The Vaillant F75 error is quite a common fault code with Vaillant ecoTEC, eco TEC plus and other boilers. In most cases, it can be fairly quickly and easily fixed without the need to call a plumber. But if you reset the boiler and don't hear the rumbling noise, your pressure sensor or pump may have a defect that needs to be seen by a qualified engineer

How to reduce boiler pressure - [ Ссылка ]

Vaillant’s manual list this fault as below:
Fault Code - F75.
Means - no pressure change detection when starting the pump.
Possible reasons for fault - Water pressure sensor and/or pump defective, air in heating installation, too low pressure in boiler; connect external expansion vessel to return.

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