Thanks for the tips. I bought the bigger fan anyway, my plan is for that to vent the room with a 250w during first weeks of veg (they're still stuck in tiny plastic cups under some flours and not too happy about it). I'm thinking I'll easily keep general house temps with that. The other fan was less than half of this, and when I tried it with the 250HPS for half an hour with the door shut, it only reached 23C.
During that time I'll prepare the 400w with a cooltube, and add that after a few weeks as needed, and especially for flowering.
I'm hoping that will work.
I also plastered, sandpapered and painted the room with a whole new coat of matte white after making those whacking great holes in the wall, and did the hallway at the same time

Heh, this new fan is a pretty hefty toy, though VROOOOOOOOOOOM!!!!!!!