I plan to use our existing masonry chimney for a wood furnace in the basement. So, I need to vent the propane furnace (4" single wall) and propane water heater (3" single wall) which currently use the masonry chimney. Can they be combined with a single power vent? What type should I be looking for? Would a 5" vent be sufficient so I won't need a priority switch? Thanks for any help. -Dan