This gas furnace flue is not venting fumes (carbon monoxide) safely outside. Carbon monoxide is the by-product of the combustion process and is known as the silent killer because there are no indicators of its presence without a gas detector.
Must house here in south Texas have gas appliances that all create carbon monoxide, each one of these appliances is equipped with a flue that is rated for heat (b-vent). The job of the flue is to allow a draft to get that by-product out of the home and in the open air. The problem that is seen more often than not is the improper installation of flues. In the picture, you can see this flue is not connected at all to the elbow fitting right below the roof sheathing. Common practice is to use a foil tape at connection points but one bump can create this problem. Carbon monoxide is heavier than air and will sink to the ground so when the furnace is running, in this case, all that by-product is finding its way into the home creating a dangerous living environment for the homeowners. As homeowners, you need to walk your home once a year and check for safety concerns, be proactive and catch potential safety concerns.