A Vaillant F75 fault means the boiler started its pump but detected no change in water pressure, so it locked out. Vaillant names the water pressure sensor, the pump, air in the heating installation and too little water in the appliance as the causes. Diagnosis is a Gas Safe engineer’s job, not a leak specialist’s, unless pressure keeps falling after every top-up.
A Vaillant F75 is a confusing code to land on, because the boiler is not telling you what has broken. It is telling you what it failed to see: a change in water pressure the moment the pump started. Here is what the code means, the faults behind it, what you can safely check, and the one pattern that turns an F75 into a leak question rather than a boiler one.
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What a Vaillant F75 fault actually means
Your central heating runs as a sealed circuit. The pump pushes water round the radiators and back through the boiler, and a water pressure sensor watches what happens inside. When the boiler calls for heat it starts the pump and expects the sensor to register a small, immediate change in pressure as water begins to move. That change is the boiler’s proof that water really is circulating.
If it does not arrive, the boiler stops. Vaillant’s installation and maintenance instructions for the ecoTEC range word the code as no pressure change detection when starting the pump, and list the water pressure sensor, the pump, air in the heating installation and insufficient water in the appliance as the causes. The adjustable bypass, and on some installations an external expansion vessel connected to the return, are named too.
The lockout is deliberate: firing a burner into a circuit where water may not be moving risks overheating the heat exchanger. That is why an F75 boiler often resets, runs briefly, then locks out again. The code appears across the ecoTEC range, and older units are the likelier candidates, because sensors and pumps have had years to collect debris.
The usual causes behind the code
A handful of faults sit behind almost every F75, and they are not equally likely. The table sets out what each looks like from outside the boiler and who deals with it.
| Cause | What you tend to notice | Whose job it is |
|---|---|---|
| Water pressure sensor blocked or failed | Pump clearly audible, boiler still locks out | Gas Safe registered engineer |
| Pump seized or failing | A faint hum, or silence, where water should be moving | Gas Safe registered engineer |
| Too little water in the system | Gauge reads below the normal cold range | You can repressurise; a repeat drop needs investigating |
| Air or sludge restricting circulation | Gurgling, slow warm-up, cold spots, dark water when bleeding | Bleeding may help; anything persistent needs an engineer |
| Bypass or expansion vessel set-up | Often begins after work has been done on the system | Gas Safe registered engineer |
In practice the first two are the ones that come up most often. The pressure sensor sits in a stream of heating water that is rarely as clean as it looks, and magnetite, the dark sludge produced as steel radiators and pipework corrode internally, settles on it until it stops reading changes accurately. Both parts sit behind the casing, so both are engineer work.
What you can safely check yourself
A few checks can be made without touching anything inside the boiler, and they are worth doing first, because they tell you which trade you actually need.
- Read the pressure gauge with the heating off and the system cold. Vaillant’s ecoTEC instructions give a cold filling pressure of 1.0 to 2.0 bar, and the boiler flashes a low-pressure warning below 0.8 bar. Check the gauge markings or the manual for your model.
- If it reads low, repressurise through the filling loop. That is a normal homeowner task on most modern boilers, and our guide to how to increase boiler pressure walks through it.
- Reset the boiler once and listen for the pump: water moving, a hum, or nothing at all.
- Write down today’s date and the pressure reading. The pattern over the next fortnight is what separates a boiler fault from a leak.
- Stop there. Do not take the casing off, and do not keep pressing reset. Vaillant’s own instructions cap restart attempts at three.
Pump or pressure sensor? What the noise tells you
Listening narrows the fault down before anything comes apart, and you can do it from outside the boiler. Reset it and stand next to it. If you hear the pump running and water moving for a spell before the boiler stops, tries again, then locks out on F75, the pump is doing its job and the sensor becomes the likelier fault. A faint electrical hum, or nothing that sounds like water at all, makes the pump the stronger suspect. Treat that as an indication rather than a diagnosis, and not a reason to buy a part. An engineer confirms it by testing before anything is replaced.
Why a Vaillant F75 often traces back to the sensor
The water pressure sensor sits permanently in system water. On systems never flushed or dosed with corrosion inhibitor, that water carries fine black magnetite and scale, and the residue builds on the sensor until small pressure changes stop registering. A small change is exactly what F75 is looking for.
When F75 is a leak problem, and when it is not
Most F75 faults have nothing to do with a leak. The code is about a missing pressure change at pump start, not about water escaping. But one of the listed causes is too little water in the system, and if the system is short of water, the question becomes why. The test costs nothing but patience: repressurise to the figure your manual specifies, then watch the gauge.
| What the gauge does after a top-up | What that means | What you need |
|---|---|---|
| Holds steady for weeks | The system is sealed. Pressure was low for a one-off reason, such as bleeding radiators or a recent repair | Nothing on the water side. If F75 persists, it is a boiler fault |
| Slides down over a few weeks | Water is leaving the circuit, or the expansion vessel or relief valve is at fault | An engineer to rule out the vessel and valve, then leak detection |
| Drops within days, nothing visible | Water is escaping from a concealed part of the circuit | Leak detection on the heating circuit |
A sealed heating system does not consume water, and nothing evaporates out of it. A gauge that keeps sliding back down means the water is going somewhere, and if there is no puddle, it is going somewhere you cannot see: a joint under a solid floor, a fitting inside a stud wall, or a radiator valve weeping behind a skirting board.
Two innocent explanations mimic a leak and are worth eliminating first. A failed expansion vessel lets pressure spike as the system heats and pushes water out through the relief valve, so the system genuinely loses water with no pipe damaged anywhere. A weeping relief valve does the same more slowly. Our guide to what to do when a boiler keeps losing pressure runs through the full list.
Where the vessel and the valve check out and the pressure still falls, water is escaping from the pipework, and that is what central heating leak detection exists to find. Thermal imaging, acoustic equipment and tracer gas locate the escape point without lifting every floor, and our walkthrough of how to find a leak in a central heating system sets out what that involves.
Who to call, and in what order
Fault codes sit across two trades, and calling the wrong one wastes a week.
- The boiler will not fire, F75 is on the display, and the gauge reads normal and holds there. That is a Gas Safe registered heating engineer’s job.
- The pressure keeps dropping, you are topping up again and again, and no water is visible anywhere. Once an engineer has cleared the expansion vessel and the relief valve, that is leak detection work.
We do not repair boilers, replace pumps or pressure sensors, or service heating systems. What we do is find where water is escaping from a system that will not hold its pressure, and document it for an insurer. If your F75 turns out to be a pump or a sensor, a heating engineer is who you want.
Older systems in Cornwall and Devon
Two things about the housing stock down here make this code a regular caller. The first is age: we still see plenty of ecoTEC units fitted in the 2000s, which have spent fifteen years or more circulating water through radiators that were never dosed with inhibitor. That is the recipe for a sludged pressure sensor.
The second is the buildings. Granite and cob cottages, converted barns and farmhouse extensions rarely have neat, accessible pipe runs. Heating pipes get chased into thick walls or buried under solid floors during a refit, and a weeping joint in one of those places can pull system pressure down for months without a damp patch showing where it is. We see these calls cluster in the first properly cold week of autumn. Holiday lets and second homes get a harder version, because nobody is in front of the boiler each morning to notice the gauge creeping down.
Frequently asked questions
What does F75 mean on a Vaillant boiler?
F75 means the boiler started its pump but detected no change in water pressure, so it locked out instead of firing. Vaillant’s ecoTEC instructions list the water pressure sensor, the pump, air in the heating installation and insufficient water in the appliance as the causes.
Can I fix a Vaillant F75 fault myself?
Only at the edges. If the pressure gauge is low, topping the system up through the filling loop is a homeowner task and may clear it, and bleeding trapped air from the radiators is also something you can do. Everything else behind the code sits inside the boiler casing, and anyone you employ to work on a gas appliance must by law be Gas Safe registered.
Why does the F75 come back after a reset?
Because the boiler repeats the same check every time it tries to fire. A reset clears the display but changes nothing about the pump, the sensor or the water in the system, so the fault returns within seconds or minutes. Repeated resetting will not fix the underlying cause.
Does an F75 fault mean I have a leak?
Usually not. Most F75 faults are a pressure sensor or pump problem with no water escaping at all. A leak becomes likely only when pressure keeps falling after you top the system up, so you have to top it up again over days or weeks, with no visible water anywhere in the house.
Is F75 the pump or the pressure sensor?
Listening after a reset gives a strong hint. If you can hear the pump running water through the boiler before it locks out, the sensor is the likelier fault. A faint hum or silence points at the pump. An engineer confirms which before replacing anything.
What pressure should my boiler be at to avoid F75?
Vaillant’s ecoTEC instructions give a cold filling pressure of 1.0 to 2.0 bar, with a low-pressure warning below 0.8 bar, so check the gauge markings or the manual for your model. Too little water is one of the listed causes of F75, though a correctly pressurised system can still throw the code if the sensor or pump has failed.
Topped it up more than twice this month?
Pressure that falls again within days of a top-up, with nothing visible on the floor, means water is escaping the heating circuit, and finding it is the job we do across Cornwall and Devon. If the gauge holds steady after one top-up, your F75 is a boiler fault and a Gas Safe engineer is who you want.
Call Dickie on 07822 025 911