venting high efficiency heater

i'm thinking about getting a high efficiency furnace instead of replacing my gas furnace, but i'm not completely familiar with the direct wall venting requirements.

i was told that you need 6' clear (both measured vertically and horizontally) from all windows to do it. if i don't have the 6' clear (there is a window opening located 3' up and 4' over from the only possible venting location), do i have any other options?

thanks ahead of time for any help.