Neighbor's furnace exhaust
The water vapor is slightly acidic but not enough to damage plant life in my experience.  At 20’ horizontal and then 7-8’ up the carbon monoxide should be well diluted  but this is probably what you notice, 
almost all of the systems I have installed exhaust to the roof the one that was in a wall was at 9’ , but that may be a requirement for my area as it was specified on the prints. 

The one thing I would question is if the moisture stream is actually hitting your house?  if it is ask them to install a 45 or 90 to direct the moisture to the ground as horizontal moisture acidic or not could cause issues.