You should be able to vent the new furnace out through the roof as you did the old furnace. You may have to install a stainless steel liner. This is a fairly common practice.

Ah - ok I was hoping that was an option. Do I have to run the PVC pipe the lenght through the existing B vent up and out the chimney cap?
Many thanks

Yes the PVC would effectively be the liner. I am not sure you would use PVC. There are flexible liners made for this purpose that would be easier to install.

I turned on the DVR to set a record time last week. I watched for a few minutes as they pulled a vent system down the old chimney for a high efficiency gas furnace. It was flexible and had integral, flexible stand-offs that center it in the chimney cavity. The combustion air is drawn down through the cavity in the chimney around the plastic pipe for flue gas. The new chimney cap snapped onto the flue gas pipe and had vents around it for air intake.

Maybe this is what mike_home wrote about.

Ionized,

I saw the same episode of this Old House. That set up may work.

I was always under the impression that high efficiency furnaces had to be vent out the side of the house. But a PVC pipe up through the roof also works. I think the key is the connections have to be water tight due to the condensation.