703 on an alpha innotec heat pump: Antifreeze. Please call installer: Air/water units only: heat pump running and flow temperature < 5 °C - frost danger detected
Short answer
On an alpha innotec heat pump, 703 means: Antifreeze. Please call installer: Air/water units only: heat pump running and flow temperature < 5 °C - frost danger detected. 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: Antifreeze. Please call installer: Air/water units only: heat pump running and flow temperature < 5 °C - frost danger detected.
When it shows: Air/water units only: heat pump running and flow temperature < 5 °C - frost danger detected.
Likely causes:
- Insufficient heat pump output.
- Defrost valve fault.
- Heating system problem.
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).
- Air source units: check the outdoor unit/air inlet and outlet are not blocked by snow, ice, leaves or debris.
- Check the heating system pressure gauge reading and report it to the installer.
What the engineer will check
- Check heat pump output.
- Check defrost valve.
- Check heating system.
Who to ask for: heat pump / heating engineer.
What you might notice
Sources
- Operating manual Luxtronik heating and heat pump control Part 2 (UK) 83055400lUK - 2.1 ↗ · 83055400lUK 2.1 · page 52 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026