Viessmann F5 Fault (Problem With The Gas Pressure)

February 15, 2023

A Viessmann F5 fault code indicates a problem with the gas pressure, and the system would not function correctly if left unfixed. Many other codes, such as a "low or high outdoor temperature," can be set, including a fault code which only means there was a power failure during storage of the heating system. This can occur when there is an interruption of the power supply to an appliance whose control system was not able to switch itself off and on again at start-up / shut-down automatically. When this occurs, the control system in an F5 display will most often show ”FF” with F5 as the individual fault code. This article will focus on the possible causes of the Viessmann F5 Fault.

What does the F5 fault code mean on my Viessmann boiler?

The F5 fault code is a problem with the gas pressure. The problem can be that there is too high a pressure in the system, which causes the boiler to switch off automatically. This can be caused by a blocked orifice, a clogged condensate drain line, or if the boiler has been switched off and the pressure has not yet been relieved. The most common cause of overpressure is a blocked orifice, which is an opening on your system that helps regulate gas flow into your boiler. If it's blocked (for example, with limescale), it will cause overpressure in your system, which will make your boiler shut down to protect itself.


The possible errors causing the Viessmann F5 Fault

The Viessmann F5 fault code (problem with the gas pressure) is displayed on the control board of your hot water system if there is a problem with the gas pressure. The possible causes for this error are listed below:


Limescale build-up

Suppose your system contains limescale deposits in the heat exchanger. In that case, it can cause an imbalance between incoming and outgoing gas pressures, resulting in a fault code being displayed on your control board. To resolve this issue, strip all components from your system, and reassemble them in their original position. When you finish, run a hot water cycle to clear any remaining limescale deposits from the system before attempting to start up again.

Defective or disconnected wiring

It is also possible that there is a problem with the wiring of your heating system, which will also cause a lack of pressure and, therefore, an F5 error code. Check all of the wires and make sure they are connected securely.

Airway blockages

If air gets into your system, it can cause various problems with your boiler. Air can enter via the airways in the radiators themselves; if this happens, you'll see steam coming out of them when you open up the radiator valves. It can also get in through any damaged pipework or joints, where there may be a small gap or crack in the system, which allows air to enter and mix with the water in your pipes. This is likely to cause back pressure inside your boiler and may even prevent it from operating.

Faulty pressure release valve (PRV)

If the pressure release valve is faulty or not working correctly, it will cause an overheating problem. The PRV releases excess pressure from your system to prevent damage to your boiler or heating system. In this case, it may be time to replace your PRV.

Pump failure

The first thing to check is whether your gas boiler has failed. The most common reason for this fault is that there is no gas pressure coming into the boiler, so you need to check that there is gas coming into your home and that the gas pressure isn't too low. You can do this by checking your meter when it's on standby (meaning no heating) and making sure that it reads at least 3 bars. If you have high pressure, try turning off all appliances in the house, including any lights or electrical devices, and make sure you don't use any water at all during this time. If, after 10 minutes, it still doesn't read 3 bar, then there is something wrong with your boiler!

Failed thermistor

If your gas pressure is ok, but you're still having problems with this fault, then it's possible that there could be a problem with your thermistor. This prevents air from getting into the system when the unit isn't running and stops any air from being introduced into your boiler.


Low refrigerant level

If you have a gas-fired boiler, it uses a certain amount of gas to heat water. When this gas runs out, it will automatically shut down until an engineer replaces it. If your boiler has run out of gas, you will need to call an engineer to refill it with more.


Can my gas valve be repaired?

The gas valve is the most common part of a boiler to fail, and it is usually due to overheating.

If your boiler has failed due to the gas valve, it can be repaired, and this can be done by replacing the complete valve or just replacing the faulty parts.

Replacing the complete valve is cheaper, but you will need to find an engineer who has worked on this type of boiler before. If they don't have experience with this model, they may not be able to repair it properly and could cause further damage as a result, so be keen to pick an experienced engineer.

How to improve your boiler’s life

A boiler is an essential component of your home's heating system, and it warms the water in your home, which then heats the radiators or other heating systems throughout your house.

It's not uncommon for boilers to have problems, especially if they're old. Many boilers suffer from overheating and pressure issues and require repairs or replacement. If you want to prevent this from happening, here are a few tips:

Is it safe to reset a boiler?

Resetting a boiler is a simple process that anyone can do. The process involves opening the valve and allowing water to drain from the system. Once the water has drained out of the system, you close the valve and turn on your heating system as usual.

You should always consult a professional if you have any doubts about resetting your boiler, but for most boilers, there isn't anything to worry about. Is it safe to reset a boiler? The short answer is yes, but you need to get some advice before doing it yourself.


Increase Boiler Efficiency: Lower the Stack Temperature

The stack temperature refers to the temperature at which water vapor is released from your boilers into the atmosphere. The higher this temperature, the more money you spend on fuel bills. To lower this temperature, install an economizer or add insulation around your hot water tank or pipework. Reducing heat loss will increase the efficiency of your system efficiency and save money!


Install an Economizer

An economizer is a device that uses ventilation air instead of combustion air when it is cold outside. This helps reduce fuel consumption and emissions, which means it will save you money on your heating bills and prolong the life of your boiler.


Tune the Burner Regularly

The burners are the part of your boiler that burns fuel such as natural gas or oil to heat water in your home. If these burners are not tuned properly, they will not work as efficiently as possible, and this can lead to an overheated boiler which may cause damage to other parts of your system and even break down completely. By checking or tuning your burners regularly, you will reduce the chance of any problems occurring later on due to poor combustion efficiency.


Install a Variable Frequency Drive

A variable frequency drive (VFD) is an electronic device that controls how fast your boiler’s burner turns on and off. A VFD can help prevent overheating or overfiring by adjusting how quickly or slowly the burner cycles on and off. This allows you to save money on heating bills while improving the life expectancy of your boiler by preventing damage caused by overheating or overfiring.



To get to the bottom of the problem, we first need to decide if it is indeed a fault with the conventional boiler and not with the system as a whole. Generally speaking, a faulty conventional boiler can lead to problems with other parts of your central heating. A common cause of this may be signs of water leaks within the equipment or lack of sufficient pressure in gas supply pipes. In this case, a technician will need to disconnect your boiler from the gas supply and use an infrared camera to view defects and faults (if any) on internal surfaces.

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