Bosch error code F2 on Greentherm tankless models indicates a flue temperature fault — the unit's flue sensor reads outside acceptable range, either too high (overtemperature lockout) or showing a sensor open-circuit. F2 is a safety lockout and the unit will not operate until cleared.
What F2 means
The Greentherm series uses a flue gas temperature sensor (thermistor or thermocouple depending on generation) that monitors combustion exhaust temperature. The sensor protects against: scaled heat exchangers (raises flue temp because heat isn't transferring to water), partial blockages, and sensor failures themselves.
Diagnostic steps
- Visual inspection of vent: open the unit cover and verify the flue path is clear. Look for debris, insect nests, or any obstruction. Check the exterior termination for blockage.
- Heat exchanger scale check: if the unit has been in service in hard-water areas without descaling, scale on the heat exchanger increases flue temperature. Annual descaling is critical — if you've skipped years, the heat exchanger may need professional cleaning.
- Sensor continuity: with power off, disconnect the flue sensor and measure resistance with a multimeter. Specs vary by model; consult the service manual. An open-circuit (infinite ohms) or shorted (near zero ohms) sensor needs replacement.
- Verify combustion air supply: insufficient air for combustion causes incomplete burning and elevated flue temperatures. Verify intake and exhaust both clear.
Heat exchanger descaling
If F2 was preceded by gradually increasing recovery times or reduced flow at hot fixtures, scale on the heat exchanger is the likely culprit. Bosch publishes a specific descaling procedure using their recommended descaler solution and a circulation pump setup connecting cold and hot ports. The procedure runs about 45–60 minutes plus flush. After descaling, F2 may resolve immediately or require manual reset followed by normal operation cycle.
Sensor replacement
Flue sensor replacement is an OEM part order from Bosch authorized parts. The sensor is typically located in the flue collection area between the heat exchanger and vent. Replacement requires disconnecting wiring, removing the sensor mount, and installing the new sensor with thermal compound. 20-minute DIY for someone comfortable opening the unit; otherwise authorized service.
When F2 keeps recurring
If F2 returns shortly after every reset despite cleared flue and recent descaling, the heat exchanger itself may be partially failed — internal sooting from previous combustion issues, hairline cracks affecting heat transfer, or sensor mounting issues. Authorized Bosch service inspection is the next step; heat exchanger replacement is expensive ($600–1,200) and may push toward whole-unit replacement decision for older units.