8 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution HM gen 1: Too high temperature (>90 C) on hot water sensor (BT6); blocks hot water via outdoor unit and additional heat
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 8 means: Too high temperature (>90 C) on hot water sensor (BT6); blocks hot water via outdoor unit and additional heat. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Too high temperature (>90 C) on hot water sensor (BT6); blocks hot water via outdoor unit and additional heat.
Part involved: High HW temp.
When it shows: BT6 > 90 C.
Likely causes:
- Contactor to internal electric heater defective; incorrect external heat source setting.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: yes (switch SF1 off/on per user manual).
What you can check
- Note the alarm number/text shown on the display. Check the consumer-unit breakers/isolators for the indoor and outdoor units are on. The user manual allows resetting by switching the indoor unit off and on with switch SF1; if the alarm cannot be reset, hold the operating-mode button to select 'Add. heat only' (immersion heating) and contact the installer. Recurring alarms mean an installation fault.
What the engineer will check
- Indoor-unit checks per HM-T-193 flowchart: check sensor characteristics (BT6/BT19), external heat source settings, contactors QA1-QA3.
Who to ask for: Heat pump / refrigeration engineer (manufacturer-trained).
Certification: F-gas certification for refrigerant-circuit work; electrical competence (Part P/18th Edition) for PCB/wiring.
What you might notice
Sources
- Technical Manual HM-T-193 - Air to Water Heat Pump (Hydrolution) ↗ · '13 HM-T-193 (updated December 18, 2014) · page ML p.159 (printed 158); flowchart p.165 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026