4052 on a Worcester Bosch Bosch Compress 5800i AW: Thermal disinfection unsuccessful (Thermische Desinfektion misslungen)
Short answer
On a Worcester Bosch heat pump, 4052 means: Thermal disinfection unsuccessful (Thermische Desinfektion misslungen). 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: Thermal disinfection unsuccessful (Thermische Desinfektion misslungen).
Part involved: thermal disinfection.
When it shows: Thermal disinfection unsuccessful (Thermische Desinfektion misslungen).
Likely causes:
- Constant DHW draw-off/leak; DHW sensor mispositioned; cylinder coil not vented; wrong pipework; excessive circulation losses; faulty DHW sensor; electric booster output too small for volume.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: Clears when cause removed.
Manual reset: Reset/acknowledge via UI 800 fault menu after cause removed.
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
What the engineer will check
- Stop constant draw-off, reposition/vent, check pipework and circulation, check sensor, check/increase maximum duration (0...30...180 min).
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
- Installationsanleitung Bedienfeld UI 800 (Luft-Wasser-Wärmepumpe) 6721833375 ↗ · 6721833375 (2023/09) DE, AT, CH, LU · page 20 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026