Worse yet in residential hvac systems, you have a closed system. When the furnace kicks on, it pulls inside air to heat it up. That air displacement creates a a negative air pressure inside the home. The exaust plumbing is typically nothing more than pvc pipes that are placed right at the exterior wall of the house. This just causes all that exhaust from the furnace to just get sucked back into the home. Diluted with outside air, yes, but a significant amount of it is getting sucked back in. Now it's winter, you are running your dryer, and frying some bacon on the stove with the exaust. Some one is in the bathroom with the exaust fan on too. All that co2 is getting pumped right back into your home and then some.