GT8 on a Worcester Bosch Greenstore ground source heat pump: Heat transfer fluid out max T8 (GT8)
Short answer
On a Worcester Bosch heat pump, GT8 means: Heat transfer fluid out max T8 (GT8). 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: Heat transfer fluid out max T8 (GT8).
Part involved: T8 flow.
When it shows: Heat transfer fluid out max T8 (GT8).
Likely causes:
- Insufficient flow over heat pump; blocked primary filter.
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 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; 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
2 owner and engineer reports mention this.
- No heating on the Greenstore ground source heat pump (original, System & Combi): 2 reports
Sources
- Worcester Greenstore 6/7/9/11 kW System Installation, Commissioning & Servicing ↗ · 8-716-112-860 Issue c · page 38 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026