3 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution HM gen 1: Temperature limiter in indoor unit has tripped (blocks both outdoor unit and electric heater)

What it means

The manual says: Temperature limiter in indoor unit has tripped (blocks both outdoor unit and electric heater).

Part involved: TB alarm.

Likely causes:

• Temperature limiter tripped during transportation; high temperature in indoor unit; blown circuit fuse; tripped miniature circuit breaker.

Is it safe?

Switch the heat pump off at its isolator and call an engineer.

The heat pump stops until the fault clears or is reset.

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: measure across temperature limiter FD1, reset/replace limiter, check immersion sensor and contactors QA1-QA3, relay board AA6.

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

• Display backlight flashes and status lamp red; some alarms switch to 'Add. heat only' at minimum supply temperature (low heat / higher electricity use)

Sources

• Technical Manual HM-T-193 - Air to Water Heat Pump (Hydrolution) ↗ · '13 HM-T-193 (updated December 18, 2014) · page ML p.155 (printed 154); flowchart p.160 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026