750 on an alpha innotec heat pump: Return sensor external. Please call installer: Break or short circuit in the external return sensor
Short answer
On an alpha innotec heat pump, 750 means: Return sensor external. Please call installer: Break or short circuit in the external return sensor. 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: Return sensor external. Please call installer: Break or short circuit in the external return sensor.
When it shows: Break or short circuit in the external return sensor.
Likely causes:
- Sensor defective.
- Plug/cable fault.
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.
Resetting itself: no.
Manual reset: manual acknowledgement required (rotary pushbutton 7 s) - display usually says "Please call installer".
What you can check
- Note the error number and text shown on the Luxtronik display.
- Acknowledge the message by pressing the rotary pushbutton for about 7 seconds (as the Luxtronik manual permits); if the same error reappears, contact your installer or ait customer service with the error number.
- Check the heat pump breakers/isolator at the consumer unit are on (do not open any casing).
What the engineer will check
- Check external return sensor, plug and connection line.
Who to ask for: heat pump service engineer (electrical/controls).
What you might notice
Sources
- Operating manual Luxtronik heating and heat pump control Part 2 (UK) 83055400lUK - 2.1 ↗ · 83055400lUK 2.1 · page 53 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026