I recently started to change the old single wall vent pipe on the furnaces in a duplex to b-vent since a friend had 90 % of the pipe I needed as extra. The problem I am having is the appliance adapters do not fit the exit on the furnaces. One furnace is an Ultra, I believe an Amana knockoff, the other furnace is a Goodman. Both are 80 + efficiency with vent fans. Do I need the Rheem-type flanged adapter to fit the B-vent to the furnaces?