Vaillant Boiler F84 Fault – Error Code Guide

The Vaillant F84 error code affects the ecoTec plus models of Vaillant boiler.

The F84 error code means that the boiler has detected a permanent temperature difference between the flow and return pipes.

This can be caused by wrongly installed or calibrated thermistors, water damage and leaks or faulty wiring. You won’t be able to fix this error yourself, and it may result in parts needing replacing.

What does a Vaillant F84 Error mean?

An F84 error means that the boiler has detected a permanent temperature difference between the flow and return NTC thermistors.

Thermistors monitor the temperature of the water leaving and re-entering the boiler via the flow and return pipes.

When the boiler is operating correctly, the return temperature will be lower than the flow temperature as some heat is absorbed by the heating system, and the difference between the two readings will fluctuate during the boiler’s operation.

When the temperature reading between the two thermistors remains at a permanent difference, then this will trigger the F84 error.

What causes a Vaillant F84 error?

Incorrect installation of thermistors

Although very uncommon, it’s possible that the thermistors are giving incorrect readings because they have been installed on the wrong pipes. If this is the case, it is likely to have been a manufacturing issue.

Incorrect calibration of thermistors

If the NTC thermistors of the flow and return pipes are incorrectly calibrated, then this can cause them to give incorrect resistance readings. 

Incorrect calibration can be the result of age or of water damage, which would signify a leak within the boiler.

Damaged or faulty wiring

If the wiring connecting the thermistors with the PCB is loose or damaged then this means the thermistors cannot communicate the necessary data for the boiler to operate, or they can only do so intermittently.

Faulty wiring can be caused by age and wear and tear or become loose over time because of the vibrations produced by the boiler’s moving parts.

Heat exchanger leak

Heat exchangers are prone to limescale build up, which creates a ‘hot spot’ within the component, eventually leading to it cracking and leaking.

The resulting leak could make its way to the thermistors and PCB, resulting in water damage which causes incorrect readings and could cause the thermistors and PCB to read the permanent temperature difference between the flow and return thermistors.

PCB fault – water damage

Especially if you have noticed water beneath your boiler or other signs of a leak, your PCB needs to be tested when this error code occurs. 

Even when the thermistors have been determined as the fault, a communication issue could also be down to an error with the PCB, which as another electrical component could have experienced water damage. 

Although it is unlikely, a damaged or faulty PCB may lead you to consider investing in a new boiler as they are very expensive to replace.

How to Fix a Vaillant F84 error

Thermistors can be easily replaced if they need to be, and the parts alone are inexpensive – depending on the model, they can cost as little as £30 each.

It’s important that all wiring surrounding these parts is also tested for faults or damage because new thermistors don’t come with fresh wires – if the wiring is the problem, then replacing the thermistor won’t resolve the problem. 

Additionally, if the thermistors are working incorrectly due to water damage, then a Gas Safe Engineer must test the PCB to determine whether it has also been damaged – otherwise, you may end up paying for replacement parts and repairs only to find that your boiler runs into more problems later due to a faulty PCB.

Similarly, if the problem is due to water damage and the cause of the leak is identified as being the heat exchanger, then this becomes a much more expensive problem to fix as the heat exchanger will also need replacing.

However, a problem this severe will usually only occur when your boiler has reached old age, and it’s probably time to start thinking about a new one.

Can you fix a Vaillant F84 error yourself?

You will not be able to diagnose or resolve an F84 on your own.

The presence of a leak can contribute to the F84 error code, and you may be able to identify this problem yourself if there is a low-pressure reading or even water collecting beneath your boiler. 

You can combat the effects of the leak itself by repressurising the boiler and bleeding radiators and towel rails, but this will not fix the F84 error code.

Does an F84 mean my boiler is broken?

This error code does not mean that your boiler is broken. It can frequently be fixed by a Gas Safe Engineer repairing the wiring or replacing the thermistors.

Do I need to reset my Vaillant boiler after an F84?

A Gas Safe Engineer will reset your boiler for you after resolving the issue.

Constantly resetting the boiler to clear the error code and then using it until it locks out again can actually cause damage to your boiler and will not resolve the issue.

Instead, when you see this error code you should call a Gas Safe Engineer to resolve it for you.

How much will fixing an F84 cost me?

Fixing an F84 may require parts replacement of the NTC Thermistors and/or the surrounding wiring. In terms of parts alone, this isn’t very expensive.

The cost of labour increases the price up to around £150, but this is one of the cheaper errors to repair.

Will fixing leaks resolve the issue?

Although a leak within the boiler can contribute to the F84 error code showing, the resulting damage caused to the internal parts is what actually triggers the code rather than the loss of water pressure itself.

This means that repressurising the boiler and bleeding radiators and towel rails will not help to clear this error code.

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