A01-811 on a Worcester Bosch heat pump: DHW provision: thermal disinfection unsuccessful
Short answer
On a Worcester Bosch heat pump, A01-811 means: DHW provision: thermal disinfection unsuccessful. 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: DHW provision: thermal disinfection unsuccessful.
Part involved: thermal disinfection / DHW sensor.
When it shows: DHW provision: thermal disinfection unsuccessful.
Likely causes:
- Constant DHW draw-off (leak/taps open); DHW sensor mispositioned; DHW priority deselected so output insufficient; cylinder coil not vented; incorrect boiler/cylinder pipework; cylinder primary pump undersized; excessive DHW circulation losses; faulty DHW sensor; booster heater too small for volume (increase maximum duration 60-240 min).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: Clears when cause is removed.
Manual reset: User guides (7001i/7400i AWE, 3400i AWS, section 3.5): user may confirm the fault by pressing the selector; if it persists call the contractor with fault code, sub-code and user-interface 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 no hot tap is left running and there is no visible hot-water leak.
- Confirm/acknowledge the fault once at the controller (press selector) as the user guide allows; if it returns, call the installer.
What the engineer will check
- Check sensor position/resistance, coil venting, primary pump capacity, circulation losses, DHW max duration setting.
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 instructions User interface HPC 410 ↗ · 6721832682 (2021/02) · page 17-18 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026