I am very interested in getting a heat pump water heater whenever our current water heater fails. I see the 1000 cu. ft. requirement which makes it an obvious choice for basements or attics and that you aren't supposed to put it in a closet. I wonder how much that rule can be bent. My current 75gal water is in the closet of my sun room. If I were to install vents in the door (like make the whole door a vent), could that work? Our sun room meets the 1000 cu. ft. requirement but obviously we aren't going to put the heat pump in the middle of it. The sun room is 1026 cu. ft. not counting the closet space. Are there any heat pump water heaters with a remote heat exchanger? It would be very easy for us to run lines out from the water heater to the outside. It's right there.

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fwww.greenandsave.com%2Fgreen_news%2Fgreen_building%2Fsave_money_and_energy_with_geyser_water_heaters&ei=mo9lVdOsMceMNp77gMAM&bvm=bv.93990622,d.eXY&psig=AFQjCNGmj4C5cxKOoO5hSiXhjPISYHygoQ&ust=1432805643142354

Dunno what lineset length is.
Space requirement has to do with over cooling surrounding air which could still occur even with louvered doors. At the very least efficiency will suffer and recovery which is going to be slower than the old tank to begin with.

IMO, a fully louvered door (and the resulting convection) would go a long way towards keeping the temperature and efficiency reasonable. Exactly how much is hard to say - a few degrees? At this point, it may be more efficient that being located in a basement (typically a cooler place).