Bosch Boiler: Start the emissions test (chimney sweep) mode
Start the emissions test (chimney sweep) mode on a Bosch boiler: Holding the emissions test button starts boiler test mode for gas pressure and combustion checks; a brief tap toggles between maximum set heating output and maximum nominal output.
Applies to: Greenstar ZBR/ZWB 21-42-3A wall-hung boilers and ZWB 28-42-3A combis
How to fix start the emissions test (chimney sweep) mode: step by step
- Keep the emissions test button pressed until it lights
- Tap it to switch between max set heating output and max nominal output
- Keep it pressed until the light goes out to return to the supply temperature
If the unit shows an error code, look it up: Bosch Boiler codes
Models
ZBR21-3AZBR28-3AZBR35-3AZBR42-3A
Source: Bosch Greenstar wall ZBR-ZWB 21-42-3A service 6721853955 (2025-11) (same content in combi service 6721853953), 8.3 Dynamic gas pressure test p.50. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.