So an FAQ update today to the Core Rule book FAQ says the following
Quote:
Temporary Hit Points: Do temporary hit point from the same source stack?
No. Generally, effects do not stack if they are from the same source (Core Rulebook page 208, Combining Magical Effects). Although temporary hit points are not a "bonus," the principle still applies.
This prevents a creature with energy drain (which grants the creature 5 temporary hit points when used) from draining an entire village of 100 people in order to gain 500 temporary hit points before the PCs arrive to fight it.
Temporary hit points from different sources (such as an aid spell, a use of energy drain, and a vampiric touch spell) still stack with each other.
—Pathfinder Design Team, today
Note the last sentence. This is different from how I have been playing - but not entirely sure why I was always assuming that temporary hit points always overwrote each other. So, for example, if you have False Life up and do something that would also grant you Temporary Hit Points you would only get the higher of the two - your remaining False Life temporary hp or the new source but that they wouldn't stack.
But according to this FAQ it appears that temp hp from different sources do stack - so False Life & Vampiric Touch & Death Knell & Energy Drain & Aid etc will all stack.
[which reminds me of another rules debate I had with folks at a table a while ago - do spells cast by summoned creatures like Aid by summoned archons remain in effect after the summons ends? As a GM I don't like the headaches of spells that have TWO clocks on them - a duration of the spell and some other effect that will result in the spell going away sooner - but there are some indications that RAW that is what happens]
