HVACopediaBosch · Boiler · Freeze protection
Frost protection setup
What happens, in order
- Heating system leave boiler on with the boiler high limit dial at 95 F (35 C) or higher
- Or leave boiler off, put approved anti-freeze in the heating water and drain the DHW circuit
- DHW tank leave boiler on, high limit at 95 F or higher and turn the DHW thermostat fully counterclockwise (50 F / 10 C)
Timings and thresholds
| Item | Value |
|---|---|
| Minimum boiler high limit | 95 F (35 C) |
| DHW thermostat for tank frost protection | 50 F (10 C) |
Applies to: Greenstar 3A series
Models
More Bosch boiler sequences
- First power-up purge and trap filling · Startup
- 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
- Thermal disinfection of DHW tank · Other
Related
Take it to the job
Sequences of operation, error codes and tips work offline in the HVACopedia app.
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.