Te epay o kirios?If you look at your install instructions for this furnace you speak of (installers should have left it with you the homeowner). The manufacturer allows side wall vent termination's with clearances listed to items such as windows, doors and overhangs.
If it is burning properly your CO level would be around 2 to 8% PPM (parts per million). There is other stuff in what we call “products of combustion”. Mostly it’s water. Did you know if you burn a 100K BTU furnace for 1 hr it will make 1 gal of water (H2O). The high efficiency units have a condensate drain to drain off some of this water. Some of it escapes out the flue pipe, that is why it has to slope back to the furnace so water inside the pipe can drain back.
80% AFUE furnaces also make 1 gal of water per 100K BTU’s in one hr. They are designed to vent 100% of this water out the flue pipe in the form of water vapor. So the 80% models keep the flue gases above due point until they exit to the outside. Sometimes when it’s cold you can see water vapor coming from these vent pipes too.
Hope this answers your questions.