I have a condensing tankless water heater and a condensing furnace. The WH vents using 3″ PVC for intake and exhaust. The furnace uses 2″ PVC for intake and exhaust. I would like to combine the venting for these units, and use 1.5″ pvc pipe for intake, and 1.5″ pvc pipe for exhaust and connect both units to those vents. Is this OK to do? It seems logical.