What does F75 mean on a Vaillant boiler?
When a Vaillant boiler starts its pump, it expects the water pressure sensor to register a small rise in pressure. That is how it confirms water is actually moving round the system before it lights the burner. Vaillant’s ecoTEC plus fault table describes F.75 as “No pressure change detection when starting pump”.
It is a safety lockout. A boiler that fires without proper circulation can overheat quickly, so the boiler refuses to run until the cause is fixed.
What causes F75?
Vaillant lists these causes:
- Faulty water pressure sensor. The sensor may be reading wrongly or have failed altogether.
- Faulty or seized pump. A pump that is stuck, weak or not powered will not create the pressure change.
- Air in the heating installation. Air in the pump or pipework stops it moving water properly.
- Not enough water in the boiler. If the system pressure is low, the pump cannot do its job.
The manual also points engineers to check the bypass and, on some systems, the position of an external expansion vessel. Sludge in older systems can make a weak pump worse.
What can you safely check yourself?
- Check the pressure gauge. If it is low, top up the system following your user manual. Low pressure on its own normally shows as F22, but the two can appear together.
- Bleed the radiators with the heating off, starting downstairs, then recheck and top up the pressure.
- Listen to the boiler when it starts. A hum with no sound of water moving, or a grinding noise, is worth mentioning in your enquiry.
- Reset once using the reset button.
If F75 appeared after work on the heating system, such as a new radiator or a drain-down, air is the most likely cause and the bleed-and-top-up steps often help.
When should you get an engineer?
If F75 returns after bleeding, topping up and one reset, the pump or pressure sensor needs testing. The engineer will check the pump is turning and powered, test the pressure sensor, look for air and sludge, and confirm the bypass is working. Pumps and pressure sensors are common replacement parts on ecoTEC boilers, and you will be given a price before any work begins. If the boiler is also noisy or radiators are cold in places, mention it: see radiators cold.