E2X-T8T9-delta on a Worcester Bosch Greenstore LECP ground source heat pump: High temperature difference heat transfer fluid E2X
Short answer
On a Worcester Bosch heat pump, E2X-T8T9-delta means: High temperature difference heat transfer fluid E2X. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: High temperature difference heat transfer fluid E2X.
Part involved: T8/T9 delta (>13 K).
When it shows: Difference between T8 and T9 exceeds 13 K (measured 10 min after start; 3 min delay).
Likely causes:
- Difference between T8 and T9 exceeds 13 K (measured 10 min after start; 3 min delay).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: no.
Manual reset: Deactivated on acknowledgement.
What you can check
- Read and note the full alarm text and category letter 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.
What the engineer will check
- Diagnose per chapter 21: Difference between T8 and T9 exceeds 13 K (measured 10 min after start; 3 min delay).
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
Sources
- Installation, Commissioning and Servicing Instructions Worcester Greenstore LECP GSHP (System/Combi 6-11 kW) ↗ · 6 720 806 768 (2013/02) · page 62 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026