"Mikepier" wrote in message
...
This is a roof mouted fan, not a whole -house fan.

I don't have issues with my furnace since in the cold weather, the
attic fan is off anyway. Its my water heater I have issues with.

I have a pull-down attic stairs in the second floor hallway. I know
it's not 100% sealed, but for the most part its pretty tight.

You also may want to look into sizing of the flue, meaning the chimney is
_too_ big. It may have been ok for the previous WH, but not this one.
Here's an article, which explains it better. Around here, we are not
allowed to vent into a brick or masonry chimney.
http://www.totalhomeinspection.com/hints_chimneys.shtml