Will the H100 fit on the back of the phantom 820 case where the 140 mm fan is?
Nope, that spot is for changing where the air goes, and cannot fit two 120s, nevermind a 2*120mm rad. You can however, put it on the top, or the bottom.
ok, one other problem would the lines for the radiator reach that far?
To the top or to the bottom?
either one doesnt matter. i just found out that corsair just came out with the H100i so the options should be the same for that right?
Yes. I will definatly fit to the top, and maybe the bottom, its hard to tell.
edit: but you have to take out a HDD rack for it to work on the bottom, or you have to take out the 200mm fan at the top to use that.
ok thanks ill just experiment when i get it