I had two similar units side by side in a house with a smoking fireplace. When either kicked in, the Fp would smoke like crazy. As I remember it, there was about -15 Pascals WRT the basement while only about -5 to 6 WRT outdoors. While in the basement with one unit running, I held my CO analyzer near it and it starting climbing fast. It was backdrafting the unit 3 feet away. Not only that, it was backdrafting the DHW heater 6 feet away, which was B-vented. The Bldr's Super almost passed out! He thanked me and followed my recommendations on correcting the house and the fireplace then worked. The extra 12 feet of chimney they added on had no effect. As you make chimneys taller but they are now hanging in the breeze, it cools the stack so very quickly you hit a point of diminishing returns unless they also extend the chase to protect the pipe from the wind.

I've seen these intake snorkels sucking in fumes from nearby paint cans, laundry chemical fumes and dryer lint. This arrangement really ought to be banned. Should pull from outdoors 100%. At least it is usually fairly easy and inexpensive to correct.

__________________
disgusted with some people on this forum. Out of here!