This Vaillant boiler has been suffering from error codes F.75 and low pressure on the pressure gauge. There was also a visible leak to the right-hand side of the boiler. When I opened the boiler, I found a source of the leak – it as scaled Automatic Air Vent on the boiler pump.

First call of action was to replace that leaking AAV. Below you can see an existing leaking part on the left and a new part on the right.

With the boiler drained down, I also checked the expansion vessel pressure – it was zero.

I repressurized the expansion vessel to 0.9 bar. I also replaced the Pressure Relief Valve – as with a flat expansion vessel, those valves usually pass as well. New PRV pictured below.

With the boiler fully drained down, the pressure sensor should be sensing no pressure and the expected error message on the boiler display is usually F.22 or 0.0bar – or both messages flashing one after another. However, in this case the boiler was showing an F.75 error message. What do those error messages mean?

F.22 – low water pressure, boiler will not fire. Top up boiler pressure or find a leak and repair the leak on the system or on the boiler itself

F.75 – no pressure increase detected when the pump starts. This prevents the boiler from firing as the PCB assumes that the pump is not working. This error according to the Vaillant manual should point to the faulty pump. In most cases however, this is a faulty or blocked pressure sensor – as those little sensors are notoriously unreliable.  The pumps on those boilers are very reliable and fail very rarely.

Boiler below showing error code F.75 with no pressure in the boiler. Expected error code in this case should be F.22 / o.o bar.

On the picture below, you can see the faulty pressure sensor in situ.  It is this black round object with a white connector and black and red wires attached to it.

I removed the sensor (member the boiler needs to be drained down), and as you can see from the picture below it is fully blocked.

I do not recommend trying to unblock those sensors. Always replace them with a new sensor. I fitted the new sensor, and turn the boiler on, still with no pressure in the boiler.
Now the boiler started showing correct message for the low pressure – F.22 alternating with 0.0bar.

This means that the pressure sensor is operating correctly. It was time to repressurize the boiler to 1 bar and make sure there are no more leaks. This concluded the repair.

I also recommended that the system is power flushed, as dirt in the system blocked the pressure sensor and also caused a leak on the pump AAV.

Parts replaced on this repair:

– pressure sensor

– auto air vent

– pressure relief valve

– expansion vessel repressurized