I know that normally pex shouldn’t be installed within 6" of a single walled vent connector. This oil-fired water heater (and boiler) was power vented, which to my way of thinking, reduces the vent temperature considerably. Do power vented appliances have different pex/vent distance requirements? BTW - The AHJ approved the installation.
Almost all of the power vent appliances I have seen use PVC vent pipes and it does look like this one is as well. I don’t see an issue.
The vent connector is single walled metal. I was concerned about the pex being too close to it.
you require 6 inch clearance