F.820 on a Glow-worm GeniaAir: The communication with the building circuit pump has been interrupted
Short answer
On a Glow-worm heat pump, F.820 means: The communication with the building circuit pump has been interrupted. 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: The communication with the building circuit pump has been interrupted.
Part involved: The communication with the building circuit pump has been interrupted.
Likely causes:
- Pump does not report any signal back to the heat pump.
- Glow-worm trade page: Building circuit pump connection fault Fault in the wiring harness to the building circuit pump, faulty building circuit pump.
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
- Check the cable to the pump for defects and, if required, replace it. 2. Replace the pump.
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.34 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026