121 on an Atlantic heat pump: Flow temperature for heating circuit 1 (HC1) not reached
Short answer
On an Atlantic heat pump, 121 means: Flow temperature for heating circuit 1 (HC1) not reached. The heat pump keeps running. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the Alféa Excellia / Excellia Duo (R410A split, 11-16, pre-A.I.)
The manual says: Flow temperature for heating circuit 1 (HC1) not reached.
Likely causes:
- Flow temperature of the heating circuit not reached.
On the Alféa Extensa + / Extensa Duo + (R410A split, 5-16)
The manual says: Flow temperature for heating circuit 1 (HC1) not reached.
Likely causes:
- Flow temperature of the heating circuit not reached.
On the Alféa Hybrid Duo (heat pump + integrated fuel-oil boiler)
The manual says: Heating circuit 1 start temperature not reached.
Likely causes:
- Flow temperature of the heating circuit not reached.
Is it safe?
On the Alféa Excellia / Excellia Duo (R410A split, 11-16, pre-A.I.)
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: 1.
On the Alféa Extensa + / Extensa Duo + (R410A split, 5-16)
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: 1.
On the Alféa Hybrid Duo (heat pump + integrated fuel-oil boiler)
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
What you can check
- Check the heating system pressure gauge reading (typically 1-2 bar per Atlantic maintenance guidance).
- Check the outdoor unit is not blocked by leaves, debris or snow.
- Note the error number/LED pattern shown and give it to your installer.
What the engineer will check
- Check heat pump output, flow rate, backup heaters and heating curve settings.
What you might notice
Sources
- alféa excellia duo Maintenance Document (U0546279_1738_EN_16) ↗ · U0546279_1738_EN_16 · page 68 · Reviewed 23 September 2026
- alféa extensa + Installation and operating manual (doc 1734-10, 1734-EN) ↗ · 1734-10 · page 74 · Reviewed 23 September 2026
- alféa hybrid duo Installation and commissioning instructions (doc 1440-13) ↗ · 1440-13 · page 65 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026