The Vaillant F75 fault codes typically indicates a problem with the water pressure in your Vaillant boiler. This fault code is commonly found in Vaillant ecoTEC and ecoMAX boilers. When the F75 code appears, it means that the boiler is not detecting adequate water pressure, and it may result in your heating and hot water not working properly. Here's how to fix it:

1. Check the Water Pressure: The first thing you should do is check the water pressure on your boiler. You can usually find a pressure gauge on the front panel of the boiler. The ideal pressure range is typically between 1 and 2 bar. If the pressure is too low (below 1 bar), you'll need to repressurize the system.
2. Repressurize the Boiler: To repressurize your Vaillant boiler, follow these steps:

a. Turn off the boiler and wait for it to cool down. b. Locate the filling loop, which is usually a flexible silver or braided pipe that connects the cold water supply to the boiler. c. Open both ends of the filling loop by turning the valves until they are in line with the pipe. d. Slowly open the valve at one end of the filling loop to allow water to flow into the system. Keep an eye on the pressure gauge. e. Once the pressure reaches the recommended range (usually between 1 and 2 bar), close both valves on the filling loop. f. Turn the boiler back on, and the F75 fault code should clear if the pressure was the issue.

• Check for Leaks: If you find that you need to repressurize the boiler frequently, it's essential to check for any water leaks in the system. Leaks can lead to a drop in pressure, and you'll need to fix the source of the leak to prevent the F75 error from recurring.
• Call a Professional: If you've checked the water pressure, repressurized the boiler, and still encounter the F75 fault code, it's advisable to contact a qualified heating engineer or a Vaillant service technician. The problem may be related to a faulty pressure sensor, expansion vessel, or another internal component that requires professional diagnosis and repair.

Remember that working on a gas boiler can be dangerous, so if you're not confident in your ability to perform these steps safely, it's always best to seek the assistance of a qualified heating engineer or technician to resolve the F75 fault code and ensure your boiler operates safely and efficiently.