Is it ok to have high efficiency condensing furnace in the garage or attic of a house in an area that experiences below-freezing temperatures? I've heard that having a furnace in a spot without "conditioned air" can lead to frozen components that somehow break the furnace.
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.