U1 on a Grant heat pump: Compressor overheat protection relay operation
Short answer
On a Grant heat pump, U1 means: Compressor overheat protection relay operation. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manual says: Compressor overheat protection relay operation.
Part involved: compressor overheat protection relay / refrigerant charge.
When it shows: Detected by heat pump/controller PCB; error reset type printed as 'Manual'.
Likely causes:
- Compressor overheat protection relay faulty.
- Gas leakage.
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: engineer presses - and + on the remote controller together for 3 s, or Reset SW on the Terminal PCB (mains OFF/ON possible but not recommended).
What you can check
- Note the exact code shown on the controller display and tell your installer or Grant-trained engineer.
- Grant advises homeowners NOT to reset Aerona3 fault codes themselves - leave the reset to the engineer.
What the engineer will check
- Check relay resistance by tester; replace if blown.
- Check for excess gas / gas leakage at service valve and refrigerant circuit (pipe); collect refrigerant and recharge with prescribed mass.
Who to ask for: Grant-trained (G1) heat pump engineer.
Certification: F-gas certified refrigeration engineer (refrigerant circuit work).
What you might notice
What owners and engineers report
10 owner and engineer reports mention this.
- No heating on the Aerona³ R32: 5 reports
- No hot water on the Aerona³ R32: 4 reports
- No heating on the Aerona³ (R410A): 1 report
Sources
- Grant Aerona³ Air to Water High Efficiency Heat Pump Range - Installation and Servicing Instructions (UK DOC 0136) ↗ · Rev 2.4 · page 56 (DOC 0154: 55) · Reviewed 23 September 2026
- Grant UK Aerona³ Air to Water High Efficiency Heat Pump Range - Installation and Servicing Instructions (UK DOC 0109) ↗ · Rev 1.1 · page 55 (IRL DOC 0001: s11.4-11.5, printed pp55-57) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026