I am buying a house that's been split into two apartments. One apartment has a new 80% furnace. The second apartment has an old gravity furnace that I am replacing with a 90% furnace. Both of the existing furnaces vent into the masonry chimney. I don't think that there is a condensation problem now since the gravity exhaust is so warm. However, when I install the 90% furnace, what is my best venting solution? Stainless Steel flue liner? Or can the 80% and 90% both go out a PVC vent?