bill wrote:
most of the time you see the vapor cloud above the metal chimneys but
this one had a stream of water. nothing like it on the other similar
houses. maybe he's got a real humid envirnoment inside?

Possibly, but since metal, I'd venture to guess it may be double-walled
where others aren't and so is condensing more external moisture than
others.

Also, relative efficiency of the furnaces could have an effect -- a
low-efficiency furnace would use more fuel in a comparable house...is
there a cap, perhaps that is a condensation point? If it's
flue-moisture, it should just be vented as steam--the water (if from
combustion or internal humidity) has to have some surface on which it
either condensed or collected to run from.