In my 100-year-old house, the hot water heater and boiler, both gas-fired, currently vent through an old masonry chimney (see photo below). I want to use the chimney for a new wood-burning fireplace and reroute the other two vent pipes (4" HWH; 5" boiler) through a side wall. Space is tight so I'm wondering if I can use a single vent pipe and a common connector -- or is there some prohibition against mixing the two kinds of flue gases? Also, can I use PVC instead of metal for any portion of the run?
Thanks -- John