Vaillant Boiler F72 Fault – Error Code Guide

The Vaillant F72 error code affects the Ecotec Pro, Ecotec Plus and Ecotec Exclusive models of Vaillant boiler. It means that there is a fault with the flow and return thermistors.

Either the parts themselves are faulty, or communication between the thermistors and PCB is inconsistent due to faulty or damaged wiring.

If the thermistors are faulty, then these will need replacing but if not, an engineer should be able to resolve this issue without needing to replace any parts.

What Does A Vaillant F72 Error Mean?

The F72 fault code shows when there is a fault with the flow and/or return NTC thermistor. It means that the thermistors are providing incorrect readings and that the difference between the two thermistors is consistently too great, causing the boiler to shut off.

Thermistors are responsible for monitoring the temperature of the water leaving the boiler via the flow pipe and re-entering the boiler through the return pipe.

NTC thermistors work by altering their resistance in response to temperature; the resistance decreases at higher temperatures and increases at lower temperatures (meaning that the higher the temperature the more electrical current can pass through).

The PCB, the boiler’s control panel, converts these resistance readings into temperature measurements and signals whether the water needs to be heated or if the temperature is correct.

When the thermistors are not providing an accurate resistance reading, this prevents the boiler from regulating the temperature of the water. 

Essentially, this error means that the boiler thinks the water entering the system is too hot when in reality it has not reached full temperature. This causes the boiler to stop heating water before it should and leaves you without heating or hot water.

What Causes A Vaillant F72 Error?

The symptoms of an F72 error with your boiler might include the boiler repeatedly shutting off before the central heating or hot taps reach full temperature, that your hot water is too hot, or that the boiler repeatedly shuts on and off.

There are three common causes of the F72 error code.

Faulty Thermistors 

The flow NTC thermistor and/or the return NTC thermistor are faulty and giving incorrect resistance readings. 

They could have been defective to begin with or, if you have had your boiler for a few years, the parts may have degraded over time. 

Either way, this would cause incorrect and inconsistent communication with the PCB, triggering the F72 code.

Wiring Issue

There could be a wiring issue between one or both thermistors and the PCB. The wires may have degraded over time, but boilers are filled with vibrating parts which, over time, can simply cause loosened connections.

Degraded or loose wiring will mean that the PCB is not receiving the necessary information from the thermistors. 

This would result in inconsistent communication between the parts.

Installation Error

When the boiler was installed, the flow and return thermistors may have been connected to the wrong pipes. 

Although this is a possible cause, it is highly unlikely because boilers are tested as part of the installation process and this kind of error would have caused the boiler to shut off, so it would have been caught and remedied immediately

How to Fix a Vaillant F72 Error

If the problem is that the thermistors have been connected to the wrong pipes, this will most likely be noticed during the installation testing process and resolved immediately.

If loose wiring and connections are causing the issue, then an engineer will be able to tighten up connections and the issue will be resolved. However, if the wiring is degraded or faulty then it may need replacing.

In order to test the thermistors themselves, an engineer will manually measure the temperature in the flow and return pipes to see if this matches the reading given by the thermistors.

If the engineer discovers that the thermistors are providing incorrect readings, then the thermistors are faulty and will need replacing.

Can You Fix A Vaillant F72 Error Yourself?

No. In the case of an F72, power needs to be isolated before the parts are tested and this should be done by a professional.

Electrical faults are tricky to diagnose, and attempting to resolve such an issue without the appropriate accreditation is not only unsafe but could result in a costly misdiagnosis and possibly damage to your boiler.

A registered Gas Safe Engineer should be called to resolve this problem.

What Else Can Cause An F72 Error to Show?

As with most boiler error codes, sometimes the code shown is not an exact diagnosis but the closest code the boiler knows to show – so sometimes, the thermistors may not be the cause of the issue at all.

Another possible cause of the problem is limescale build-up, which can cause parts to degrade and incorrect temperature readings. 

Limescale build-up can be resolved by a professional, who will dose the system with a limescale inhibitor as well as fitting a scale reducer to prevent future build-up.

Do I Need to Reset my Vaillant Boiler After An F72?

Once the fault has been resolved, the boiler will need resetting to get back up and running again. The engineer who dealt with the issue will usually do this for you.

How Much Does A Thermistor Replacement Cost?

Fortunately, thermistors are among the cheaper parts of a boiler and will cost between £10 and £30 to replace. 

However the replacement can be a little time consuming, and including labour costs, the replacement will cost £150-£200.

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