F84 Vaillant fault code causes, meaning and how to fix it
In the UK the Vaillant F84 error code is for ecoTec plus models.
F84 Vaillant error code is one that indicates that the boiler detected a permanent temperature difference between flow and return pipes.
These can be caused by incorrectly calibrated or installed thermistors, water damage, leaks, or faulty wiring. As with most Vaillant boiler fault codes, this boiler fault is not something you can fix yourself and may require parts to be replaced.
Read our guide to how much a new Vaillant boiler costs here for more information.
What does the Vaillant F84 error mean?
An F84 error indicates that the boiler detected a permanent temperature difference between flow and return NTC thermistors. These temperature sensors ensure that your boiler is working properly and not hover heating.
The boiler temperature is monitored by the thermistors via its flow and return pipes. When the boiler is working properly, the return temperature will be lower that the flow temperature because some heat is absorbed into the heating system. This difference will fluctuate throughout the boiler’s operation and if it is not working properly this is when you will see one of the Vaillant fault codes.
If the temperature difference between the two thermistors is not corrected, the Vaillant boiler F84 error will be triggered.
What are the NTC thermistors for my Vaillant boiler?
The NTC thermistors monitor the temperature of water in your heating system. One is connected to the flow, and one to the return.
The boiler’s water temperature will be monitored by the flow thermistor. If the water temperature is not at the right temperature, the PCB (printed circuit boards) will tell the gas valve to open and provide more fuel.
The boiler’s return thermistor monitors water temperature. This temperature will be lower than the flow temperature because radiators and pipework absorb heat.
The thermistors do not work like standard temperature gauges. They measure resistance in ohms. They should read 12k ohms at 20C
What are the most frequent Vaillant faults in NTC thermistors?
In the case of the F84 fault code, there are specific thermistor faults to look for:
- Failure of wiring or connections
- Faulty NTC thermostors (replacement required)
- Incorrect installation NTC thermistors
- PCB detection fault
- Limescale build-up
What causes Vaillant boiler wiring and connections to go stale?
Age of boiler components has a lot to do with faults.
Vibrations, in addition to wiring that has become faulty over time are common causes of faults. Small vibrations can be caused by the boiler’s many moving parts (such as fan and pump).
These small vibrations can over time help loosen connections. The signals that these vibrations provide to the PCB are intermittent if the connections become weak.
The circuit board might receive signals occasionally, while others may not. Even more important, signals could be wrong.
This could lead to intermittent boiler operation. This could lead to intermittent boiler operation.
- Hot water from taps, showers, or baths does not reach the right temperature
- Boiler turns off before radiators reach temperature
- Before locking out, the boiler turns on and off continuously and doesn’t heat up either hot water or radiators.
What is the cause of a Vaillant F84 error?
Incorrect Installation of Thermistors
It is possible, although it is rare, that the thermistors are not giving the correct readings due to incorrect installation. This could have been an issue with manufacturing.
Incorrect calibration for thermistors
Incorrect calibration of NTC thermistors for the flow or return pipes can lead to incorrect resistance readings.
An incorrect calibration could be due to age or water damage. This would indicate a boiler leak.
Broken or defective wiring
If the wiring connecting thermistors to the PCB is damaged or loose, then the thermistors can’t communicate the required data to the boiler. Or they may only be able to do this intermittently.
Faulty wiring may be due to wear and tear, or can become loose from the vibrations generated by the boiler’s moving parts.
Heat exchanger leak
Limescale buildup in heat exchangers can lead to cracking and leaking. You will get the F84 fault code if this leak affects the thermistor. You may get other fault codes first too hopefully before this happens.
A heat exchanger leak could result in water getting into the thermistors or PCB. This could lead to inaccurate readings at the internal pressure measuring point.
PCB fault – water damage
If you notice water under your boiler, or any other signs of a leakage, it is important to test your PCB when this error code appears.
Even if the fault is determined to be with the thermistors, it could still be due to a communication problem. This could also be caused by an error in the PCB which, as an electrical component, could have suffered water damage.
Even though it’s unlikely, a damaged or defective PCB could be a reason to invest in a new boiler. They are extremely expensive to replace.
How do you fix a Vaillant ecoTEC F84 fault
If they are damaged or worn out, the parts can be replaced easily.
Because new thermistors are not equipped with fresh wires, it is important to test all wiring around these parts for any faults or damage. If the wiring is the problem then you will need to replace the thermistor.
If the thermistors fail to work correctly due to water damage, a Gas Safe Engineer must inspect the PCB. Otherwise, you could end up having to pay for repairs and replacement parts only to discover that your boiler is experiencing more problems due to a faulty or defective PCB.
If the problem is caused by water damage, and the source of the leak is the heat exchanger then it will be much more costly to fix. The heat exchanger will also have to be replaced. A problem of this severity will typically only develop when the boiler is old and needs to be replaced.
Can you fix a Vaillant F84 error yourself?
The Vaillant F84 fault will not be diagnosed or solved by you on your own.
A leak could be the cause of F84 error code. You may be able identify this problem by taking a reading at low pressure or water collecting under your boiler.
Although you can reduce the damage caused by the leak by turning off the boiler, flushing radiators and towel rails and repressurizing the boiler, this will not resolve the F84 error code.
Is an F84 a sign that my Vaillant boiler is damaged?
The error code does NOT mean your boiler is damaged. This error code can often be fixed by a Gas Safe Engineer who will repair or replace the thermistors. It would likely be worth getting the Gas Safe registered engineer to check other parts such as the water pressure sensor, heat exchanger, temperature sensors and gas supply pressure.
After an F84 fault, do I need to reset my Vaillant boiler?
After resolving the problem, a Gas Safe Engineer will reset your boiler. The Vaillant boiler can be damaged if it is constantly reset to clear an error code. You should instead call a Gas Safe Engineer if you see the error code.
What will it cost to fix an F84?
Repairing an F84 might require the replacement of NTC Thermistors or the wiring. This is a relatively inexpensive repair in terms of the parts. Although labour costs can increase the cost up to £200, this is one of the most affordable errors to fix.
Can fixing leaks solve the problem?
Limescale can make a primary heat exchanger defective and then a boiler leak can cause the F84 error code to appear. However, it is the damage to the boiler’s internal parts that triggers the code and not the loss of water pressure. This means that the F84 error code cannot be cleared by repressurizing the boiler or bleeding radiators or towel rails.
Does F84 on a Vaillant boiler mean I need a new one?
If you have an old Vaillant boiler and you are starting to see various fault codes, then it could me the start of the end. Fixing a boiler fault code could mean that there are further issues, especially if there has been a limescale build as it will not be limited to one specific part. If your Vaillant ecoTEC boiler is over 10 years old we strongly suggest getting a new boiler.
