What does the Ontario Building Code require in terms of distance to windows / doors when venting a natural gas appliance?
I have heard 6', however I do not have a single point on my exterior walls in which I can vent 6' away from an operable opening. The best I can do is a little over 3' horizontally from the main entrance and 1' from the garage door. The garage is attached and is sealed from the rest of the house by an exterior-type door and is unheated. So the garage door would not be considered an operable opening into the house as far as I'm concerned.
The manual for the unit advises 2' from any operable opening. It is a Bosch tankless water heater so it won't be running all that much.
I believe the 6' is for power vented products not direct vented products. As Beenthere stated check the local codes officials. If you need help I could make a phone call and find out Monday morning. I am sure the rep we have would know.