If your Vaillant boiler is showing an F75 fault, it usually means the boiler has not detected the expected change in water pressure when the pump starts. This is often related to the pressure sensor, pump performance, or the way pressure is behaving in the system.
What the Vaillant F75 fault usually means
When the boiler starts up, it expects a small but clear change in pressure as the pump begins to move water around the system. If that change is not detected, the boiler logs F75 and shuts down to protect itself.
Common root causes include a faulty pressure sensor, a sticking or weak pump, low or unstable system pressure, or debris in the system affecting circulation. Repeatedly resetting the boiler might clear the fault briefly, but it does not address the cause and can increase wear on components.
If F75 is appearing more than once in a short period, treat it as a fault that needs attention rather than something to keep resetting and ignoring.
Safe checks you can do before calling an engineer
There are a few simple checks most homeowners can safely carry out without taking the boiler apart. These can help you rule out basic issues and give clearer information to an engineer.
Confirm system pressure when cold: Check the pressure gauge on the boiler when the system is completely cool.
Listen for gurgling noises: Air in radiators can affect circulation and pressure readings.
Check filling loop valves are shut: After topping up pressure, the valves should be fully closed.
Note when F75 appears: Does it happen on heating, hot water, or at random?
Check the boiler pressure is in the correct range
Look at the pressure gauge when the system is cold. Most Vaillant systems are usually set to around 1.0 to 1.5 bar when cold, but always refer to your user manual for the recommended range.
If the pressure is near zero, the boiler is likely to lock out. You can usually top up using the filling loop as per the instructions in the manual. After topping up, recheck that the reading is stable and not dropping over the next few hours.
Bleed radiators if you hear gurgling
Gurgling, sloshing or trickling noises in radiators suggest air in the system. Excess air can confuse the pressure readings and reduce circulation, which in turn can trigger faults like F75.
With the heating off and the system cool, you can bleed radiators using a radiator key and a cloth. Work through them methodically, starting on the lower floor. After bleeding, you will usually need to check and top up the boiler pressure again.
Make sure the filling loop is fully closed
If you have recently topped up the system, double check that the filling loop valves are back in the closed position. A partially open filling loop can cause pressure to creep up or down and may contribute to intermittent lockouts.
Ensure both ends of the filling loop are turned fully off, then keep an eye on the pressure gauge. If the pressure steadily falls or rises with no heating demand, that is useful information for an engineer.
Note patterns in when the fault appears
It helps to pay attention to when F75 shows on the display. Patterns can give strong clues about where the problem lies, even before anyone opens the boiler.
Make a note of whether the code appears only when the heating first starts from cold, after hot water has been used, during long heating runs, or completely randomly. Also record whether the pressure gauge swings about when the boiler starts, or if it looks stuck.
What an engineer is likely to check and fix
Once the basic user checks are done, further investigation should be left to a Gas Safe registered engineer. The F75 fault often involves components and tests that should not be handled by an unqualified person.
Pump performance and operation
The engineer will typically check whether the pump starts cleanly, runs quietly, and provides enough circulation. A sticking, noisy or seized pump can prevent the expected pressure change when the boiler fires.
Depending on findings, they may clean or free off the pump, adjust settings, or recommend a replacement if the unit is weak or failing.
Pressure sensor testing and replacement
The pressure sensor (or transducer) is critical to how the boiler decides if it is safe to run. If it is blocked, contaminated or failing electrically, the boiler may show F75 even if the pump is working.
An engineer can test the sensor, check for blockages in the connecting pipe, and replace the part if necessary. They will also clear any trapped sludge that might be affecting readings.
Expansion vessel and pressure stability
The expansion vessel absorbs changes in water volume as the system heats and cools. If the vessel has lost its air charge or has failed, pressure can swing wildly, leading to repeated lockouts and F75 errors.
Engineers can check the vessel’s charge, re-pressurise it if suitable, or replace it if it has failed. They will also check the safety valve and other related components.
System debris, sludge and blockages
Sludge, rust flakes and limescale can all build up inside a heating system, restricting water flow and affecting pumps and sensors. This is a common background issue behind F75 and other circulation-related faults.
Your engineer may recommend system cleaning, new filters, or localised pipework work to restore proper flow. They can also advise on inhibitors and ongoing protection to reduce future build up.
When to stop troubleshooting and call an expert
There is a clear point where home checks should stop and professional diagnosis should start. Safety should always come first.
F75 or other fault codes causing frequent lockouts or resets
Pressure dropping quickly, even after topping up
Any signs of leaks from the boiler or pipework
Unusual smells, burning odours, or loud banging/whining noises
If any of these apply, switch the boiler off and arrange a visit from a Gas Safe registered engineer rather than continuing to reset or top up.
Next steps to get your boiler back to normal
If your Vaillant boiler is showing F75 and the simple checks above have not solved it, it is time for a proper diagnosis. A structured inspection will usually pinpoint whether the pump, pressure sensor, expansion vessel or system debris is behind the problem.
To get your heating and hot water running reliably again, contact BOG STANDARD PLUMBING AND HEATING LTD to book a boiler repair visit on 03301132248. An experienced engineer can safely trace the cause of F75 and carry out any necessary repairs.
To reduce the chances of repeat lockouts in future, it is also sensible to schedule regular servicing. Annual boiler servicing with BOG STANDARD PLUMBING AND HEATING LTD on 03301132248 helps keep components clean, spot issues early, and keep your heating system running as smoothly as possible.
