What It Is, Why It Happens and How to Fix It
If you’ve recently noticed an F75 error code on your Vaillant boiler, you’re not alone. Many homeowners encounter this fault, often causing worry about costly repairs or complicated fixes. In this guide, we’ll explain what an F75 on a Vaillant boiler means, why it occurs, and what you can do to address it safely. We’ll also discuss why handling a gas boiler can be dangerous and why calling a qualified gas engineer is often the safest option.
What Does F75 Mean on a Vaillant Boiler?
The F75 error code on a Vaillant boiler indicates a problem with the flow temperature sensor or the domestic hot water temperature sensor. Essentially, your boiler is detecting that the temperature of the water is too high, too low, or fluctuating outside of normal ranges.
In simple terms, the F75 code acts as a safety feature, preventing your boiler from operating in unsafe conditions that could damage the system or even pose a risk of overheating.
Common Symptoms of an F75 Fault
You might notice the following when your boiler displays the F75 fault:
The boiler won’t ignite or keeps shutting down.
No hot water is being produced, or the heating is intermittent.
The display panel shows F75 alongside a flashing red light (depending on your Vaillant model).
Sometimes, the boiler may make unusual noises before shutting down.
If any of these symptoms occur, it’s crucial not to ignore them, as continuing to use the boiler may worsen the issue or cause safety risks.
Why Does an F75 Fault Appear on a Vaillant Boiler?
The F75 fault can appear due to several underlying causes, usually related to the boiler’s temperature sensors or water flow. Some common reasons include:
Faulty Flow Temperature Sensor: The sensor responsible for monitoring the heating system may have failed or become disconnected.
Domestic Hot Water Sensor Issue: If your boiler struggles to detect hot water temperature correctly, it may trigger F75.
Blocked or Sludged Heat Exchanger: Poor water circulation can prevent accurate temperature readings.
Pump Problems: A malfunctioning pump may lead to uneven water flow, triggering the sensor warning.
Electrical Issues: Loose wiring or faulty connections within the boiler can result in inaccurate sensor readings.
In most cases, the sensor itself is either malfunctioning or detecting abnormal readings, but other system faults can trigger F75.
F75 Boiler Fault How to Fix It
It’s important to note upfront: you should never attempt to repair a gas boiler if you are not a qualified engineer. Gas appliances carry serious risks, including fire and carbon monoxide poisoning. Always call a registered gas engineer to inspect and fix your boiler.
That said, here’s what a professional will typically do to resolve the F75 fault:
Reset the Boiler
Many minor faults can be temporarily resolved with a reset.
Refer to your boiler manual to safely reset it, usually by holding the reset button for a few seconds.
Warning: If the fault keeps returning, do not attempt repeated resets.
Inspect and Replace Sensors
A gas engineer will check the flow temperature and domestic hot water sensors.
Faulty or failing sensors are replaced to ensure accurate readings.
Check Boiler Circulation and Pump Function
The engineer will verify that the pump is operating correctly and that water is circulating properly.
Sludged or blocked heat exchangers may be cleaned or flushed.
Examine Electrical Connections
Any loose or corroded wiring can cause sensor errors.
Connections will be tightened or replaced to restore normal operation.
Full System Diagnostics
A qualified engineer may perform a complete diagnostic to identify hidden issues, ensuring the boiler is safe and efficient.
DIY Checks You Can Safely Do
While gas and internal boiler repairs should only be done by professionals, there are a few safe checks you can do:
Ensure water pressure is within the recommended range (usually 1–1.5 bar).
Check for visible leaks around radiators, pipework, or the boiler.
Make sure radiator valves are fully open and not obstructing flow.
If these basic checks don’t resolve the issue, it’s time to call a professional.
Why You Should Call a Gas Engineer
Handling a gas boiler yourself is extremely dangerous. Incorrect repairs can lead to:
Gas leaks
Carbon monoxide exposure
Boiler explosion or fire
Further costly damage to your system
A registered gas engineer has the skills and tools to safely diagnose and repair F75 faults. They can also perform checks to prevent the issue from recurring.
The F75 error on a Vaillant boiler is typically linked to temperature sensor issues or water flow problems, and while a reset may temporarily help, it’s not a permanent solution. Gas boilers are dangerous to tamper with, so calling a qualified gas engineer is the safest way to fix an F75 fault.
If your boiler shows an F75 code, don’t wait contact our expert team today to get your heating system running safely again.
Need Help with an F75 Fault?
Call Our Expert Gas Engineers Today!
Don’t risk your safety or damage your boiler if your Vaillant boiler is showing an F75 fault, call our Gas Safe engineers today. We provide fast, reliable repairs and maintenance to get your heating and hot water back on safely. Book your service now!