Some sort of heat exchanger?

During my third mock inspection I ran into this. The water heater flu goes through a box and out through the chimney. There is also a 4 inch vertical down tube that goes from the box down to about two inches off the floor. I assume that there is some sort of baffle in the box. Could this arrangement allow co into the basement. I am trying to post a photo, but I’m not having any luck. I tried the photo again…if it is here it looks like poor quality

Thanks for your help

Photo would help tremendously. Usually it is a problem with the size of the photo. Try resizing and attaching. The photo will not show up on your post until you submit it.

Download the “Image Resizer” tool if you do not already have it…

Looks additional draft hood.