Temp HP never ever stack. Period.

That's not stacking, though. You always get Temp HP equal to MAX( what I have right now, what this power would give me ). You never add Temp HP. You always overwrite them....Except when a Battlerage Vigor fighter hits with an Invigorating power...
...Except when a Battlerage Vigor fighter hits with an Invigorating power...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.