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I am doing a basement bathroom reno and wish to install an exhaust fan in the ceiling. Since I have had the furnace replaced and now its direct vent out the side of the house , I have an existing furnace flue in the area that is no longer in use.(5 inch) Is it possible to vent the exhaust fan through this ? As there is windows, furnace vents,gas lines at the side it is almost impossible to add a vent on the side of the house. The rise for the old furnace flue is about 16 feet and I will need to add a couple of elbows to get to the flue.

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I see no problem with using the old furnace flue for the bathroom vent as long as that vent has no other connections such as off to a gas water heater.

The 16' rise of the old furnace flue can represent a challenge for a small bathroom fan to push air through. The column of air presents a back pressure that the fan must overcome in order to be able to produce positive upward flow out of the flue pipe. A small capacity fan may be able to cause air flow to occur but it may be way less that what is needed to adequately vent your bathroom.

So I would suggest selecting a bathroom fan that has a greater than normal flow capacity than would be needed for your installation under normal circumstances.

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