Vokera boiler losing ptressure?

I have installed in the loft of our bungalow a vokera linea series combi-boiler, (domestic & heating
In the owners manual it advises that we keep the pressure between 1 to 2 psi, it achieves this when it is functioning,ie heating is operating or running the bath etc.
When the unit is at idle the pressure gauge needle comes to rest on the dial needle stop ie below zero, I then repressurise to 1.5 psi, after a few days depending on usage I need to top it up again, is this normal, if I did not repressurise, would the boiler run dry and eventually blow up