Do you have the directions? What do they recommend. If not can You go to their web site?
I am not trying to PO you. With the new furnaces that are out with the ultra efficiency some are being vented with PVC, Others use double or triple wall ss or steel liners.
As Smoke suggested power vents, or side venting may be an option.
Last , what does your local building regulations state? I know you probably want them in your plan.....but should something happen it would be nice to be compliant even if you did not get a permit.....russ