venting water system

hi

sorry to ask such a daft question

since fitting a new heater matrix I can't get it working, have vented it with the engine off

here my daft question is it better to vent the system with the engine up to temperature and running so the thermostat is open , I would only crack the vent open very slightly but wondering with the engine running if I would suck more air in, not sure if the vent is on the positive or negative side

is it better to take the expansion cap off from cold, then start the engine and allow it to warm up, would the water just pour out of the expansion tank with the engine running and the thermostat opening

also noticed, on the expansion tank there is a small pipe at the top and with the engine running looks as though air is passing back into the expansion tank, or maybe it's an optical illusion

in a few weeks time going to do a full engine service and drop all the water out in order i can refil it with fresh antifreeze

just want to get it up and running before I do anything to it

thks guys and sorry once again asking a daft question

went through the hole system, the flow and return pipes on the heater matrix are both hot, fans running and top rad hose nice and hot, temp gauge at its midway level

i now suspect that one of the heater control cables have become dislodged

oh happy days, lol

It's not unknown for there to be a blanket that's not been removed?, easy enough to check whip the out pipe off and see if coolant is getting through it?