Can somebody explain this interaction to me as though I were really slow?
Sipping Demon (Su)
A 13th level, a hungry ghost monk gains 1 temporary hit point each time he hits an enemy with a melee attack. The monk gains a number of temporary hit points equal to his Wisdom modifier when he scores a critical hit. The maximum number of temporary hit points the monk can have is equal to his monk level. The temporary hit points disappear 1 hour later. The monk can only use this ability when he has at least 1 ki point in his ki pool. This ability is a proscribed manipulation of ki considered by many good monks to be a corruption.The ability replaces diamond soul.
Lifesurge Weapon
...In addition, whenever the wielder receives temporary hit points from any source, he adds the weapon’s enhancement bonus to these temporary hit points; this does not stack for temporary hit points from multiple sources, and these points are lost if the lifesurge weapon is not being wielded.
So, even though Sipping Demon stacks, the bonus temp HP from lifesurge does not stack? I assume each attack giving temp HP counts as HP from multiple sources.
So, at level 20, with a +5 lifesurge weapon: I attack and hit, gain 6 temp HP. Every attack after that adds just 1 temp HP, up to a max of 20.
Is that correct?
The way I read it is that the life surge bonus only works with a single source. Sipping Demon seems like it would be considered a single source. If that is the case you might get 6 temporary HP per strike. I could also see limiting it to 6 temporary HP on the first strike of the round and then 1 per strike after that. That is really going to be up to the GM.
A +5 Lifesurge weapon is a +7 weapon, so should be powerful.