Common Causes of Vaillant F75 Fault Explained

A Vaillant F75 fault is the pressure sensor in the boiler not sensing a pressure change when the pump starts.

When the pump starts to move the water around the boiler expects to see a slight increase in pressure.

If the pressure rise isn’t detected your Vaillant boiler will give an F75 fault.

The top 3 reasons for an F75 fault are:

Blocked or faulty pressure sensor

Faulty circulating pump

Low pressure

Blocked or faulty pressure sensor

Your Vaillant boiler knows the water pressure inside your central heating system through the use of a small pressure sensor. The sensor is located in one of two positions in the boiler.

In older systems the sensor is in the left hand manifold.

Vaillant pressure sensor, the main cause of F75 faults.

In newer boilers (or those that have had the update kit fitted) it is just underneath the main heat exchanger.

Vaillant pressure sensor near the main heat exchanger, the main cause of F75 faults.

Unfortunately, the hole that allows the water into the sensor is prone to clogging through any dirt in the system.

A blocked Vaillant pressure sensor, the main cause of F75 faults.

It’s pretty straightforward to clean the dirt from the hole. However I find that the dirt gets stuck on the inside of the sensor which causes them to fail.

The best course of action is to replace it.

Faulty circulating pump

The pump that circulates the water around the central heating system can fail. If the pump fails then a f75 fault will be displayed on the boiler.

Replacement Vaillant pump with a new cable.

This is the least likely of all the reasons for a F75 fault but is definitely a possibility. When the boiler starts you normally hear a whirring noise along with the sound of water moving around the system. No noise means the pump is seized

When it is stuck you can sometimes hear a buzzing sound when the pump is trying to turn but isn’t strong enough.

Low pressure

The pressure should between 1 and 1.5 bar. If the pressure is too low the sensor can fail to notice the pressure rise when the pump starts up.

The boiler should display F22 when the pressure is too low. This is another clue to a pressure sensor problem.

Low pressure on a Vaillant boiler can cause an F75 fault.

The (several) times I’ve seen this happen are when the pressure sensor has gone out of calibration. It is far enough out that a low pressure situation (below 0.5bar) doesn’t generate an F22 fault.

Even though it’s out of calibration it can still sense the change in pressure generated by the pump starting up.

If the pressure is too low the pressure doesn’t change when the pump starts so an F75 fault is displayed.

Topping up the pressure should clear this fault but you still need a new pressure sensor.

How to fix a Vaillant F75 fault

The first step is to press the reset button on the front of the boiler. On older boilers it looks like a flame with a cross through it.

The reset button will normally clear F75 faults. However clearing the fault doesn’t mean it won’t come back.

Fixing a pressure sensor F75 fault

Resetting the boiler will clear the F75 fault. The boiler will then go through its start up sequence. If you hear the pump run and the water move around the boiler the fault is probably the pressure sensor.

There is a good chance that the boiler will start up and work normally at least for a while. Eventually though the F75 fault will return.

Changing the sensor is a simple job. Turn off the power. Drain the pressure from the boiler. Remove the front case. Unclip the sensor and fit the replacement. Top up the pressure then put the case back on.

Replacing the sensor doesn’t interfere with any gas or flue components. However you do have to remove the boiler case. This job can only be completed by a competent gas safe engineer.

With the new sensor the boiler should work normally.

Fixing a F75 fault from the pump

Can you hear the pump run?

Resetting the boiler will clear the F75 fault. The boiler will then go through its start up sequence. If you don’t hear the pump run and water move around the boiler the fault is most likely the pump.

If the pump has failed completely you won’t hear water moving around the boiler. You may also hear a noise like the pump is straining and buzzing.

It is possible that the pump is just temporarily stuck. This can be fixed by removing the silver cap on the front face of the pump. Using a small screwdriver try to spin the impellor. This will either be free or jammed. With the pump powered a short spin gets it running.

Try to stop the pump spinning with a screwdriver jammed in the front. If it stops spinning the pump is too weak to work properly.

If the pump is jammed or burnt out completely then it must be replaced. Vaillant have recently updated their replacement parts so all pumps must be the high efficiency types. This means a straight swap of pump head only isn’t guaranteed anymore.

Two types of pump

If the old pump has bigger (generally silver coloured) allen screws, the pump head and body will need to be changed.

If the old pump has smaller (generally black) allen screws the head can be changed by itself.

Vaillant recommend changing the head and body together which is the route I always go down.

When ever you change the pump you also have to replace the wire that feeds power from the boiler to the pump. This is because the connector on the new pump is not compatible with the old style. Without the wire you can’t change the pump.

Replacing the pump doesn’t interfere with any gas or flue components. However you do have to remove the boiler case so this job can only be completed by a competent gas safe engineer.

Next Steps

Once you’ve reset the fault the boiler will either come back on or won’t. Either way more investigation is required to figure out which part is faulty.

Just because it has reset doesn’t mean the pump is fine. It may well have stuck when it was hot then become unstuck after cooling down.

The best and most reliable solution is to get a gas safe engineer to visit to diagnose the fault properly and replace the faulty part.

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