My neighbor's house sits 20ft from mine. Their furnace exhausts out the side of their house about 3 ft above the ground. The exhaust is blowing onto the side of my house and into the attic vent. Their furnace is a newly installed unit. Is this dangerous or can it damage my house?
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.