HVACopediaViessmann · Boiler
F.7 DHW storage tank sensor open circuit
What the control is telling you
No DHW heating; broken lead on the DHW tank temperature sensor.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Likely causes
- Open circuit on tank sensor or its wiring
Test and repair steps
- Verify the DHW setting made in the commissioning assistant.
- Check tank sensor at plug 5, wires 3 and 4.
- Measure the sensor input on the electronics module: about 3.3 V DC with the sensor unplugged.
- Replace the faulty part.
Models that show this code
4 models in Vitodens 200-W B2HE
Other codes on the same units
- F.673 Thermostat-function sensor TS3 short circuit
- F.68 Flame signal present at burner start
- F.69 Ionization current outside permitted range (valve/electrode check)
- F.691 No combustion air supply feedback
- F.70 BCU internal error
- F.71 Fan speed too low
- F.72 Fan idle state not reached
- F.73 BCU internal communication error
Save it for the next call
Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.
Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.62-73. 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.