HVACopediaBosch · Boiler · Startup
First power-up purge and trap filling
What happens, in order
- Switch the boiler on with the ON/OFF switch
- On the first ever power-up the heating zone pump cycles on and off for about 4 minutes (purging sequence)
- On combi models open the automatic vent during this time and close it when it finishes
- Condensate trap filling runs when the burner has been idle 28 days or more, after switching between summer and winter mode, or after the ON/OFF switch was used
- On the next heat or DHW call the boiler is held at low output for 15 minutes to fill the trap
- Display alternates a symbol with the supply temperature while trap filling is active
Timings and thresholds
| Item | Value |
|---|---|
| First purge | about 4 min |
| Trap filling run | 15 min at low output |
| Trap filling trigger | burner idle 28 days |
Applies to: Greenstar 3A series
Models
More Bosch boiler sequences
- Heating call, anti-cycle and pump post purge · Heating
- Combi comfort and economy modes · Domestic hot water
- Tank priority (ZBR with DHW tank) · Domestic hot water
- Frost protection setup · Freeze protection
- Thermal disinfection of DHW tank · Other
Related
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Source: Bosch Greenstar 6721853955 (2025-11) and 6721853953 (2025-11), Heatronic service functions and operation sections. Written in our own words; timings can differ by model and software version, so check the manual for your exact unit.
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.