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Hi. Hope this is the right forum category. I want to install a bathroom fan in my master bath, which shares a wall with another bathroom that already has a fan exhausted to the outside. Can I just add a "T" to the other fans exhaust line and connect the new fan so that they both share the same exhaust line? My main concern was that the air being pulled through the new fan would come out in the other bathroom instead of going out through the roof vent. I hope this makes sense. Thanks!
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You could connect the two lines into one however you must keep in mind that a fluid (liquid or gasseous) will always take the path of least resistance.
If both the fans were connected to a tee with the side opening of the tee going to the outside discharge vent, the path of least resistance would be for the air to pass straight through the tee and discharge out the opposite fan.
Many fan fixtures have a small sheetmetal damper that is attached to the top of the air duct and swings open to allow air to discharge but falls downward and restrict outside air from backflowing into the fixture in the same manner as the flapper door in a laundry drier vent. In this case as the air tries to backflow to the second fan the damper would close and the path of least resistance would then by out the proper discharge vent.
The is another solution that would work much better. Instead of attaching the two lines with a tee attach the two lines with a very low angle Wye connection. In this manner the direct flow from either input would be directly towards the discharge line. In turn, the venturi principal states that as the velocity of a fluid increases the pressure decreases proportionally, thus by passing the powered discharge from one fan through the Wye, the direct line of flow through the WYE has the greater velocity and proportional decreased pressure. The end result is that the standard atmospheric pressure in the second input line is greater than the air pressure in the line of powered flow, thus the atmospheric pressure would then push from the second bathroom in toward the WYE rather than out toward the second bathroom.
Any HVAC sheet metal shop could easily assemble the necessary WYE for a nominal fee.
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