GT8-GT9 on a Worcester Bosch Greenstore ground source heat pump: High Heat transfer fluid delta T8/T9 (GT8/GT9)
Short answer
On a Worcester Bosch heat pump, GT8-GT9 means: High Heat transfer fluid delta T8/T9 (GT8/GT9). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: High Heat transfer fluid delta T8/T9 (GT8/GT9).
Part involved: T8/T9 temperature difference.
When it shows: High Heat transfer fluid delta T8/T9 (GT8/GT9).
Likely causes:
- Insufficient flow over heat pump (pump jammed, valves closed, pump speed); blocked primary filter.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
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 the heating system pressure gauge reading (Bosch guidance is typically 1-2 bar when cold); report low pressure to your installer.
- 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
- Check pump, valves, speed; bypass/external main pump per p.13; clean filter.
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