The F75 Fault code on the modern range of Vaillant Boilers (including the EcoTec Plus, Ecotec Pro, Ecomax and EcoFit) and the F9 / F75 Fault code on modern Glowworm boilers (including the CXI, SXI, Flexicom, Energy and Betacom) is probably the most common fault that we attend.
It is usually a fairly simple fix, but can become costly if your engineer guesses at the fault rather than following a logical fault diagnosis process.
At Boiler Boffin we are experts in Vaillant and Glowworm boiler repairs and cover all of Hertfordshire and North London. We actually carry out repairs on behalf of Vaillant Group Service themselves.
We carry a wide range of Vaillant and Glowworm Spare parts on our vans and we can usually get your boiler up and running again in one visit.
What does the F75 Fault code on a Vaillant boiler and F9 / F75 Fault Code on a Glowworm Boiler mean?
Vaillant and Glowworm boilers are made by the same company (Vaillant Group) and are made in the same factory in Derbyshire. The logic used in their Printed Circuit Boards are almost identical.
The F75 / F9 code means that when the boiler runs through its start up tests, it has failed to see a rise in the system pressure when it tries to spin the pump.
What possible causes are there for an F75 / F9 fault?
The main reason for the F75 / F9 being one of the most common faults that we attend, is that it is a code that covers a pretty wide range of potential issues. You will also often find that you actually have 2 or more of these issues occurring at the same time.
The most common causes of this fault code are;
1 – Lack of Boiler Servicing
The F75 / F9 issue is one that can almost certainly be completely avoided if your boiler is regularly serviced by engineers that have a thorough understanding of these boilers.
All of the fault conditions below are things that would be tested for and in some cases rectified during a routine Vaillant / Glowworm Boiler service if carried out by Boiler Boffin.
2 – Blocked or partially blocked Water Pressure sensor.
If this sensor becomes blocked then it will fail to register an increase in pressure when the pump spins and after a few attempts the boiler will lock out.
If this is the cause of the F75 / F9 then it is worth noting that your boiler may need flushing and / or a system filter fitting in order to prevent the new sensor from blocking, and to prevent the debris in the system from damaging any other components.
This is something that we would be testing for and checked during a routine Boiler Boffin service.
3 – Leaking components
This fault actually goes hand-in-hand with having a blocked pressure sensor.
If you have a leak on your system then your system pressure will drop. The pressure sensor should notice that the system pressure has dropped and display an F22 fault code.
But if your sensor is blocked then it will ‘stick’ at a certain point (so the system pressure may actually be 0.3 Bar, but your sensor will not drop below 1.2 Bar) .
When your pump spins, the pressure sensor should register at least a 0.2 Bar increase in pressure, but if the pressure it is displaying is way above the actual pressure then the pump will not be able to create enough of a pressure uplift to activate the sensor – Causing an F75 / F9 code to display.
4 – Pump failure
If your pump is seized, running slowly or the power supply to the pump is faulty then this will also cause an F75 / F9 fault code.
If this is the fault it will usually mean a new pump is required. If your Vaillant boiler was fitted after 2012 then this is usually just a case of removing the pump head and replacing it. But if you have an older Vaillant Ecotec you may require the entire Hydroblock section that the pump head connects on to be replaced as well.
You can tell if this is required by looking at the pumps data plate – If you have a data plate with a white background and black writing then you require the hydroblock and pump replacing. If you have a black data plate with white writing then you will usually just require a new pump head.
4 – Expansion Vessel Depletion
The expansion vessel in a sealed system is a major component and it is vitally important to ensure that it is correctly charged and sized for the system. Unfortunately this is one of the most basic and most common issues that we see when attending breakdown visits.
If the charge of air is either too low or is extremely high, then it will affect the amount of positive pressure that the pump is able to put onto the pressure sensor, and can cause an F75 fault.
Again this is something that would be tested for during any Boiler Boffin service – It is actually something we check on every single visit where we need to drain any water from the system.
5 – Blocked Pipework / Heat exchanger
This is a less common fault – although it is often diagnosed as the cause, if mis-diagnosed then it can cost customers a huge amount of expense for work that is not required.
This fault is more likely to happen in older versions of the Vaillant Ecotec range (models built before 2010), where they were previously manufactured with rubber hose sections on the Flow and Return pipes. The rubber in these pipes tends to attract magnetite build up far quicker than copper pipework and could become blocked, meaning the pump is unable to push water through them.
* Whatever boiler / system type you have – we always advise fitting a magnetic filter to your heating system in order to help catch any loose debris that is circulating and prevent magnetite from building up inside your boiler.*
Boiler Boffin usually advise customers to swap out any rubber hoses for the re designed copper sections during a service, as they had a tendency to burst after becoming blocked and spray water all over the inside of the boiler (usually causing it to be written off).
But if you haven’t had your boiler serviced for a long time, or if the person servicing the boiler hasn’t mentioned the fact that these hoses have been superseded then they could be the component that is causing your F75 issue.
The Heat Exchanger could also be blocked and causing an F75 fault code to appear, although you will almost always be forewarned that the system is contaminated as the pressure sensor should become blocked a long time before the heat exchanger does.
If you are constantly changing pressure sensors because of an F75 fault, then you need to have your system thoroughly flushed, because if the heat exchanger later becomes blocked, then it is often not financially viable to replace it. That could mean you have to spend out several times what a flush would cost in order to have a new boiler installed.
6 – Faulty PCB
The last cause is a very rare one – but it can happen, and has caught engineers out several times in the past because it is not something that they expect to see.
If the PCB is failing to send 230v of power to the pump, then the pump will not spin and after 3 start up attempts the boiler will lock out showing F75.
Most of the time that the pump is not spinning, it is just assumed that it is because the pump is seized, but Boiler Boffin will always test the pump to ensure the correct power is being delivered before diagnosing a new pump being required.
Boiler Boffin are specialists in Vaillant Boiler Repairs and we carry a wide range of spare parts on our vans, allowing us to repair most boilers in one visit.
If you live in Hertfordshire or North London you can book an appointment with us via our website, or would can call us on 01920459575