F5 Viessmann Fault Code Causes, Meaning & Fixes

The F5 Viessmann fault code can send alarm bells ringing however, if your Viessmann boiler is displaying an F5 fault code, don’t worry too much. 

Fault codes can be a common occurrence in boilers as they age. Our article aims to provide answers to any questions you may have about your Viessmann boiler problems, as well as a step-by-step guide on how to fix it.

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What is the Viessmann F5 Fault Code?

The F5 fault code on a Viessmann boiler indicates a faulty air pressure switch, which is responsible for measuring the gas pressure entering the boiler and sending readings to the PCB. If the switch sends a signal to the PCB indicating high or low gas pressure, the system will be locked out to prevent unsafe operation.

Older Viessmann manuals may also list a sticking mixture damper as a potential cause for the Viessmann F5 code, but this part is no longer included in Viessmann’s range. 

The mixture damper is a small metal flap that seals the outlet of the fan, which is a crucial component of the boiler’s safe functioning. If the fan fails to start, the PCB will prevent further heating stages from taking place.

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Causes of F5 Fault Code on Viessmann Boilers:

The Viessmann F5 fault code can be caused by several factors. One possible cause is a problem with flow of gas into the boiler from the mains supply into the home. This therefore can prevent the gas pressure switch from registering sufficient levels within the boiler. The gas meter regulator is a common source of such problems, as it can become frozen in cold weather and prevent gas from flowing.

Faulty Gas Valve

Another potential cause is a faulty gas valve, which can become stuck in the closed position and prevent gas from flowing into the unit. If the gas pressure switch itself is faulty, it may not be able to measure the gas pressure within the boiler, which can cause the PCB to prevent further gas from flowing into the unit.

Stuck Mixture Damper

In older models that feature a mixture damper, the F5 fault code can also result from a jammed damper. The rubber seal connecting the flap to the outlet can start to stick as it gets older over time, This will then cause further issues such as stopping the fan pressure from building up and opening it.

Faulty PCB

Finally, issues with the PCB can also cause the F5 fault code, as it is needed to control each electrical process within the boiler. 

Over time wiring, connections and joinings can become loose therefore damage can be caused by moisture from leaks within the unit. In some cases, the faulty PCB may have ceased to function entirely, or may be displaying an inaccurate fault code as it fails.

Is it possible for me to repair my Viessmann boiler with the F5 fault?

No, as this is related to the gas supply, it is a legal requirement for a licensed Gas Safe engineer to carry out the diagnosis and repair.

It’s not recommended to attempt a DIY fix for this issue as it requires working inside the unit and may disrupt the gas supply. Remember, there is a mixture of gas water and electricity all in one place.

Step-by-Step Guide on How to Fix a Viessmann F5 Fault

We obviously strongly advise you not to attempt to fix a Viessmann boiler unless trained, however, here is the potential process for an engineer to fix the Viessmann F5 issue.

1. Call an engineer to diagnose the problem and test gas pressure at every stage from the mains supply to the boiler unit itself.
2. If the issue is caused by a frozen regulator, the engineer will defrost it and may recommend preventative measures like boxing in the metre or adding insulation.
3. If the gas valve is faulty, the engineer will test it and may remove and free the valve before reinstalling it. However, it’s often more cost-effective to replace the valve entirely.
4. If the gas pressure switch is faulty, the engineer may visually inspect the leads, reconnect them if loose, and test the switch and surrounding leads with a multimeter. If a replacement switch is required, it will cost under £100.
5. If the mixture damper is stuck, the engineer may replace the part entirely or remove the rubber seal with a knife and leave the flap in place. A new damper costs under £30, but the labour costs will vary.
6. If the PCB is faulty, the engineer can test it using a multimeter and repair loose connections if necessary. If there is damage, a new PCB is required and may cost around £500.
7. Once the issue is fixed, the engineer may recommend regular maintenance and servicing to prevent future faults.

Note: Always ensure that any modifications suggested by the engineer don’t affect your Viessmann boiler warranty or another cover with Viessmann.

Other common Viessmann fault codes

• Viessmann F1 fault code
• Viessmann F2 fault code
• Viessmann F3 fault code
• Viessmann F4 fault code

Do I need a new Viessmann boiler? 

The likelihood of needing a new Viessmann boiler is low with the F5 error code if caught early, however, if your boiler is older, say around 10 years old it may be time to consider a new one.

The price of repairs is usually around £250 on average, so it may be viable to repair your boiler once, but after this, if you consider the latest boiler prices, it may be cheaper in the long run to invest in a new Viessmann boiler.