Vaillant Boiler F72 Fault Code
The Vaillant fault code F72 boiler NTC thermistor problem Q & A or require an engineer for an emergency boiler repair.
The Vaillant Boiler Fault Code F72 Error Common Q&A
Vaillant Boiler F72 Fault Code
- What does the F72 fault code on my Vaillant boiler mean?
- What is a NTC thermistor?
- What are the symptoms of a failing, or failed NTC thermistor?
- What are the most common causes of a NTC thermistor temperature differential?
- Is this a problem that can be fixed on a DIY basis?
- How can a boiler engineer determine the correct resistance reading, if the NTC thermistors are faulty?
- What can be done if the resistance reading is incorrect on the NTC thermistors?
- How much does it cost to replace the NTC thermistors, and how long will it take?
- What can cause an incorrect reading on NTC thermistors, and the F72 fault code to be displayed?
- Is there anything else that can cause damage?
- How will an engineer know if the NTC thermistors are fitted to the wrong pipes?
- What else could cause a F72 fault code to be displayed?
- What needs to be done if limescale has caused the NTC thermistor fault?
What does the F72 fault code on my Vaillant boiler mean
According to Vaillant, the F72 fault code is a NTC thermistor fault.
There is a permanent temperature difference between the flow and return NTC thermistors, causing the F72 error to be displayed.
What is a NTC thermistor
A boiler is controlled by a central electrical unit, the PCB (printed circuit board).
Every part of the boiler gives signals to and receives signals from the PCB.
A few examples might be:
- The air pressure sensor gives a signal that pressures are within working range for the boiler to ignite
- The pump receives a signal to begin circulating hot water
Like these parts, the NTC thermistors give and receive signals. They monitor temperature on the flow pipe (hot water leaving the boiler) and return pipe (water returning the boiler to be reheated).
Essentially, they are temperature measuring devices. As heat rises, resistance measured reduces. As heat decreases, resistance increases.
The PCB converts this into a temperature reading to know if water needs to be heated, or whether to switch the burner off due to water being up to temperature.
What are the symptoms of a failing, or failed NTC thermistor
The F72 fault code is the first sign. But, some newer boilers will display multiple fault codes on the control panel; so, the fault might not be limited to the NTC thermistor.
These temperature control sensors on a boiler dictate when the boiler fires up, and when it shuts down after getting to temperature.
If there’s a fault with the NTC thermistor you might find:
- The boiler shuts off before getting radiators or hot water up to temperature
- On the flip side, hot water burns hotter than it should
- The boiler cycles on and off constantly, failing to get up to temperature
What are the most common causes of a NTC thermistor temperature differential
Faults that cause a temperature differential on the flow, return, or both thermistors include:
- An incorrect resistance reading on the flow and return thermistors
- A wiring issue between the NTC thermistor and the PCB
- Thermistors that have been fitted to the incorrect pipes
Is this a problem that can be fixed on a DIY basis
All work concerning gas, LPG and oil boilers should be completed by a professional, and not just for safety.
Electrical problems with boilers are not easy to diagnose. And a misdiagnosis could mean replacing the wrong parts, only to have the problem persist.
In this case, power needs to be isolated before testing.
How can a boiler engineer determine the correct resistance reading, if the NTC thermistors are faulty
A boiler engineer will be able to determine the correct reading on the flow and return thermistors, which will vary depending on the temperature the boiler is running at.
They’ll need to manually measure the temperature of the flow and return water/pipes.
Knowing the resistance reading they need (in ohms), they’ll use a multi-meter to check this reading.
What can be done if the resistance reading is incorrect on the NTC thermistors
If they can confirm the resistance reading is incorrect and this is limited to a thermistor fault, these will need to be replaced
How much does it cost to replace the NTC thermistors, and how long will it take
Luckily, these parts are not particularly expensive. Depending on your boiler, each thermistor will cost between £10-30.
Typically, you should expect to budget approximately:
- 1 hour for diagnostic
- 3 hours for repair
- Plus parts
Depending on where you are in the country (and labour rates), expect the total cost of repair £150-200.
What can cause an incorrect reading on NTC thermistors, and the F72 fault code to be displayed
Like all internal components in your boiler, the thermistors can degrade over time.
This could be the part itself, or degraded wiring.
Either fault could cause incorrect and intermittent signals to the PCB.
If the PCB doesn’t receive the signals it needs, this is when it will shut off before it should, not heat up hot water/radiators correctly, or overheat them.
And remember, boiler parts (such as the pump and fan) do vibrate.
Over years, this can weaken or loosen wired connections. In some instances, securing these connections is enough for the PCB to get the signals that it needs to operate correctly, and clear the F72 fault.
Is there anything else that can cause damage
The number one problem that causes damage to electrical components is water damage.
There are various parts that can leak in a boiler, but the most common are the:
- Heat exchanger
Some leaks are so small that you won’t notice them. But, due to the internal heat of the boiler, condensation is created and this settles on these electrical components.
This can cause water damage to wiring and affect the signals going to a from the NTC thermistor, causing the F72 fault code to be displayed.
Leaks need to be found and fixed, before moving onto the issue at hand; the thermistors.
How will an engineer know if the NTC thermistors are fitted to the wrong pipes
Although Vaillant states this as a possible cause, it’s highly unlikely.
Each boiler (legally) must be tested. For it to be tested, it must work.
As soon as the PCB receives a signal to show the NTC return thermistor is receiving lower resistance reading (and therefore, a higher temperature), it will cut out.
It’s getting this reading because the return thermistor has been fitted to the flow side (i.e. where hot water leaves your boiler).
There is no device on the boiler to determine which side the NTC thermistors are fitted. So, all it knows is the temperature readings are incorrect.
It will lock out and display a F72 fault code.
What else could cause a F72 fault code to be displayed
A boiler’s fault code system isn’t full proof. Whilst it will attach a relevant fault code where possible, there is the chance:
- Signals mean the fault code displayed, isn’t the most relevant
- There’s no fault code for the specific fault, and the closest fault code is a F72
If there’s another error code displayed, especially one that relates to maximum differential between the flow and return pipe thermistors, it’s likely it’s a different fault entirely.
Boiler parts including thermistors, are prone to having problems generated by limescale build-up.
Above, we’ve mentioned that heat exchangers can leak. This can be due to old age, but typically it’s a build-up of limescale. This leads to hot spots and the heat exchanger eventually cracking.
When the limescale (which comes from mineral deposits in water) attaches itself to NTC thermistors, it can cause temperature sensing issues.
Typically, it will create a hotspot, so the temperature and resistance readings aren’t a true representation of overall water temperature.
What needs to be done if limescale has caused the NTC thermistor fault
The process to clear the fault code is simple:
- Employ a qualified gas engineer
- Isolate the problem to limescale build-up
- Replace the thermistors
- Dose the heating system with inhibitor (to break down limescale)
- Fit a scale reducer (a filter to catch limescale deposits)
- Reset the boiler to clear the F72 fault code
This will help get rid of the limescale in the system, but also stop it from returning.
Other Vaillant Fault Codes And Error
If your looking for the list of error codes relating to the Vaillant boiler then please read the article Vaillant Boiler Fault Codes And Cures