Originally the 2nd floor chimney was located near the South edge of the roof. Our prevailing wind is from the Southwest, resulting in positive pressure on the South side of the house. During windstorms, the 2nd floor chimney would not draft, instead blowing smoke back into the house. That got old, so the 2nd floor chimney was moved to the center of the house and has worked fine ever since.
Last winter both chimneys plugged up in late winter. Actually, the chimneys weren't plugged, but the spark arrestor screens were plugged. Fortunately, we had a few sunny days that allowed me to climb up on the roof and clean them. But, I don't want to do any more mid-winter cleanings.
Winter is almost here again, so time to maintain the chimneys. They are already semi-dirty.
Looking into the cap.
Another pic of the cap. It got dinged when I tossed it off the roof rather than carry them down the ladder. It'll be straightened later.
Windows cut into the screen so there will still be some flow even if the screen plugs. Downside -- I may have to cover the caps in the summer to prevent birds from nesting inside the caps.
Modified chimneys doing their job.