I have a power-vented hot water tank that vents outside the house through some brickwork. It is all on an upward slope. However, the exhaust is calcifying/corroding the brickwork around the vent. Is it possible to add pipe to the elbow (outside the house) to extend the vent below the brick (but still 12" above grade)?
Do you still have the installation manual? It should specify what the toal length of run for the venting can be, and how many elbows etc. that you can use.
Generally, you aren't allowed to run your venting downwards, only upwards. It would also be a concern as to how many feet of venting material would end up being outside (it could freeze up in the off cycle)
I have used a reducing coupling at the vent termination to increase the velocity of the exhaust stream to push it further away from the house with some measure of sucess in the past, but you may want to check with a local gas fitter and ask what options are permissable in Ontario.