What Does an F75 Fault Mean on a Vaillant Boiler?

The official explanation of the Vaillant error F75 is that no pressure change can be detected when the pump starts up. This means that, when the pump is functioning, the boiler cannot detect the pressure that it should. In general, this means there is something wrong with either the pressure sensor or the pump itself.

Causes of the F75 Fault Code

In the simplest scenarios all that needs to be done, is to add water to the system via the filling loop.

However, if the system pressure is dropping, this indicates there could be a leak. This needs to be fixed before you increase your boiler’s pressure, otherwise your property/boiler could be subject to water damage.

Fixing the F75 Fault Code

1. Test The Pressure Sensor

If your boiler is sitting between 1-2 bar (1.5 bar is ideal), pressure isn’t the problem. However, that isn’t to say your boiler knows that the pressure is correct.

It’s the job of the water pressure sensor to tell the printed circuit board (PCB) whether the pressure is too high or low. If it’s not doing its job and giving the PCB false signals, the boiler won’t work, and the F1 fault code will be displayed.

So, your heating engineer will want to test the water pressure sensor; it might need replacing.

2. Lack of Pressure, or Pressure Loss?

If the pressure is lacking (i.e. sensor isn’t the issue), you’ll need to determine whether the problem is a lack of pressure, or loss of pressure.

A lack of pressure will require the boiler to be topped up with water via the filling loop (see how to increase boiler pressure below).

However, if the boiler pressure is consistently dropping, that’s a sign of a leak. You’ll want to get the leak fixed before topping up the boiler’s pressure, otherwise you could have expensive repair bills for internal boiler components due to water damage, or even plaster/floorboards if the leak is coming from pipework.

You’d expect a boiler to stay stable at 1.5 bar. Below 0.5 bar it’s going to stop working. So, is the boiler dropping pressure after it’s topped up?

3. F75 Fault Code Faulty Pump

1. Reset the boiler by pressing the reset button which is the one with the image of a flame on it.
2. Wait for a few minutes and then run a hot water tap causing the boiler to fire up.
3. The heating pump should now kick in and send hot water through to the tap.
4. If there’s a pump defect, then you won’t hear or feel anything resulting in no heat or hot water
5. If the pump is working, which usually means that it runs for a couple of
   seconds then falls silent for 20 seconds, then the F75 fault code is due to a fault with the pressure sensor.

In most cases, This would be beyond a simple fix and would require a competent gas engineer, Call us on 07855382189