We are planning on a new gas furnace next month. We have gotten
several estimates and some conflicting advice. One person told us to
get a 90% efficient furnace. It will require venting from the side of
the house. The second person said that these vents can freeze up in
the winter and cause flooding in the basement and possible carbon
monoxide troubles. He suggested the older type, an 80% efficient
furnace which can use our existing roof vent.

Apparently the water being sent out by the newer types is from
condensation? This surprised me since our house is always SO dry in
the winter that we need to use a humidifier.

As for the problem with freezing - if we went away for a month could we
expect to come back to a basement full of water? Naturally we would
want to set the heat down low (65 or so).

The 80% furnace is cheaper but will require some type of air exchange
installed. Guess what we have doesn't meet code. That means that
getting the 80% won't really save us much money over the 90%. On the
other hand I don't want to have to worry about problems. That's why we
are getting rid of our old furnace in the first place.

Advice and/or experiences would be much appreciated.