This question has been asked before i am sure but instead of doing a search i am asking again because this house is a bit different than most
It has two floors the bottom floor uses radiators ( you know the ones you bleed with a key) the top floor uses the kind that goes the length of the wall
the raidiator floor bleeds easy but the top floor does not. When i try to bleed the top floor i get no hissing of air if i turn on the heat then air comes out then water for a bit then air then nothing no hissing , why? I turn off the heat and hissing again followed by water coming out then nothing trun on the heat and the same pattern. If i leavr rthe heat on for 10 minutes the pipes are still cold which tells me i have lots of air. How should i bleed with the heat on or off? Why does air get inside a sealed system in the first place? I was bleeding the top floor for an hour and still having problems.
I think you will get more responses if you post this in the hvac forum, as for my guess, you should check your pressure gauge as to whether you have enough pounds of pressure to get good circulation on the 2nd floor.
As to what is enough, I'll leave that for the experts to tell you.