An f75 fault on a Vaillant boiler is related to the pressure sensor in the boiler. When the pump starts the boiler expects a pressure change.
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.
In newer boilers (or those that have had the update kit fitted) it is just underneath the main heat exchanger.
Unfortunately, the hole that allows the water into the sensor is prone to clogging through any dirt in the system.
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.
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. If you don’t hear those noises the pump is likely to be stuck.
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
If the pressure is too low in the system the pressure sensor can fail to notice the pressure rise when the pump starts up.
This is a bit of an odd situation because the boiler should have already gone off with an f22 fault. However as is always the case with boilers weird things happen that shouldn’t.
The (several) times I’ve seen this happen are when the pressure sensor has gone out of calibration enough 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 most likely in 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 system. 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 so 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
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 the 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.
Stuck pump giving F75 fault
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 and spinning it with a screwdriver.
When (or if) you get the pump spinning the general rule as to whether the pump needs to be replaced. It is that if you can stop the pump spinning with a screwdriver jammed in the front the pump is too weak to work properly.
Broken pump giving F75 fault
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.
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.