Robert - Most installation specs I've seen call for the exhaust to slope back to the furnace. There should be a condensate pump or other means of dealing with the condensate inside. Having said that, I've never seen one slope that much. Try finding the install specs online. For instance, in the specs I have for a Nordyne condensing furnace it says: "Vent piping must be sloped upwards not
less than 1/4” per foot in the direction from the furnace to the terminal."