Cold-air return ductwork not needed on high-efficiency furnace?
Here in Kansas City, MO, I qualify for a free home repair and so they will be replacing my furnace and ductwork. A guy came out and took pictures of the current furnace and ductwork and asked if I wanted a standard 80% efficiency furnace like the one already installed, or if I wanted a 97% efficiency model. He explained that with the high-efficiency model, the exhaust would go out to a nearby side-wall instead of through the roof, and there would be a fresh-air intake installed on the side wall too. He also said they would not put in any cold-air return ductwork, just leaving existing holes in the floor without any ductwork attached.
This is a 2 story house with basement. Currently, there are 8 vents, 4 on each floor, and 2 large cold air returns on the first floor, but none on the 2nd floor. All the rooms on the 2nd floor are closed off, but have fairly big gaps in the doors that do allow air-flow. I don't know how well the current system works because the furnace has always been "red-tagged" by the gas company.
Does there way make sense - just remove the existing cold-air return ductwork and not replace it when installing a high-efficiency furnace?
I would prefer they installed new ductwork up to the 2nd floor for cold-air returns in all the 2nd-floor rooms accept the bathroom. Wouldn't that be a better way to go and also keep the 2 cold air returns connected to ductwork on the first floor. Wouldn't that be a better install?
In general, why would they install cold air return ductwork on 80% efficiency models, but not on 97% efficiency models?