F.520 on a Glow-worm GeniaAir: Building circuit flow temperature sensor signal invalid
Short answer
On a Glow-worm heat pump, F.520 means: Building circuit flow temperature sensor signal invalid. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Building circuit flow temperature sensor signal invalid.
Part involved: Building circuit flow temperature sensor signal invalid.
Likely causes:
- Flow temperature sensor at the heat pump is defective or not connected.
- Glow-worm trade page: Building circuit flow temperature sensor fault Fault in the electrical connection for the building circuit flow temperature sensor, building circuit flow temperature sensor defective.
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 - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Glow-worm's MiPro Sense operating instructions say always contact your competent person in the event of a fault message; until they arrive you may select a limp-home (back-up heater) setting on the control, which is expensive to run.
What the engineer will check
- Check: Plug, wiring harness, sensor, PCB.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
Sources
- GeniaAir Appliance Interface installation and maintenance instructions ↗ · 8000043014_01 · page HPIM 7 8000043014_01 p.31 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026