GT9 on a Worcester Bosch Greenstore ground source heat pump: High return to the heat pump T9 (GT9)
Short answer
On a Worcester Bosch heat pump, GT9 means: High return to the heat pump T9 (GT9). 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: High return to the heat pump T9 (GT9).
Part involved: T9 return (~57 C limit).
When it shows: High return to the heat pump T9 (GT9).
Likely causes:
- Temperature setting too high; DHW temperature set too high; radiator/UFH valves closed (bypass fitted); heat pump flow greater than system flow.
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: yes (restarts when temperature drops).
Manual reset: Press Ackn. on the control panel; heat pump restarts within 15 minutes if there is demand. Some causes need a reset button/MCB on the heat pump (engineer).
What you can check
- Read and note the full alarm text and code in brackets shown on the controller and pass it to your installer.
- Check thermostat / time program / operating mode settings on the controller.
- Press the Ackn. button once as the manual describes (manual: no risk in resetting an alarm); contact technical support if the alarm repeats.
What the engineer will check
- Lower settings; open valves; adjust pump speeds.
Who to ask for: heat pump installer / heating engineer.
Certification: None specific for hydraulic checks; MCS-certified installer recommended for system changes.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- Insufficient heating (house not warm enough) on the Greenstore ground source heat pump (original, System & Combi): 1 report
Sources
- Worcester Greenstore 6/7/9/11 kW System Installation, Commissioning & Servicing ↗ · 8-716-112-860 Issue c · page 37 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026