HVACopediaBosch · Boiler
9.b Service function 9.b - current fan speed
What the display is showing
Displays the present fan speed in revolutions per second. Read-only.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
What to do
- Reach the second level and scroll to 9.b
Models that show this code
4 models in Bosch Greenstar
Other codes on the same units
- 8.b Service function 8.b - code plug number
- 8.d Service function 8.d - gas burner control (GFA) fault
- 9.A Service function 9.A - constant operating mode
- 9.C Service function 9.C - current boiler output
- A7 DHW temperature sensor fault
- A8 Communication fault
- b1 Code plug not detected
- b2/b3/b4/b5/b6 Internal data error
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Source: Bosch Greenstar wall ZBR-ZWB 21-42-3A service 6721853955 (2025-11) (same content in combi service 6721853953), 11.3 Description of the service functions pp.59-64. Summarized in our own words; check the original manual for your model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.