High efficiency furnaces recognisable by the white PVC flue pipe, installed in an unheated attic is not your best choice of location, it may be the only place to put it which does not change the fact that it is not ideal. Yes, many builders and furnace installers are installing them in the attic without giving a thought to a long power outage during a super cold snow storm. They will work perfectly so long as you have power just like less efficient units that do not require a condensate drain. The problem is that should the equipment get super cold due to a long cold power outage the condensate may freeze causing the system to shut down until it thaws. So installers are putting heat tape wrapped around the drain pipes which may not always adequately heat the pipe. The best place for a high efficiency furnace is inside the habitable envelope.
