My neighbour just informed me that they had to change the vent on their high efficiency furnace. The furnace vents between our two houses and it used to have an elbow on it directing it along the walkway but she said the elbow was icing up and they had to change it. It is now directed straight at the brick on our house. She wants to lean a sheet of plastic, the type that's used as roofing material on decks, against our house to protect it. My question is, will the emissions cause damage to our brick over time? We are about four feet away. I don't want to have a sheet of plastic leaning there.
The combustion byproducts of natural gas also include nitrogen and sulfur oxides. Those oxides combine with other combustion byproducts to create acidic compounds.
If those corrosive vapors condense on your brick wall, it will do damage. That is why high efficiency gas furnaces can not use unlined brick chimneys.
I have a HE tankless boiler that exhausts via a PVC vent onto my driveway. I often park next to it and haven't noticed any damage on my car, though the car does move around a bit. However, where the exhaust has condensed and dripped onto a small concrete sidewalk below it, there is a spot with some clear corrosion. This is directly under the exhaust pipe, where water that condenses drips down.
Based on that I'd say you're at some risk of damage from corrosion, especially when your outer wall is cold and acidic water in the exhaust condenses onto it. That's during most cold weather, which is also when the furnace will run most often. At four feet, there might be enough distance for this to damage to be negligible; it depends on a lot of other factors like wind conditions. But there is a risk and you should at least keep an eye on it. Over time, it could eat through your bricks and mortar. But you could give it a couple of months and see.
If you're looking for a less ugly protector, a sheet of clear acrylic fastened to your wall should also work.
If you don't want to have a sheet of plastic leaning against your house, then tell her so.
Also I don't know what building codes are applicable in Toronto but in the USA most states use some version of the IRC, which requires vents with exhaust fans to be 10 feet from property lines or other buildings.