Vaillant F75: Pressure change too low when the pump starts

F.75 on a Vaillant boiler means: pressure change too low when the pump starts. The boiler has detected that the water pressure in your heating system is too low to run safely, so it has stopped.

Often caused by low pressure or air in the system; can also point to the pump or pressure sensor.

This code applies to Vaillant ecoTEC. Codes can mean different things on different models, so check the display against your boiler's user manual.

What you can safely check

• Look at the pressure gauge on the front of the boiler. With the heating off and the system cold, it should normally read 1 to 1.5 bar.
• If it is below 1 bar, you can usually top it up yourself using the filling loop. Your user manual shows where it is and how to use it — close it again once you reach about 1.5 bar.
• Look for drips around radiators, valves and visible pipework, and check the pressure relief pipe outside for signs of water.
• Press reset once, following the instructions in your user manual.

When to call an engineer

If the pressure keeps dropping after you top it up, water is escaping somewhere — from the pipework, a radiator, the pressure relief valve or inside the boiler — or the expansion vessel needs attention. A Gas Safe engineer can find and fix the cause.

Never remove the boiler casing yourself — everything behind it must be handled by a Gas Safe registered engineer. If you smell gas, open windows, turn the gas off at the meter, and call the National Gas Emergency number on 0800 111 999.