Options for venting 90+ furnace

RM575

I will be changing out an old furnace and replacing it with a 91% downflow. The existing furnace is located in a 1st floor closet in the interior of the home. I have a 2 story home with crawl space below the closet. The existing flue is 4" double wall and vents up, approx 15', through to the roof. Outside air is provided into the closet from a 5' X 14" floor vent to the crawl space and an 8" vent in the closet ceiling that terminates out an exterior wall by a 10' horizontal run.

My questions:
The new furnace will use PVC to vent. Should I remove all 4" double wall that goes to the roof and replace the entire length with PVC?

Or, can I run the PVC to where the 4" double wall goes into the closet ceiling and send a short length up into the 4" vent?

What is normally done for venting when retrofitting with a newer furnace?

gregzoll

The instruction manual should show the specs on whether you can go down and then horizontal, or have to go up and horizontal. You do not vent 91% furnaces out of the past exhaust ductwork, due to that is not how they are made to perform.

RM575

Thanks for the reply. I agree, the best way is PVC all the way. I may remove the old metal double wall flue and replace it with the PVC all the way to the roof. As far as the combustilble air supply, there is an 8" round vent that goes horizontal about 10' in the finished ceiling to an outside wall. I don't see a problem with putting a PVC pipe the entire length of the round vent, as a raceway, to connect to the furnace.