Vaillant Boiler F23 Fault Code
The Vaillant fault code F23 boiler stats Q & A or require an engineer for an emergency boiler repair.
The Vaillant Boiler Fault Code F23 Error Common Q&A
Just below, we have a list of common questions relating to the F23 fault code on the Vaillant boiler.
- What does the Vaillant F23 fault code on my boiler mean?
- Is the F23 fault the same as a boiler overheating?
- Should the flow and return pipes on my boiler be the same temperature?
- What causes the F23 fault code to be displayed?
- How can the pump create a F23 fault?
- We didn’t have problems before, haven’t had a new pump, and now the pump is hot, is this the problem?
- How do I know if I have an airlock in my boiler?
- My pump’s bleed screw is on the top, is that a problem?
- How do I know if I have a blockage in my boiler?
- How do I get rid of an airlock in my heating system?
- My boiler keeps making whistling noises, is this the problem causing the F23 fault?
- What are boiler thermistors?
- How do I check if the thermistors are faulty?
- I don’t have a multi-meter, now what?
What does the Vaillant F23 fault code on my boiler mean
According Vaillant, the F23 fault code relates to a differential in temperature between the flow (hot water going out of your boiler), and return (water circulating back to the boiler to be re-heated).
There are various scenarios where a boiler will display a fault code and lockout, due to the conditions not being within the boiler’s working capacity (or at least, it’s safe working capacity).
If your boiler is showing a F23 fault code on the display panel, it’s because the temperature difference between the flow and return pipes has reached (or exceeded) the maximum of 35 degrees.
Is the F23 fault the same as a boiler overheating
The problems, symptoms and fixes are similar, but not completely the same.
If your boiler is overheating, it’s likely you’ll be shown a different code on the display panel, such as F20.
Should the flow and return pipes on my boiler be the same temperature
The flow and return pipes on a boiler will always differ in temperature.
The difference will be most noticeable when you first fire the boiler up.
The flow will be extremely hot (within a matter of minutes), but the return will be cold or Luke-warm.
As the entire heating system comes up to temperature (i.e. your radiators start getting hot), the difference in temperature between the two will begin equalising.
The reason is simple. The water from the boiler flow needs to travel through pipework and radiators.
When you first switch on your heating, pipework and radiators are stone-cold. So, they absorb the heat from the flowing water.
As they heat up, they need to absorb less and less of the heat to stay at temperature, so the difference in temperature between the flow and return decreases.
What causes the F23 fault code to be displayed
The F23 fault code isn’t an easy one to diagnose, as it can relate to more than one issue.
Issues causing a F23 fault include a:
- Heating pump that isn't working correctly
- Blockage or airlock in the boiler
- Blockage or air lock in your heating system (for instance, radiators or towel rails)
- Heat exchanger fault
- Faulty boiler thermistor
How can the pump create a F23 fault
A pump fault is likely on large heating systems, in large properties, with lots of radiators and towel rails.
As we’ve already mentioned, radiators, towel rails and pipework will drain heat from the flow water.
This should happen less and less as the heating comes to your desired temperature.
But, the pump’s flow rate will affect how quickly water is pumped around the system.
If the flow rate is set to low (or the pump’s maximum flow rate is too small for your system), the water won’t be pumped around quick enough, and will keep losing temperature.
In this case, the pump’s flow rate needs to be adjusted.
On Grundfos pumps, there are 3 flow settings.
For large properties, it’s likely the fastest flow setting needs to be used.
For smaller properties (such as 1-3-bedroom apartments), the lowest setting will be fine.
If you’ve had a new pump fitted, there’s a good chance the flow rate specs from the pump aren’t high enough, or the flow rate setting isn’t correct.
We didn’t have problems before, haven’t had a new pump, and now the pump is hot, is this the problem
If the pump is completely non-operational, it’s unlikely to cause the F23 fault.
For this fault to occur, there must be a temperature difference (that’s large) between the flow and return.
A non-operational pump will lead to heating not working altogether (and therefore, cold flow and return, so no differential in temperature).
If your pump is hot, or making noises, internal parts could be seized or malfunctioning. The pump may still be working (slightly).
Any malfunctioning part can lead to a flow rate that’s below what is considered optimal.
That means the water isn’t pumped around the system quick enough, and has time to cool, creating the differential in temperature by the time it reaches the return pipe.
How do I know if I have an airlock in my boiler
The most common place to get a get an airlock in a boiler, is the pump.
If the pump is airlocked, it’s likely it makes the odd banging or tapping sound.
Most newer pumps have a bleed screw integrated into the pump. You’ll notice this on the side.
This can be opened slightly to let air escape, and then closed when water starts dribbling out. That’s a sign all the air has disappeared.
My pump’s bleed screw is on the top, is that a problem
A pump’s shaft needs to be horizontal. If it’s not, it will put excess pressure on the shaft’s bearing.
This means it will wear out before it should, be noisy and get airlocked. With the pump shaft horizontal, the bleed screw should be on the side.
If your pump shaft isn’t horizontal, it needs to be re-installed correctly.
How do I know if I have a blockage in my boiler
A blockage can also contribute to the F23 fault code being displayed.
Typically, actual blockages come in the form of central heating sludge (rust from radiators and pipes).
This can find its way into the pump and heat exchanger. It’s hard to know if sludge is in the system, without taking the components apart and inspecting them.
However, there’s never any harm in cleaning internal boiler parts.
How do I get rid of an airlock in my heating system
You can bleed all radiators and towel radiators using a radiator bleed key.
Like the heating pump, you’ll open the valve slowly until you hear hissing. Don’t open it any more.
Once the hissing stops, you’ll see water start to dribble from the radiator or towel rail.
This signifies that air has been released from the system, and you can close the bleed valve.
My boiler keeps making whistling noises, is this the problem causing the F23 fault
If you can hear whistling or kettling noises, there’s a fine line between this being related to an F23, and an F20 fault.
The noises you are hearing are due to limescale build-up. Usually, limescale finds its way onto the heat exchanger.
Where limescale settles, hotspots are created.
In a way, it’s like how oil residue on skin that touches a lightbulb, can create a hotspot and cause a lightbulb to break.
The difference, is a heat exchanger isn’t cheap to replace.
As the kettling noises are down to water being heated (in spots) more than it should be, this can lead to flow water being too hot.
But if it’s above 97 degrees, the boiler will lockout and display a F20 fault, not the F23.
For the F23 to be caused by a heat exchanger, the water must leave the boiler below 97 degrees, but come back via the return, at 35 degrees lower than that.
So, whilst you may have a kettling problem, it’s likely there’s another problem too.
An example might be a circulation fault with the pump (mentioned above).
The simple fact is, if a heat exchanger (or other parts of the boiler) are kettling, they need to be fixed.
A boiler can only withstand kettling for so long.
The kettling will fry important internal components (like the heat exchanger).
You’ll end up with an expensive repair bill, or be looking at a complete new boiler unit.
What are boiler thermistors
Thermistors register the flow and return temperature of water. That’s how the boiler knows if the water is too hot going out of the boiler, or too cold coming back.
Without them, it doesn’t know when to provide heat (or to turn off).
Essentially, it how it keeps your property in line with the temperature you’ve set your heating at.
How do I check if the thermistors are faulty
You’ll need a multi-meter to check the thermistors on a boiler. This checks for an electrical current (which the thermistors need to work).
The reading should be 12K ohms @ 20C. Bear in mind, the temperature you measure at is important.
As temperature goes up, the resistance in a thermistor will come down (and vice-versa). You’ll get different readings, at different temperatures.
You need to be measuring at 20C.
I don’t have a multi-meter, now what
We’d not advise trying to test the differential in temperature between the flow and return by hand.
As we’ve already mentioned, the pipes can get to 97 degrees before the boiler locks out, and that’s going to burn your hands.
A gas safe engineer will be able to test the NTC thermistors for you.
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