I recently replaced my old furnace and water heater and have a question about how they should be vented. The furnace is an 80% 75K btu unit. The water heater is a standard 40 gallon unit. Currently, these two appliances are venting into a common flue. There is a 4" vent pipe from the furnace and a 3" vent pipe from the water heater that connect to the common 5" flue (type b vent), which extends about 15' vertically to the roof. The furnace connects to the common flue below the water heater. Should these two appliances be vented together into a common flue or vented separately? So far the contractors that have provided bids say that a common flue is ok and venting separately isn't necessary. However, some of what I've read on the internet says that these two appliances should be vented separately. Can someone help clarify this for me? I am asking this because I am particularly sensitive to indoor air pollutants and am concerned that common venting the furnace and water heater could allow leakage of the flue gases into the living space.