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Fault Code F 75
So what is Fault Code F 75, and why has this happened?
Fault Code F 75 is a boiler safety mode and locks the boiler out so as not to cause any internal damage to the boiler.
This fault code is not normally due to the boiler being at fault, it’s normally down to an external problem.
When you read through the Vaillant book-work, it gives you reasons on why the fault code has displayed on the boiler screen.
It states the water pressure sensor and or the pump are faulty, or possible air within the system.
With all Vaillant Eco Tec fault codes, the reasons given are just a guide on what to look for, it doesn't mean that these parts at fault and need to be replaced.
The most common reason for the fault code F 75 fault code is the system pressure has dropped below 0.8 bar. For the boiler to work it needs to have a minimum pressure of 0.8 bar. The normal working pressure when the system is cold should be around 1.2 bar.
The Vaillant Eco Tec has a built in water pressure switch which senses the system pressure.
When the boiler starts up, the internal pump spins and increases the pressure around 0.2 bar, the water pressure switch senses this and sends a signal to the PCB to start the ignition sequence.
If the sensor does not sense enough pressure the boiler will display fault code F 75 and go to lock out.
To overcome this problem check the pressure gauge on the boiler, which is located on the bottom right hand side.
Make sure the needle is above the red zero and is just into the grey area.
Another way to check the pressure is go to the boiler display and press any of the top four buttons, you will notice the display light comes on, now press the top right hand button on the panel, you should now see a radiator sign, press the same button again and it will display the bar sign, press again and it will show you the system pressure in digital.
Some of the new Vaillant boilers now only come with this type of pressure display so this will be the only way to check the pressure.
Should the boiler pressure be correct then we now need to check the water pressure sensor itself?
If the system has a build up of internal sludge, the sensor can get blocked and is not able to sense any pressure, so the boiler will not fire.
Vaillant moved the position of this sensor, it used located at the bottom of the boiler, now it is fitted directly below the heat exchanger connections on the left hand side.
The reason they moved it was when it was located at the bottom, when the boiler was off the sludge could enter the sensor and cause it to block, by moving it to just below the heat exchanger it stops the sludge.
If you find you cannot reset your Vaillant Eco Tec then the best advise I can give is to contact a local Vaillant specialist, you can find them on the Vaillant website.
Vaillant boilers in my opinion are of good quality and build, but like any boiler they need to be looked after, otherwise they will eventually stop working.
