Get a copy of the GAMA vent tables. Plus a copy of the IGFC.
It will have vent size tables.
As others have said. It sounds like your flue gas is cooling too much as it goes up the flue.
Oversize flues/chimneys cause much of this.
While not as often. If you install a properly sized furnace in a house that had amplely sized duct work. Its also posible to have too much air moving across the heat exchanger. And also cool the flue gasses too much.
When ever possible. If you can find an extra install manual for a furnace, weather the brand you install or not. Grab it, and keep it.
The install instructions for one, are not always the same as the next brand.