I just want to summarize everything.
HotS account level = sum of levels of all heroes - overall number of heroes in game.
Whenever you level up a hero, you level up account level.
For every account level, one common loot chest is given.
For every 5th account level, one rare loot chest is given instead of a common one.
For every 25th account level, one epic loot chest is given instead of a rare one.
For every 10th level of any hero, one hero loot chest is given in addition to a chest for account level.
I. e. when you level up a hero to 10m level and at the same time you account level becomes 25n, you will get an epic and a hero chests.
There is a bug now that displays incorrect number of chests granted after a match. The real number of chests granted is correct.