Heat pumps · Error codes · Fujitsu
121Fujitsu · WATERSTAGE split (Comfort, High Power, Super High Power)
121 on a Fujitsu WATERSTAGE split: Flow temperature HC1 / mixing circuit too low
Short answer
On a Fujitsu heat pump, 121 means: Flow temperature HC1 / mixing circuit too low. 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: Flow temperature HC1 / mixing circuit too low.
Part involved: hydraulic unit controller.
Likely causes:
- Flow temperature for HC1 not reached.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: no.
What you can check
- Note the error number shown after pressing the Info (i) key on the Waterstage user interface. Check the heat pump breakers/isolators are on. Check the heating system pressure gauge (manual: recommended 1-2 bar) and that radiator valves are open. The user manual RESET key/'Reset HP' is for installers ('do not use during normal operation') - contact your installer if the fault remains.
What the engineer will check
- Check heat pump output, flow, settings.
Who to ask for: Heat pump / refrigeration engineer (manufacturer-trained).
Certification: F-gas certification for refrigerant-circuit work; electrical competence (Part P/18th Edition) for PCB/wiring.
What you might notice
Sources
- Maintenance Document - Air to Water Heat Pump Split single service and integrated DHW type (Super High Power) ↗ · 2105_EN_1 · page SHP SM p.5; DTM p.110; 1768-3 p.68 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026