Condensing furnace in garage or attic safe for winter?
A condensing gas furnace needs to discharge it's condensate "waste" water.  If the furnace is in an environment that experiences freezing temperatures, this can become difficult. Typical condensing gas furnaces don't have provisions to protect the condensate line against freezing temperatures, so the line can freeze up and break.

This is one of the reasons you won't typically find furnaces installed in attic spaces in cold climates.