704 on an alpha innotec heat pump: Error hot gas. Reset in hh:mm: Maximum hot-gas (discharge) temperature in the refrigerant circuit exceeded; automatic restart after the displayed time
Short answer
On an alpha innotec heat pump, 704 means: Error hot gas. Reset in hh:mm: Maximum hot-gas (discharge) temperature in the refrigerant circuit exceeded; automatic restart after the displayed time. 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: Error hot gas. Reset in hh:mm: Maximum hot-gas (discharge) temperature in the refrigerant circuit exceeded; automatic restart after the displayed time.
When it shows: Maximum hot-gas (discharge) temperature in the refrigerant circuit exceeded; automatic restart after the displayed time.
Likely causes:
- Low refrigerant quantity.
- Evaporation problem.
- Superheat.
- Flow/return conditions.
- Heat source minimum.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: automatic.
What you can check
- Note the error number and text shown on the Luxtronik display.
- The controller resets this automatically after the stated time - wait for the automatic restart before calling.
- 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 refrigerant quantity.
- Check evaporation and superheat.
- Check flow, return and HS-min.
Who to ask for: F-gas certified refrigeration / heat pump engineer (R290 flammable-refrigerant competence for propane units such as LWD).
Certification: F-gas certification (R407C/R410A units); flammable refrigerant training for R290 units.
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