High-temperature-difference-heat-transfer-fluid on a Worcester Bosch heat pump: Warning window: High temperature difference heat transfer fluid
Short answer
On a Worcester Bosch heat pump, High-temperature-difference-heat-transfer-fluid means: Warning window: High temperature difference heat transfer fluid. The heat pump keeps running. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Greensource air to water heat pump (monobloc with HWDU)
The manual says: Warning window: High temperature difference heat transfer fluid.
Part involved: T8/T9 delta.
When it shows: Insufficient flow over heat pump; blocked particle filter.
Likely causes:
- Insufficient flow over heat pump; blocked particle filter.
On the Greensource Split HP Hydrolight / Hydrocomfort
The manual says: Warning: High temperature difference heat transfer fluid.
Part involved: T8/T9 delta (>13 K).
When it shows: Insufficient flow; blocked filter.
Likely causes:
- Insufficient flow; blocked filter.
Is it safe?
On the Greensource air to water heat pump (monobloc with HWDU)
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: yes.
Manual reset: Clears automatically; warnings may be acknowledged.
On the Greensource Split HP Hydrolight / Hydrocomfort
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: yes.
Manual reset: Acknowledge by pressing the rotary selector (Acknowledge) on the Hydrolight/Hydrocomfort user interface.
What you can check
- Read and note the full alarm text shown on the controller and pass it to your installer.
What the engineer will check
On the Greensource air to water heat pump (monobloc with HWDU)
- Check primary pump, valves, pump speeds; clean filter.
Who to ask for: heat pump installer / heating engineer.
Certification: None specific for hydraulic checks; MCS-certified installer recommended for system changes.
On the Greensource Split HP Hydrolight / Hydrocomfort
- Check pump, valves, speeds; 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
3 owner and engineer reports mention this.
- Insufficient heating (house not warm enough) on the Greensource air to water heat pump (monobloc with HWDU): 3 reports
Sources
- Installation Manual Air to Water Heat Pump with DHW distribution unit Greensource 6/7/9.5 kW ↗ · 6 720 641 467 (2010/01) · page 43-44 · Reviewed 23 September 2026
- Installation, Commissioning and Servicing Instructions Greensource Hydrolight and Hydrocomfort (ODU 7.5-12) ↗ · 6 720 806 629 (2013/02) · page 66 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026