Fill on a Worcester Bosch heat pump: Water pressure in the heating system is too low
Short answer
On a Worcester Bosch heat pump, Fill means: Water pressure in the heating system is too low. 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: Water pressure in the heating system is too low.
Part involved: system pressure.
When it shows: Water pressure in the heating system is too low.
Likely causes:
- Low heating water pressure.
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: Clears when cause removed.
Manual reset: CR 10 H manual: if a fault cannot be rectified note fault code and sub-code and contact contractor with ID no.
What you can check
- Read and note the full code (and sub-code) 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.
- The CR 10 H manual says the heating water may be topped up even without a specialist following the heat source documentation; only do this if your installer has shown you how and close the filling loop afterwards.
What the engineer will check
- Top up heating water per heat source documentation; investigate cause of pressure loss.
Who to ask for: heat pump installer / controls engineer.
Certification: None specific; electrical isolation required before wiring checks.
What you might notice
Sources
- CR 10 H room controller manual (multilingual) ↗ · 6720876740 (2017/08) · page 30 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026