Heat pumps · Error codes · Thermia
aux-heaterThermia · Diplomat
aux-heater on a Thermia Diplomat: Overheating protection deployed. No auxiliary heater
Short answer
On a Thermia heat pump, aux-heater means: Overheating protection deployed. No auxiliary heater. 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: Overheating protection deployed. No auxiliary heater.
Part involved: overheat protection.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Manual reset: Set operating mode OFF then back to the desired mode.
What you can check
- Note the exact alarm text. Check the heating-system pressure gauge is in the range given in the user guide (Thermia/Danfoss: 1-1.5 bar when cold) - do not top up yourself unless you have been shown how; check radiator/underfloor valves and room thermostats are open. Acknowledge once by setting operating mode OFF then back; 'Other alarm message: reset the alarm; if the alarm remains, contact a service technician' (VUGFD302 p.16).
What the engineer will check
- Reset overheat protector; check flow, sensor, immersion position. Causes per the equivalent Danfoss service instructions VMGFC302 (same Danfoss-era controller) - not stated in the Thermia user manual.
Who to ask for: heat pump engineer.
Certification: Qualified electrician / heat pump engineer (electrical fault-finding).
What you might notice
Sources
- User manual Atria, Atria Duo, Atria Duo Optimum, Atria Optimum, Comfort, Diplomat, Diplomat Duo, Diplomat Duo Optimum (G2), Diplomat Optimum (G2) - 086U6297 Rev. 6 EN ↗ · 086U6297 Rev. 6 · page printed p.41 (PDF p.43) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026