As an added complication watch out for "Aid" the spell it is a completely different mechanic that raises maximum HP and technically stacks with itself indefinitely and doesn't overwrite temporary HP. As a GM i did not at first realize this when tackling 5th edition, you should be very careful handing out scrolls and items that cast "Aid" you may find players buying them or hording them and then going at enemies with 50+ bonus HP or something.
