Existing 80% Staying, Replacing Gravity W/ 90% Venting Question

Pittsburgh

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?

Bjd

90+ is a seprate flue to the outside, do not connect to the flue.
Leave the 80% where it is just block off the old opening where the old unit connected.

Benie

Unregistered

Thanks Benie. Also, do 80% furnace exhausts need a flue liner in an old chimney?

Bjd

Is the flue on the inside of the home? if so no liner is needed.
However if the flue is on the outside of the home, you may need a liner as the flue will not be heated by the old unit.

Bernie