Increase stack size with sacrifice?

[Zechnophobe]

Actually, you cannot do a partial heal as razorflame describes.

If the second stack (being added to) is comprised of units with HP greater than the amount of health given to them, it will not work and you cannot target them.

Example:

I use a level 1 sacrifice to do 1000 damage to my Vampires. It will convert that to 400 health. If I try to give that health to my Ogre (680 hit points) it will not work, since 400 is less than 680. Whereas if you deal 2000 damage with a level 3 Sacrifice, you'll get 1200 Health back, enough to create a new ogre (Or even red or green dragon!).