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What Does the F75 Fault Code Mean on a Vaillant Boiler?
The F75 fault code on your Vaillant boiler means it’s detected a pressure or circulation problem and shut itself down to stay safe. Your boiler’s basically saying: “I can’t detect enough water pressure or flow, so I’m stopping before something breaks.”
This code appears most often on Vaillant’s ecoTEC range—including the ecoTEC Plus, Pro, and Exclusive models. It’s one of the more common faults UK heating engineers see, particularly in older systems or homes where the boiler hasn’t been serviced regularly.
When F75 appears, your boiler won’t fire up. No heating, no hot water. The good news? Many cases have straightforward fixes, and some you can check yourself before calling anyone out.
Four main issues trigger the F75 code. Understanding which one you’re facing helps you work out whether it’s a quick fix or time to call a Gas Safe engineer.
Low Water Pressure in the Heating System
Your Vaillant boiler needs enough water pressure to operate safely. In the UK, most heating systems should run between 1 and 1.5 bar when cold. Drop below 0.5 bar and your boiler won’t fire—it triggers the F75 code instead.
Pressure drops happen gradually. Small leaks, bleeding radiators, or natural evaporation over time all reduce system pressure. You might not notice the drop until your boiler suddenly refuses to work.
This is the most common cause of F75 errors and often the easiest to fix. If your pressure gauge shows low, topping up the system might be all you need.
Faulty or Blocked Pressure Sensor
Your boiler has a small sensor that monitors water pressure continuously. It tells the boiler whether there’s enough pressure to operate safely. When this sensor fails or gets blocked with debris, it sends false readings.
The boiler thinks there’s a pressure problem even when the gauge shows everything’s fine. Result? The F75 code appears and your boiler shuts down.
Sludge in your heating system can coat the sensor, affecting its accuracy. Sometimes the sensor itself simply wears out—they’re not designed to last forever.
Circulation Problems in the System
Your boiler’s pump pushes hot water around your heating system. If the pump fails or slows down, water doesn’t circulate properly. The pressure sensor detects this lack of movement and triggers the F75 fault.
Restricted flow from blocked pipes, closed valves, or a failing pump all create circulation problems. Your boiler might have adequate pressure according to the gauge, but the water isn’t moving as it should.
This is trickier to diagnose yourself. If your system pressure looks fine but you’re still getting F75 errors, circulation’s often the culprit.
Air or Sludge in the Heating System
Air pockets and sludge both interfere with water flow and pressure readings. Magnetite sludge builds up over years, settling in pipes, radiators, and even inside your boiler. It thickens the water and restricts flow.
Air can enter through small leaks, faulty valves, or when you’ve topped up the system without bleeding radiators afterwards. Both issues confuse the pressure sensor and can trigger an F75 code.
Older UK heating systems are particularly vulnerable to sludge build-up. If your system’s never been flushed and your boiler’s throwing F75 errors regularly, sludge might be the root cause.
Before booking a Gas Safe engineer, you can safely check a few things yourself. These steps take just minutes and might solve the problem.
Check the Boiler Pressure Gauge
Your Vaillant boiler has a pressure gauge on the front panel—usually a dial with numbers or a digital display. Look at it when your heating’s been off for at least an hour.
- Below 0.5 bar: Your pressure’s too low. The boiler won’t fire until you top it up.
- Between 1 and 1.5 bar: Pressure looks fine. The problem’s likely something else—a faulty sensor or circulation issue.
- Above 2 bar: Pressure’s too high. This shouldn’t cause an F75 error but indicates a different problem.
Knowing your pressure reading helps you decide what to do next. Take a photo of the gauge if you’re calling an engineer—it saves them time diagnosing over the phone.
Repressurising a Vaillant Boiler
If your pressure’s low, you can top up the system yourself using the filling loop—a flexible hose underneath most Vaillant boilers with a valve at each end.
- Turn off your boiler first. Open both valves on the filling loop slowly and watch the pressure gauge. You’ll hear water flowing into the system. Stop when the gauge reaches 1 to 1.5 bar, then close both valves tightly.
- Turn your boiler back on and see if the F75 code clears. If it does and stays away, job done.
When to stop and seek help: If the pressure drops again within days, you’ve got a leak somewhere. If topping up the pressure doesn’t clear the F75 code, the problem isn’t simple low pressure—call a Gas Safe engineer.
Resetting the Boiler (When It’s Worth Trying)
- After topping up the pressure, you’ll need to reset your Vaillant boiler to clear the fault code. Most Vaillant boilers have a reset button—often marked with a flame symbol that’s crossed out.
- Press and hold it for a few seconds. The boiler should restart and attempt to fire up. If the F75 code disappears and your heating works, you’ve fixed it.
If the code returns immediately or within hours, resetting won’t solve the underlying issue. Something’s still wrong, and you need a professional to investigate properly.
An F75 code that clears after topping up pressure but returns days later means you’ve got an underlying problem. Your system’s losing pressure, the sensor’s faulty, or there’s a circulation issue the boiler can’t overcome.
Ignoring repeat F75 errors puts stress on your boiler. It’s constantly trying to operate, detecting a fault, and shutting down. This cycle wears components faster and increases the risk of a more expensive failure.
You’ll also be without reliable heating and hot water. In winter, that’s more than inconvenient—it’s uncomfortable and, for vulnerable people, potentially dangerous.
If you’re topping up pressure weekly or seeing F75 errors despite normal pressure readings, book a Gas Safe engineer. They’ll diagnose the actual cause rather than applying temporary fixes.
When DIY checks don’t solve the problem, a Gas Safe engineer will investigate further. Here’s what they’ll look for and how they’ll fix it.
Boiler Pressure Sensor Inspection or Replacement
The engineer will test your pressure sensor to see if it’s reading accurately. They’ll compare the sensor’s reading with the actual system pressure using diagnostic equipment.
If the sensor’s faulty or giving inconsistent readings, they’ll replace it. It’s a relatively quick job—usually under an hour for an experienced engineer. Once replaced, your boiler should stop throwing false F75 errors.
Getting the diagnosis right matters. Replacing a working sensor won’t fix circulation problems or leaks, so a proper engineer checks everything before swapping parts.
Pump Checks and System Flow Testing
The engineer will check whether your pump’s working properly. They’ll listen to it running, feel for vibration, and test the flow rate throughout your system. A failing pump often makes unusual noises or runs slower than it should.
If the pump’s failed, replacement solves the problem. If it’s struggling because of sludge or blocked pipes, they’ll need to address those issues first. Sometimes adjusting the pump speed settings is enough to restore proper circulation.
System Cleaning or Power Flushing
If sludge is affecting your boiler’s performance, the engineer might recommend a power flush. This cleans your entire heating system, removing magnetite and debris that’s restricting flow and confusing sensors.
After a power flush, your system circulates properly, pressure readings stabilise, and F75 errors typically stop. Your boiler also runs more efficiently, which cuts your energy bills.
For heavily sludged systems, fitting a magnetic filter afterwards prevents the problem returning. The filter catches magnetite particles before they settle in your boiler and radiators.
If you’re renting, your landlord must fix boiler faults like the F75 error under UK housing law. Adequate heating and hot water are legal requirements for rental properties.
Report the fault to your landlord in writing as soon as it appears. Most respond quickly—a non-functioning boiler is an emergency repair that affects habitability. Give them reasonable time to arrange a Gas Safe engineer, but don’t wait weeks if you’re without heating.
For landlords: acting fast on boiler faults prevents tenant complaints, protects your property from cold-related damage, and keeps you compliant with housing regulations. An F75 error that’s ignored can escalate to boiler breakdown, which costs more to fix and causes bigger disruption.
Once your boiler’s working again, these habits reduce the chance of F75 errors returning.
Get your boiler serviced annually. A Gas Safe engineer checks pressure, tests the sensor, and spots problems early. Annual servicing is required to keep most boiler warranties valid anyway.
Check system pressure monthly. Glance at your pressure gauge when the heating’s cold. If it drops below 1 bar, top it up before it causes problems. Catching pressure loss early helps you spot leaks before they damage your system.
Fit a magnetic filter if you haven’t already. These devices trap magnetite particles, protecting your boiler and reducing sludge build-up. They’re relatively inexpensive to install and save you from costly repairs later.
Bleed radiators carefully. When you bleed radiators, you release air but also reduce system pressure. Check your boiler’s pressure gauge afterwards and top it up if needed.
These simple steps extend your boiler’s life and keep your heating reliable through winter.
If your Vaillant boiler’s showing the F75 fault code and DIY checks haven’t cleared it, it’s time to call a Gas Safe registered engineer.
Whether you’re a homeowner dealing with an unreliable boiler or a landlord managing a rental property, getting the fault diagnosed properly prevents repeat callouts and more expensive repairs down the line.
Qeeper’s heating engineers are Vaillant experts who can diagnose sensor faults, circulation problems, and pressure issues accurately. They’ll fix the root cause, not just clear the error code temporarily.