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Bosch · Tankless water heater

AD Power derate warning

Low severity Bosch Greentherm T9800 SEBosch Greentherm T9900 SE

What the control is telling you

The unit is derating (typical at altitude above 2000 ft or with long flue runs); it keeps running and the warning cannot be cleared in that case.

Applies to: Bosch Greentherm T9800 SE 160/199 and T9900 SE 160/199 / i 199 tankless water heaters

Likely causes

  • High altitude with long vent
  • Blocked air or exhaust opening
  • Dirty or clogged air filter, or filter missing and burner clogged

Test and repair steps

  1. Hold the power button down roughly 3 s to clear the stored error.
  2. Check air and exhaust openings for obstruction.
  3. Confirm the flue length is within specification.
  4. Clean or replace the air filter (if none is fitted the burner may be clogged and need replacement).
  5. Run the gas and air tuning procedure.

Models that show this code

6 models in Bosch Greentherm T9800 SE, Bosch Greentherm T9900 SE

Greentherm T9800 SE 160Greentherm T9800 SE 199Greentherm T9800 SEC 199Greentherm T9900 SE 160Greentherm T9900 SE 199Greentherm T9900 i 199

Other codes on the same units

  • F7 Control unit internal error
  • F8 Control unit internal error
  • F9 Gas valve driver fault
  • AA Flue gas temperature too high, power limited
  • BB Fan calibration after missed ignitions
  • BC Repeated fan calibration with similar values
  • CA Water flow above 9.2 gpm
  • CE Flue blockage during operation (siphon pressure)

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Source: Bosch Greentherm T9800 SE 160/199 installation 6720816815 (06.2023), Table 24 Error code diagnostics, p.65-69; Bosch Greentherm T9900 SE 160/199 | i 199 installation 6 720 813 635 (2024-06), section 9.1 Error code diagnostics, p.68-73. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

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.