[BJM]

Most of the people on this list have a much milder winter than where I am and some of what I say may be somewhat different. In my case the cool down of the house from 70 to 55 takes place in 6 hours. The heavy objects in the house barely change. Upon entering my house at around 58F one time, the furniture was still fairly warm, the outer walls and the air are what cooled off fastest. So I still save energy. The potential energy contained within the furniture doesn't change nearly as much.

As far as condensation goes, all high efficiency units I am familiar with (and they may be more common here) have 2 sections to the heat exchanger, one section of which is always stainless steel. They are condensing style furnaces. They always have water collecting in them. They drain water when running like an air conditioner. Here at least, they are easy to spot since they do not require chimneys. They use side wall exhausts and a powered fan pumps the exhaust outside. The temperature of my exhaust is barely warmer than the air in the house, the exhaust is soaking wet and forms an enormous 20 lb stalagmite of ice right below it in January.

Where I live, people have largely gotten rid of non-condensing furnaces since they are inefficient and going from mid 80's % efficiency to mid 90's% is worth hundreds of dollars per winter.

If you have a non-condensing style furnace then I would agree you should not drop your house temp too much, but you would still save energy.