F.788 on a Glow-worm GeniaAir: Building circuit pump reports internal faults
Short answer
On a Glow-worm heat pump, F.788 means: Building circuit pump reports internal faults. 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 pump reports internal faults.
Part involved: Building circuit pump reports internal faults.
Likely causes:
- The electronics of the high-efficiency pump has detected a fault (e. g. dry running, blockage, overvoltage, undervoltage) and switched off the pump as a securing measure.
- Glow-worm trade page: Building circuit pump fault Fault in the electrical connection to the building circuit pump, fault in the wiring harness to the building circuit pump, faulty building circuit pump, air in the building circuit, blocked or restricted filter or Y strainer.
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). Check that radiator/underfloor zone valves and thermostatic valves are not all closed and that the system control is calling for heat. 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
- Switch the heat pump off for at least 30 seconds (no current). 2. Check the plug contact on the PCB. 3. Check that the pump is working correctly. 4. Check the building circuit (water volume, purging.
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.33 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026