Heat pumps · Error codes · Glow-worm
S.91Glow-worm · GeniaAir (HA x-7.1 O)
S.91 on a Glow-worm GeniaAir: Service message: Demo mode
Short answer
On a Glow-worm heat pump, S.91 means: Service message: Demo mode. The heat pump keeps running.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Service message: Demo mode.
Part involved: Heat pump control (status/Live Monitor).
When it shows: Service message: Demo mode.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Information only |
| Engineer needed? | No |
The heat pump keeps running.
Resetting itself: yes - status changes automatically with operating state.
Manual reset: not applicable.
What you can check
- Operating status only - no action needed. If heating or hot water is not being delivered for a long period, note the status code and tell your installer.
What the engineer will check
- None required unless heating/hot water is not delivered; then review status history, settings and application limits.
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.27 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026