Vaillant Boiler F71 Fault Code
The Vaillant fault code F71 boiler NTC thermistor problem Q & A or require an engineer for an emergency boiler repair.
The Vaillant Boiler Fault Code F71 Error Common Q&A
Vaillant Boiler F71 Fault Code
- What does the F71 fault code mean on my Vaillant boiler?
- Is this problem terminal?
- What is a NTC thermistor?
- How can the NTC thermistors cause the F71 fault code to be displayed on my boiler?
- Can I reset my Vaillant boiler to fix the F71 fault?
- What can be done to fix the problem?
- How does an NTC thermistor become faulty?
- What’s the most common cause of a fault?
- How do we get rid of scale and clear the F71 fault code?
- How do we prevent scale build-up in the future?
- How does water damage occur and cause the F71 fault?
- How will I know if the thermistor is water damaged?
- Will the thermistor need to be repaired, or replaced?
What does the F71 fault code mean on my Vaillant boiler?
According to Vaillant, the F71 fault code will be displayed on your boiler’s control panel when there’s an issue with the NTC thermistor.
The PCB has noticed that on firing up, there’s been no recorded temperature change.
Is this problem terminal?
Your engineer will need to fault find, to figure out the exact problem. But typically, this is not a problem that’s terminal.
Your boiler won’t need to be replaced - the fix is usually simple and cheap.
What is a NTC thermistor?
Your boiler will have two thermistors. One located on the flow side (where hot water exits the boiler), and another on the return (the side where water circulates back into the boiler).
The NTC thermistors are temperature control sensors. They monitor the flow and return water temperature.
They do this by giving off a resistance reading. When transferred to the PCB, the PCB can determine whether the water is hot enough, or needs heating further.
So, when the NTC thermistor is giving off the desired resistance reading, the PCB will tell the gas valve to close. Or, when it’s dropped below desired temperature, it will tell the gas valve to open.
How can the NTC thermistors cause the F71 fault code to be displayed on my boiler?
The F71 fault code focuses mainly on the flow side of your boiler’s pipework.
The error code will be displayed if the flow sensor is:
- Not connected to the main heat exchanger
Can I reset my Vaillant boiler to fix the F71 fault?
The reset function on your boiler is designed to be used after a problem is fixed. It’s not a function used to fix a fault.
Your boiler stops working and displays a fault code to highlight a potential danger to your safety, or to the boiler.
Call an engineer to fix the problem. When the problem is fixed, they’ll reset the boiler for you.
What can be done to fix the problem?
A gas engineer will first need to check that wiring hasn’t become disconnected, and that the NTC thermistor is connected to the main heat exchanger.
To determine this, it’s a simple case of a visual check. If this is the issue, luckily, it’s an easy one to fix. The engineer simply needs to connect the NTC thermistor to the main heat exchanger.
But, they’ll also need to check the state of wiring. Any damaged wiring will cause intermittent signals to the PCB. And, this might mean the thermistors are signalling lack of water temperature, even if that’s not the case.
How does an NTC thermistor become faulty?
There are several things that lead to damaged and faulty NTC thermistors on boilers including:
- Build-up of scale, causing signal issues
- Water damage
- Connections that are worn or damaged
- Damaged wiring
What’s the most common cause of a fault?
The most common cause of a NTC thermistor fault, is scale build-up.
Scale can cause problems with all kinds of boiler parts, and the thermistors are certainly one of the them.
Limescale comes from mineral deposits in water. It can easily attach itself to boiler parts, and sections of pipework. In fact, it causes that much of a problem, that it can stop radiators heating up.
It becomes a real nuisance is when it attaches itself onto or around the thermistor on the flow side of the boiler.
One of two things will happen:
- The water in that area will burn hotter than the rest, causing resistance readings to misrepresent the overall temperature of heating water
- Scale completely blocks an area around the NTC thermistor, so no reading (or weak readings) are given out
In both cases, the boiler is likely to malfunction.
As an example, if scale is inducing higher temperatures around the thermistor, it will signal to the boiler to turn off the supply of gas too early. It thinks the heating water is up to temperature when it’s not.
And that’s going to cause things like
- Luke-warm hot water from taps and showers
- Radiators not getting up to temperature
How do we get rid of scale and clear the F71 fault code?
In most cases, simply replacing the NTC thermistor will solve the problem, as scale is attached to this area only.
But, what’s more important is that future scale build-up is prevented.
How do we prevent scale build-up in the future?
Firstly, it’s worth considering a hot chemical flush. This will flush out any dirt, debris and loose scale in your system.
Next, get a heating engineer to dose the system with inhibitor. This chemical helps to weaken and break off limescale, from parts such as radiators and towel rails. These particles will then flow around your heating system.
Finally, you’ll need a scale reducer fitted.
This is designed to catch all the particles that the heating inhibitor has weakened and broken off. Remember, these filters need to be cleaned out (with an annual boiler service).
How does water damage occur and cause the F71 fault?
If water damage has caused an NTC thermistor fault, there’s a good chance that you’ve noticed your boiler leaking.
Cracked heat exchangers and leaking pumps can cause a range of operational problems. For instance, they’ll drop the pressure on your boiler to a point that it locks out.
More importantly, when this water finds its way into electronic components, it can cause lasting damage. One of the most expensive problems it can cause, is a broken PCB.
However, in this case, it’s likely you’ve avoided an expensive repair and it’s just the NTC thermistor that has gotten damaged.
How will I know if the thermistor is water damaged?
Without electrical testing equipment, it’s hard to determine whether the fault comes from the wiring, or the NTC thermistor itself. In fact, the signal from the NTC might not be related to that part at all; it could be the PCB.
And, that’s why we always suggest using a Gas Safe engineer to fix the problem. Fixing boilers isn’t cheap, and diagnosing the wrong part could lead to an expensive repair bill, and the problem not being fixed.
Will the thermistor need to be repaired, or replaced?
There is little point trying to repair a faulty NTC thermistor, especially when water damage is the issue. There’s no telling how long it will last.
What’s more important is:
- Determining if other parts in the boiler have been subject to water damage
- Stopping the leak that’s caused boiler damage in the first place
The NTC thermistors are not internal boiler parts. So, if they’ve gotten water damaged, there’s a good chance that internal parts have been exposed too. It’s important to have an engineer check over the internal electrics and their condition.
Once they’ve done that, the next step is to isolate the boiler leak.
Typically, something like a cracked heat exchanger is going to need to be replaced. However, a pump will usually leak because of faulty pump seals.
These can be replaced to fix the pump, rather than installing a new unit.
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