I am installing a 95% condensing gas furnace to replace an oil furnace. I want to run pvc vent pipe from the gas furnace up the old chimney which is not used by any other appliance. The problem is that with the ac coil installed under the gas furnace the gas furnace vent is above the opening in the chimney. The furnace installation instructions appear to say the vent pipe can be run a maximum length of about 65 feet horizontal or vertical. I guess this is determined by the strength of the induced draft blower. To get to the old chimney opening I will have to go horizontal about two feet down 8 inches horizontal 6 inches and then vertical 14 feet. Can the blower handle this? Will I get condensation in the lower horizontal leg? The chimney is cinder block with a flue pipe liner making a new hole difficult.The advantage to this is it keeps the vent pipe out of the way. A horizontal run to outside means the vent pipe could later be enclosed if garage is finished and going up thru the attic means it is susceptible to damage when moving stuff around up there.
You’ll get a lot of condensation in this downward vent section to the point that it will completely block exhaust discharge. Assuming that the manufacturer of your furnace brand permits downward venting you’d need to install condensate trap on it. Check the manual for it or look at their website. Click the following link for a drawing of the American Standard 90+ downward vent pipe installation requirements – Downward PVC vent pipe installation American Standard
Let me know if you have any other questions.