Fault guide
Vaillant F75, the pump and pressure sensor lockout
F75 is the ecoTEC saying it cannot prove water is moving. It is a protective lockout, and it has three realistic causes rather than one.
How the boiler decides to show F75
When the ecoTEC starts the pump it expects the pressure sensor to register a small change as water begins to circulate. No change within the allowed window and the boiler assumes circulation has failed, then locks out on F75 to protect the heat exchanger.
The three causes
Fitting a sensor because the code says sensor is the classic way to end up back here in a month.
- A failed or silted pressure sensor that cannot see the change even though the pump is running.
- A pump that is air locked, seized after a summer of standing still, or electrically failed.
- Genuinely low system pressure, so there is little for the sensor to read in the first place.
What you can check
Start with the simplest of the three.
- Look at the pressure gauge cold: it should read roughly 1 to 1.5 bar.
- If it is below 0.8 bar, top up via the filling loop and reset once.
- If pressure is fine, or F75 comes straight back, stop there and call an engineer.
How we prove which one it is
We check the static and running pressure, test the pump electrically and mechanically, bleed any air out of the pump head, and compare what the sensor is reporting against the actual system pressure. Whichever component is lying gets replaced.
We also look at why: a pump that seizes and a sensor that silts up are both usually symptoms of a dirty system, so we check the water quality and recommend a cleanse and filter where it is warranted.
Questions we get asked
- What does F75 mean on a Vaillant?
- F75 means the boiler started the pump and did not detect the expected change in water pressure, so it cannot confirm that water is actually circulating.
- Is F75 always the pressure sensor?
- No, though it often is. It can equally be an air locked or seized pump, or genuinely low system pressure. All three need ruling in or out before a part is fitted.
- Can I fix F75 myself?
- You can check the pressure gauge and top the system up to about 1.2 bar cold. Beyond that it is engineer work, because it involves testing the pump and sensor on a live system.