on a Hitachi heat pump: High water pressure
Short answer
On a Hitachi heat pump, 84 means: High water pressure. 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: High water pressure.
Part involved: water pressure.
When it shows: Water pressure >= 3.7 bar at pressure sensor.
Likely causes:
- Water pressure >= 3.6-3.7 bar measured by the pressure sensor.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| 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.
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.
- Check the pressure gauge reading and that the filling loop is closed (an open filling loop can over-pressurise the system); do not vent or drain the system yourself.
- Call an engineer.
What the engineer will check
- Follow SMGB0146 section 6.7 troubleshooting flowchart for alarm 84.
- 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.656 (viewer p.682); troubleshooting flowchart printed p.733 (viewer p.759) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026