Viessmann F8 Fault

February 15, 2023

The Viessmann F8 fault presents itself when a burner is disabled/locked out, meaning it cannot ignite. This fault may be triggered for various reasons, which will be addressed in this article. The most common cause of this fault is the failure of the flame sensor, which tells the control board that there is an actual flame present at the burner. After many rounds of ignition and cooling off, the sensor’s contacts wear enough to become loose. When they are not making good contact, they report an open circuit to the control board, which disables the burner from further operation.

This article will discuss a Viessmann F8 fault code that indicates that the burner on the boiler is locked out and unable to be fired or turned on. This is not a flame failure but rather an indication that the burner has been disabled!


What does F8 Fault Mean To My Boiler?

The Viessmann boiler F8 fault code indicates that a burner has been disabled and is being prevented from starting. This can happen for various reasons, ranging from the boiler not receiving the right signals to the boiler not getting enough air to work properly.

Viessmann boilers use a combination of both water and air as fuel. The air comes in through the burners and mixes with the water to create vapor, which is then used to ignite the gas burner. The burners are housed within the furnace and are regulated by a controller that uses both pressure and temperature to ensure that they are working at maximum efficiency.

If there isn’t enough pressure, or if the temperature drops too low, this can cause the engine to overheat and cause various problems, including F8 faults.

One reason for this could be that there isn’t enough water in the system for it to run at optimum levels. Boilers need a certain level of water to run properly, so this should be checked before any other work is done on it. This could mean that you have an issue with your external plumbing or internal plumbing line, so it’s essential to check these before you begin any troubleshooting.


Causes Of Viessmann F8 Fault

The Viessmann F8 Fault (burner is locked out) is very common in Viessmann boilers. This fault can occur due to several reasons. Here are some of them:


Combustion characteristics are not ideal

The combustion characteristics can be affected by various factors, including the fuel used, the air supply, and the amount of carbon monoxide present. The flame-sensing electrode helps detect these changes, so they can be corrected by adjusting the burner settings or replacing filters and other components of your boiler system.

Condensate on the flame-sensing electrode

The condensate on the flame sensor can cause false detection of a flame. The flame sensor is located in the throat of the burner and detects whether there is a flame. If it has been installed incorrectly or if there is condensation from the air, this can cause false detection of a flame fault. You should check your installation guidelines.

Gas pressure is too low

The gas pressure must be at least 1 bar (100 kPa) during operation, even if it drops below 1 bar during start-up or shutdown of heating appliances. This ensures that an electronic control device can reliably detect faults and correctly measure parameters in the heating system. If there is no pressure, false detection of a flame failure will occur because no electronic measurement can be made until after ignition has taken place and sufficient pressure has built up in the system again.

Resolution Of Viessmann F8 Fault

The F8 fault code can also be caused by an incorrectly installed gas pressure regulator or insufficient combustion air being provided to the boiler.

To resolve this issue:


Ensure combustion air is not contaminated

Check that there is no carbon monoxide (CO) vapor in your home’s air supply and ensure that all appliances are using a fresh air supply. Check that all air intakes are clear of debris and that there are no leaks in the ductwork. If a gas appliance has been repaired or serviced, ensure that it has been appropriately re-certified.


Ensure each boiler has its gas regulator

If you have more than one boiler connected to a single gas supply and regulator, one of them may be causing the problem. In this case, disconnect all but one of the boilers from their respective regulators and see if the fault still occurs. If it does not, then you know that one of them has become contaminated with carbon monoxide and needs servicing. If it continues, there may be an issue with carbon monoxide entering your home from outside sources such as cooking or heating appliances. This can be difficult to diagnose because there are many factors involved; therefore, it is advisable to contact a professional.

The supplied burner insulation wool must be packed around the burner tube in an airtight manner, and the electrodes inside the burner door refractory must be insulated to avoid the formation of condensate on the flame-sensing electrodes.


Should I turn my boiler off if it has a Viessmann F8 Fault?

If you have a fault on your boiler and it’s not an actual fire risk, don’t turn it off! This is because many other parts of your system rely on the boiler being on to work correctly (such as the hot water cylinder).

The best thing to do is to call a qualified engineer. It may be that the fault is due to the boiler being incorrectly installed or incorrectly set up. If this is the case, then it could be that turning off the boiler or resetting it will solve it. But if you have a Viessmann F8 fault and no obvious reason, then it’s best left in the hands of an expert.



The F8 fault code is not generated by the boiler but rather by the boiler control system. The control system monitors all aspects of the operation to ensure no problems. If any issue is detected, the control system will lock out that component until it can be diagnosed and repaired.

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