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Vaillant boilers have two temperature sensors. One monitors the temperature of water that leaves your boiler (flow sensor). The other monitors the temperature of water returning to the boiler (return sensor).
The boiler's printed circuit board (PCB) constantly monitors the temperature difference between the two, which needs to be within a tight operating range.
The F72 fault code will be displayed when the boiler’s PCB notices:
Boilers use these temperature readings to keep your unit working efficiently, by adjusting:
Without readings from the flow and return that are accurate (or within tolerance), the PCB is not able to regulate your boiler in a safe way.
This could result in the likes of:
The F72 boiler fault code doesn't necessarily point to an overheating fault. The temperature measurements it's receiving aren't within tolerance, so it locks out and displays the F72 fault code as a safety precaution.
The F72 Vaillant fault code relates to an issue with NTC thermistor temperature readings. This can be misdiagnosed as an overheating fault, but that's rarely the issue.
Below, we'll cover the 4 main issues (and how to fix them), including:
Below, we'll cover each in detail, including how much each is likely to cost to repair.
If you’d prefer to replace your boiler rather than repair it, jump over to our new boiler quotation tool here.
The most likely issue causing the F72 fault code to be displayed on your Vaillant boiler, is a faulty NTC (Negative Temperature Coefficient) thermistor. There's a thermistor on both the flow and return pipes of your boiler.
As the temperature of water rises, their electrical resistance drops. This data is then sent back to the PCB to ensure the boiler is working within heat tolerances. When the PCB notices the value spread between the flow and return sensors is incorrect (or impossible), it locks out and shows the F72 fault code.
Age is the biggest killer of boilers and in particular, thermistors.
Over time, resistance levels can fluctuate, partly thanks to small cracks in the component. If this happens, the temperature readings become less and less reliable.
Typically, these readings will be correct when cold, but incorrect when hot. This is what leads a lot of boiler engineers to diagnose this as a "boiler overheating" fault even though a lot of times, this simply isn't the case.
This is a job for a Gas Safe engineer; no free fixes here. Your boiler engineer will need to:
It's most likely at least one thermistor has failed. However, best practice here is to replace both thermistors to ensure maximum accuracy.
Replacing a set of thermistors is likely to cost anywhere from £250-£350. After a replacement, resetting the boiler will clear Vaillant’s F72 error code from your boiler.
A worthy side note here is that if your boiler is under warranty, this should be fixed for free (parts and labour) by the manufacturer.
If your boiler is old, unreliable, inefficient and starting to show signs of errors, it might be worth considering a new one. New combi boilers cost as little as £1,950 (check exact prices using our boiler quote tool here).
The best and most reliable boiler brands generally offer a warranty in the region of 5-12 years, so you won’t have to fork out for expensive repairs like this one!
The PCB's job is to give and receive signals to/from boiler components and sensors. This includes the NTC thermistors on both the flow and return.
Whilst it's likely the issue relates to faulty NTC sensors or wiring, the fault could be with your PCB instead.
As part of any electrical boiler fault finding, boiler engineers always check the PCB for faults.
Faults that could affect thermistor readings include the likes of:
Typical signs a boiler's PCB is on its way out include:
Experienced boiler engineers with in-depth electrical knowledge are sometimes able to fix a PCB.
They'll be able to run a combination of a visual inspection and electrical tests to determine if the PCB can be fixed.
Unfortunately, if your Vaillant boiler's PCB can't be fixed, it will need to be replaced. Typical replacement cost is £500+.
If you'd prefer to upgrade to a new boiler (where a fix like this would be covered under warranty), you can use our clickable form here and get prices from £1,950.
Both thermistors on your Vaillant boiler send a resistance signal (converted to a temperature reading) via two thin wires, directly to the PCB.
With age, this wiring can deteriorate. This could lead to the likes of:
If any of these are present, the resistance reading will be incorrect, even if there is nothing wrong with the thermistor itself.
The PCB will see the temperature fluctuate dramatically, or not move at all. That’s what triggers the F72 error code to be displayed.
Your Gas Safe engineer needs to ensure connectors are in good condition (replace if necessary) and tight on both the thermistor and PCB.
A visual inspection is then needed to check for corrosion (green corrosion suggests exposure to moisture) and brittleness (this suggests excessive heat exposure).
The cost of parts for this fix is negligible, so you'll be mostly paying for labour. Expect labour costs to be in the region of £125-£200 if wiring is the issue.
In order for thermistors to get resistance (and therefore, temperature) readings, your boiler needs a flow of water via the flow and return.
When circulation is poor, readings are going to quickly go out of tolerance. This can lead to the F72 error code being shown on Vaillant boilers.
Typical causes of poor circulation in a boiler relate to:
First, your boiler engineer needs to check your system (including the pump) for heating sludge and limescale build-up.
This is easily done by draining some water from the system. If it's black, there's a good chance heating sludge is clogging up your radiators, pipework and pump, restricting the circulation of water.
Radiators that are cold at the bottom (which is where sludge settles) and hot at the top is also a common symptom of a blocked system.
To get rid of heating sludge, your engineer can chemically flush your system, and top it up with an inhibitor. They can also fit a limescale and magnetic boiler filter to prevent this happening again in future.
Alongside this, they'll want to bleed your system of any air. This can get trapped in radiators, pipework and the pump which leads to restricted flow.
If removing sludge, limescale and air still doesn't get your pump operating correctly, a replacement could be needed.
Expect a replacement pump to cost in the region of £300-£400 including parts and labour.
Like any fault costing £300+, it might be worth considering investing this into a new boiler, rather than an old and unreliable one.
And that’s it for our guide to the F72 Vaillant fault code.
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