Can a heat pump water heater be vented to the attic?
The cold is just air cooled in the home and it has no more moisture than the house air in-fact it is dryer than the house air. I have vented 2 into Areas that are not conditioned , I think the attic is a good option as in the summer it will help cool that space. If you have plumbing in the attic I would choose a different location as this can help the pipes to freeze. I vented mine outside but that was a convenient location for me.  You still need a drain for the moisture that builds up on the exchanger, also if a tight house you will need some makeup air allowed in normally at the furnace, my 1930’s home did not need additional vents but a Check needs to be done, the efficiency gain is reduced if inside the home so in my opinion it is a crock to require them.