Technical Service Bulletin
Issue: Burner is locked out with fault code F7
Applies to: Vitodens 100, WB1A Series (all models)
The information in this bulletin is intended exclusively for licensed heating contractors, and is NOT intended for the end user. This material is NOT intended to take the place of the information contained in the manuals shipped with the product.
Use of this information by unqualified personnel may result in property damage, personal injury or death.
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The burner is locked out, and fault code "F7" is displayed on the boiler control.
Boiler water pressure is 12 psig or greater, but the boiler controller is reading pressure lower than the permissible pressure, resulting in an F7 fault.
Faulty pressure sensor, which could be caused by damage to the sensor harness or a bad connection at the harness.
Measure the voltage on X9 terminals 1 and 2 to see what the sensor is measuring for boiler pressure – review section "Testing the water pressure sensor" below for details. If the pressure reading is lower than the actual pressure, then replace the pressure sensor.
If the sensor harness is damaged, then replace the harness.
If the problem persists after replacement, then contact Viessmann for a sensor bypass kit.
Testing the water pressure sensor
The following image displays the X9 plug on the boiler control, as viewed from the front.
Measuring between terminals 1 and 2 will reveal the return voltage from the sensor transducer to the control – this voltage depends on the boiler pressure (see graph below).
Measuring between terminals 1 and 3 will reveal the voltage going to the sensor transducer from the control – normally this should be 5 VDC.