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It is done quite frequently and can be done. Generally the only thing you have to do, is to insulate the drain line. In extreme cold climates, you may have to put a self regulating heat tape on it as well. If the furnace condensate trap hangs outside the furnace (often they do when installed horizontal) it too will need insulated but thats as far as it goes.
What part of the country are you in?
NJ is cold, so I would use a self regulating heat tape on the drain and then insulate it. There are alot of 90%+ furnace installed in attics there. Some installers are afraid to, but when the precautions are taken, its a non issue.
HAve I answered your question or do we need to go deeper into it?