Heat pumps · Error codes · Thermia
low-return-temp-1Thermia · Atec
low-return-temp-1 on a Thermia Atec: The return line temperature is too low to start a defrosting
Short answer
On a Thermia heat pump, low-return-temp-1 means: The return line temperature is too low to start a defrosting. 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: The return line temperature is too low to start a defrosting.
Part involved: return line temperature.
Likely causes:
- Return temperature too low for defrost (e.g. low system volume/buffer - buffer volume requirement 10-20 l/kW per installation instructions). (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
What you can check
- Note the exact alarm text. Check the outdoor unit is not blocked by snow, ice, leaves or debris and that airflow around it is clear. Acknowledge once (operating mode OFF then back); if it remains, contact a service technician (user manual 086L0130 p.25).
What the engineer will check
- Check system volume/buffer tank sizing, return temperature, heating demand at defrost.
Who to ask for: heat pump engineer.
What you might notice
Sources
- Service Instructions Atec (VIIFM102) - silo.tips copy ↗ · VIIFM102 · page p.10 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026