Ferroli maxima 35C?

Ferroli maxima 35C
"when I turn on a tap for hot water"
The pressure on the digital display falls below .5 bar .
The pressure in the manual gauge stays fine when I turn on the hot water it will shut the system down with F37 when I turn of the hot water and press the reset the pressure goes back to 1.5 bar on the digital display and the heating works fine. the manual gauge stays the same pressure either way

regards

patrick