Vaillant Boiler F20 Fault Code

The Vaillant fault code F20 boiler over heating Q & A or require an engineer for an emergency boiler repair.

The Vaillant Boiler Fault Code F20 Error Common Q&A

Just below, we have a list of common questions relating to the F28 fault code on the Vaillant boiler.

What does the Vaillant boiler F20 fault code mean?
How do I reset my boiler after the F20 error has appeared?
Why does my boiler lockout and show a F20 fault code?
What are the most common causes of a F20 fault code appearing?
Can I fix the F20 boiler fault on a DIY basis?
My boiler goes off after 20-30 minutes every time it fires up, what’s the problem?
How do I fix the fan and the F20 fault?
The boiler pipes aren’t hot, but my boiler still locks out and displays the F20 fault code?
My boiler pump is hot, is that causing the F20 fault on my Vaillant?
My boiler sounds like a kettle that’s boiling and whistling, what’s the issue?
The PRV was leaking. It’s now been removed or blocked to stop the leak. Could this be the problem?
The kettling noise sounds like it could be coming from the pipes to the boiler, not the actual boiler?
Are there any precautions I can take to protect my boiler and stop the F20 error appearing in the future?

What does the Vaillant boiler F20 fault code mean

According to Vaillant, the F20 fault code is displayed when a boiler overheats.

Vaillant boilers have a safety mechanism called a lockout. This is a fail-safe designed to kick in when the boiler doesn’t work within its operating range.

Problems that can cause a lockout include low pressure, high pressure, system blockages and the system overheating.

In the case of the F20 error code, the boiler’s thermistors have detected an overheat.

Water has been measured at above 97 degrees, which is outside of the operating range of a Vaillant boiler.

How do I reset my boiler after the F20 error has appeared

Most newer boilers will have a reset (or ignition) button on the front of the display panel.

For example, the EcoTec has a small button that looks like a flame with a cross through it. Pressing and holding this will reset the boiler and allow it to reignite.

As this is an overheat issue, allowing the boiler time to cool down, and then resetting it will allow the boiler to fire up again.

However, you should have fixed the problem that’s caused the F20 error on your Vaillant boiler before doing this.

Otherwise, it will lockout again.

Why does my boiler lockout and show a F20 fault code

A fail safe like a boiler lockout is designed with two things in mind:

Protect any internal parts of the boiler from being exposed to damage (in this case, frying themselves)
Protecting the inhabitants of a property from an unsafe boiler (i.e. you and your family)

You shouldn’t risk frying expensive parts in your boiler, or your own safety.

Get the issue causing the fault inspected by a qualified engineer, before resetting the boiler.

What are the most common causes of a F20 fault code appearing

Vaillant boilers that are overheating and causing a boiler lockout could have any one (or more) of the following problems:

Broken fan
Faulty thermistor
Heat exchanger Kettling
Bodged (pressure release valve) PRV's
Limescale hotspots near the thermistor

Can I fix the F20 boiler fault on a DIY basis

Gas appliances shouldn’t be worked on by an untrained professional, so call an emergency gas engineer.

More to the point, some issues that relate to a boiler lockout, overheating and a F20 error are caused by “bodge jobs” by people that have tried to fix the problem.

The knock-on effect of incorrectly diagnosing and fixing a boiler overheating, could mean that expensive parts are ruined, such as the heat exchanger.

My boiler goes off after 20-30 minutes every time it fires up, what’s the problem

Due to the amount of time it takes for the boiler to overheat in this case, it sounds most likely to be a faulty fan.

The newer your boiler, the quicker it’s likely to overheat when there’s a fault causing an F20 error, especially when its fan related.

For instance, a brand-new boiler will overheat within 10-20 minutes, with older ones taking 20-30 minutes.

That’s because newer boilers are more energy efficient (A-rated boilers are over 89% efficient), so they get to temperature quicker.

The thermistor will monitor the flow and return pipe temperature.

If it gets too hot, it communicates with the boilers computer (PCB) to switch off the heat, and activate the fan to cool down the boiler’s internal components.

If your fan is broken, the water will cool naturally (over time), but without the fan to keep internal component temperature down, the boiler will lockout.

How do I fix the fan and the F20 fault

Call an engineer to diagnose the exact problem (i.e. the fan).

Fans are expensive, so replacing it without being certain would be a big mistake.

And if you’re lucky, the fan’s components can be replaced to fix the fault, rather than the entire fan.

The boiler pipes aren’t hot, but my boiler still locks out and displays the F20 fault code

The boiler doesn’t have to overheat, for it to think it has.

As mentioned, the thermistors on a boiler monitor the heat on the flow and return of the boiler.

Once heat is below the desired level, they tell the boiler to switch on.

Once heat is above the desired level (or the boiler is overheating), they tell the boiler to switch off.

A faulty thermistor could lead to a boiler not firing at all, or not completely getting to temperature.

Essentially, it’s not monitoring temperature correctly, and shutting off your boiler.

It thinks it’s hotter than it is.

My boiler pump is hot, is that causing the F20 fault on my Vaillant

If your pump is hot to the touch, it’s usually because internal parts have seized.

Whilst this can be fixed by cleaning out the pump, the route of the problem needs to be found.

It’s likely to be:

An old pump that needs replacing
A pump full of dirt, because the heating system hasn’t been protected with a boiler filter and central heating inhibitor

The F20 fault can be caused by a faulty pump.

But, a faulty pump is more likely to relate to an “inadequate circulation” fault such as the F24.

My boiler sounds like a kettle that’s boiling and whistling, what’s the issue

Boiler kettling is more common than you might think. It makes boilers incredibly noisy, and malfunction at the same time.

In hard water areas (areas with high mineral content), limescale is a problem.

This is a build-up of minerals that attaches itself just about everywhere.

It’s common for the limescale to create hotspots on the heat exchanger.

This will mean that the water is over-heated, sometimes to a point where it’s so hot, it makes kettling and whistling noises.

There are two real problems here.

The first, is that this is going to cause the thermistor on the flow pipe to detect temperatures that cause a lockout and a F20 error on your Vaillant.

The bigger problem is the fact that kettling is happening in the first place.

The heat exchanger is one of the most important (and expensive) parts in a boiler.

If it’s kettling, the hotspots will then create weak spots. That in turn, can cause the heat exchanger to crack.

There’s no fix for a cracked heat exchanger; it must be replaced. And, this will cost hundreds of pounds.

So, if you think your boiler is kettling, it would be wise to turn it straight off, and call in an engineer.

Hopefully, you’ll have caught it in time, and won’t have to replace the heat exchanger.

The PRV was leaking. It’s now been removed or blocked to stop the leak. Could this be the problem

A boiler’s computer will display what it thinks is the most relevant fault code for the issue.

If a PRV has been removed or blocked to stop a leak (which is not the correct fix anyway), there’s a chance the increased pressure could cause a F20 fault code to appear.

Some Vaillant boilers don’t have a “pressure too high” related fault code.

As increased (rather than just generally high) pressure (causing the PRV to kick in) relates to increases in temperature of water, causing parts to expand, this could be the problem.

The fix is simple. The PRV needs to be installed correctly, or replaced.

The kettling noise sounds like it could be coming from the pipes to the boiler, not the actual boiler

Limescale can get everywhere, not just onto the heat exchanger.

You might hear small kettling sounds on the pipes that are connected to the boiler.

If limescale has built up here, just like the heat exchanger, this will cause a whistling or kettle type sounds.

Like the heat exchanger, the build-up of limescale will create hotspots.

If these are near the thermistor, it could detect temperatures that are not representative of the overall flow and return of the system.

Basically, it’s much hotter near the thermistor, than the water is overall. The thermistor then assumes all the water in the system is too hot. It will lockout and display a F20 fault.

The boiler will cool down, be reset, and the same problem will occur.

The fix is to detect the pipework that has hotspots, replace them, and replace the thermistor in case this has also got a limescale hotspot on it too.

Are there any precautions I can take to protect my boiler and stop the F20 error appearing in the future

Get an annual boiler service. An engineer will notice important things like a noisy fan (that could stop working), and a kettling heat exchanger (that’s very costly to fix).

If you are, consider fitting a limescale reducer (also known as a scale reducer). This will help to catch scale, before it settles on boiler parts and creates limescale hotspots.

Check if you are in a medium-high water area by searching 'Hard water map by Bristan.