if it is auto fill there is a problem with the valve meaning it has filled itself up. I have known of system leak well below 3.5 bar - drain it ASAP

you wont get much heat into the system at that sort of pressure, a common misconception is: more pressure = more heat. it is the total opposite (i am not saying you have done this personally)

if its manual fill there should be some sort of tap or a filling loop so you can top it up,
if it has a loop there is no way any more water could get into the system, meaning there is another problem, maybe a pressure release valve fault??

To drain down there should be a valve like this


put a hose pipe on it and then open the valve. MAKE SURE THE OTHER END IS DOWN A DRAIN

Scott