If your Vaillant boiler is showing an F75 fault code, it usually means the boiler isn’t detecting a change in water pressure when the pump starts.
In simple terms:
The boiler expects to see water movement — and it isn’t seeing it.
Sometimes it’s a simple pressure issue.
Sometimes it points to a deeper system problem.
Here’s what normally causes it.
What Causes an F75 Fault?
Low System Pressure
If your boiler pressure drops below around 1 bar, the sensor may not detect movement properly.
Check your pressure gauge.
It should usually sit between 1–1.5 bar when cold.
If it’s low, topping it up may temporarily solve the issue.
But if pressure keeps dropping, that’s a separate problem that needs investigating.
Faulty Pressure Sensor (Very Common)
Vaillant boilers use a pressure sensor to detect movement when the pump fires.
Over time, these sensors can fail.
One of the most common reasons?
Dirt and debris in the system.
Heating systems naturally collect:
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Sludge
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Rust particles
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Magnetite
If your system isn’t well protected, that debris can clog or damage the pressure sensor — causing repeated F75 errors.
Dirty System / Poor Filtration
If you don’t have a magnetic boiler filter fitted — or it hasn’t been cleaned regularly — dirt can circulate around the system.
That increases the chance of:
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Pressure sensor failure
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Pump wear
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Blockages
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Repeated fault codes
A good quality magnetic filter, cleaned annually during a service, significantly reduces this risk.
If F75 keeps returning, it may indicate the system needs:
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A proper system clean
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Or a powerflush (in more severe cases)
Pump Problems
If the pump isn’t circulating water properly:
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It may be sticking
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It may be worn
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There could be an airlock
The boiler won’t detect pressure change and will shut down with F75.
Expansion Vessel Issues
If the expansion vessel has failed or lost its charge:
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Pressure fluctuates
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Boiler may lose pressure regularly
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F75 may appear intermittently
If you’re constantly topping up your boiler, this could be the real issue.
Can You Fix F75 Yourself?
You can safely:
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Check the pressure
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Top it up if low
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Try a reset
If the fault clears and doesn’t return, it may have just been low pressure.
If it keeps coming back, it needs diagnosing properly.
Repeatedly resetting the boiler without addressing the cause isn’t a solution.
When To Call an Engineer
Call a qualified heating engineer if:
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The fault returns after topping up
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Your boiler keeps losing pressure
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You don’t have a system filter
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You suspect the system is dirty
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You hear unusual pump noises
F75 is usually straightforward to fix — but ignoring it can lead to bigger repairs later.
We Repair Vaillant Boilers in Twickenham & Surrounding Areas
At Frank Finn Plumbing, we regularly repair Vaillant boilers across:
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Twickenham
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Teddington
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Richmond
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Hampton
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Surrounding areas
If your Vaillant boiler is showing F75 and won’t reset properly, give us a call.
📞 0208 408 0743
We’ll diagnose it properly and fix the underlying issue — not just reset the code.
Preventing F75 Faults in Future
To reduce the risk of F75 returning:
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Have your boiler serviced annually
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Make sure a magnetic filter is fitted
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Have the filter cleaned during service
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Consider a system clean if your heating water is dirty
Keeping the system clean protects the pump, sensors, and boiler internals — and saves money long term.
