The F5 fault code indicates that your Viessmann boiler has “locked out”, which means you won’t be able to turn it on until the underlying problem is fixed.
While some fault codes can be straightforward to diagnose, F5 has a number of potential causes that vary in likelihood depending on the age and model of your boiler.
As diagnosis of all of these causes requires work within the boiler unit, you will always need to contact a registered Gas Safe engineer when the fault appears.
This article will help you to understand what could have caused the issue, as well as the repairs and costs that your engineer may suggest.
What Does the Viessmann F5 Fault Mean?
Viessmann lists the meaning of the F5 fault code as “Air pressure switch faulty”, with some manuals also including “Sticking mixture damper”.
This can be confusing as they are two totally separate parts within the boiler. However, if you have a new unit then you can rule out the mixture damper, as Viessmann no longer includes this part in their range.
The air pressure switch is an electrical component which measures the pressure of gas entering the boiler, and sends these readings to the Printed Circuit Board (PCB).
The PCB is responsible for confirming each stage of the heating process has taken place safely before permitting the boiler to continue.
If it receives a signal from the switch that the gas pressure is too high or low, it will lock out and display a code such as F5 to prevent the system running unsafely.
The mixture damper is a small metal flap which seals the outlet of the fan, closing by gravity and opening when the fan is running and exerts air pressure onto it.
It is also colloquially known as a flapper, jimmy flapper, jimmy slapper, or jemmy slapper. The fan is a vital part of the boiler’s safe functioning, as it pushes dangerous combustion gasses away from the unit, out of the flue pipe.
Starting the fan is one of the first steps of the heating process, so if the PCB registers that this hasn’t happened for any reason, it will prevent further stages from taking place.
Viessmann no longer includes this part in their boilers, so this meaning of the F5 fault code is only relevant to older installations.
What Causes the Viessmann F5 Fault?
Issue Affecting the Gas Supply to the Boiler
The gas pressure switch may be failing to register sufficient levels within the boiler due to an issue affecting the flow of gas from the mains supply into the home.
A common source of such problems is the gas meter regulator, a part which ensures that the correct amount of gas is flowing at any one time.
As these meters are usually located outside, in cold weather they can become frozen, leaving them stuck at an incorrect reading and preventing gas from flowing. This is most likely if your meter is not protected by a box.
Faulty Gas Valve
Once the gas has travelled through your home to the boiler itself, this valve is responsible for opening and closing to permit the appropriate flow into the unit.
It is a mechanical part, rather than electrical, and with sustained use the mechanism can begin to stick.
This is a problem whether it’s stuck in the fully open or closed position, though in the case of the F5 fault it’s more likely to be closed.
Faulty Gas Pressure Switch
If the gas from the mains supply and into the boiler is flowing as it should, there may be a fault with the switch which measures gas pressure within the unit.
As this is an electrical component, it is connected to the PCB via leads, which may have become shaken loose due to vibrations within the unit.
These leads, or the switch itself, may also have suffered a fault leading to total malfunction. If the PCB cannot register a reading from the switch to ensure that the gas pressure within the unit is safe, it will prevent further gas from flowing into the unit.
This safety measure stops excess gas within the boiler, which could ultimately lead to dangerous leaks into the home.
Mixture Damper is Stuck
This flap isn’t fitted on new models, but it’s one of the most common causes of the F5 fault on the older models that do have it. When the fan is not in use, the flap connects to the outlet by a rubber seal. Over time, the seal can become sticky from wear and tear or exposure to moisture, and eventually it can become so stuck that the pressure from the fan will not be enough to blow it open.
Faulty PCB
Issues with the PCB can easily stop the boiler from functioning entirely, as it is needed to control each electrical process.
Individual connections can become loose, or localised damage can be caused by moisture from leaks within the unit. It could also have ceased to function entirely, or even be displaying an inaccurate fault code as it fails.
Can I Fix My Own Viessmann Boiler with the F5 Fault?
There is no DIY fix for this fault. As diagnosis and repair involves work within the unit, and may interfere with the gas supply, it must legally be undertaken by a registered Gas Safe engineer.
How to Fix a Viessmann F5 Fault
When There is an Issue Affecting the Gas Supply to the Boiler
Your engineer will be able to test gas pressure at every stage from the mains supply to the boiler unit itself.
This is often done with a tool called a manometer, which can be connected to the supply, providing readings that can be checked against the appropriate level.
If a frozen regulator has caused the issue, they will be able to defrost this and will likely offer to undertake preventative measures.
These can include boxing in the meter, as well as adding insulating materials around it. This process is known as “lagging”.
Where there is no obvious issue with a part of your system, occasionally it may indicate an issue on the side of your gas supplier, which an engineer should be able to find out for you.
If the Gas Valve is Faulty
A stuck gas valve is usually easy to identify as it won’t open and close correctly when testing the boiler. In some cases, your engineer will be able to remove and free the valve before reinstalling it.
However, it is often more cost-effective to replace this part entirely, as sticking is often a recurring issue once it has begun.
They are expensive parts, costing up to £250, but this is still a worthwhile investment to prevent further costly call-outs in the future.
If the Gas Pressure Switch is Faulty
It may be possible to note loose leads with a visual inspection, after which they can be reconnected to fix the issue.
Your engineer may want to investigate other components within the boiler if they suspect that excess vibrations have caused the leads to become loose, as these can sometimes be caused by faults with parts such as the pump.
If everything is appropriately connected, engineers can test the switch and the surrounding leads with a tool called a multimeter, which takes resistance readings.
If a replacement switch is required, the cost will vary per model but is likely to be under £100.
If the Mixture Damper is Stuck
As these parts are not included in new installations, approaches to dealing with this issue vary, even at the advice of Viessmann themselves.
Some engineers may advocate for a total replacement of the part, whereas others may remove the rubber seal with a knife and leave the flap in place to continue functioning.
If your engineer suggests completely removing the part, make sure to check that this modification wouldn’t affect your waanty or other cover with Viessmann.
The cost of a new damper is usually under £30, but the labour costs will vary as accessing the part can involve time consuming dismantling.
If the PCB is Faulty
If all other potential causes have been ruled out, your engineer can test the PCB using a multimeter. This will shower whether the whole part or specific connections aren’t functioning.
In some cases, loose connections can be repaired without replacing the whole part, but if there is damage then a new PCB will be required. If moisture is to blame, they will also want to check for any leaks, which may incur further repairs.
The PCB is one of the most expensive components, so you can expect to pay around £500 for a replacement.
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