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I have an older house, the two bath exhaust fans are vented into the attic. I want to replace one of the fans and then link the two to a duct that runs out the side of the house. The distance is about 23 feet from the farthest bath fan, what is the best CFM fan to get? I was told more power the better, but Google doesn't seem to tell me what CFM will push air the furthest. Thanks for the help.
 

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Link the two and th odors from one bathroom will pushed into the other. I am not a pro, but I don't believe it's code inNA to link to bathroom fans together this way. But you are correct, you need to get the, out of the attire. I would vent them separately.
 

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My disclaimer here is that I'm just some homeowner that does my best, not up to speed with code or really even the best way to do much of anything lol.

That said... You can link them with one way flow valves. Draft blockers, damper or whatever they are called in the air flow world - basically check valves that allow air to only flow one way but,

1) codes may not agree
2) you may have issues running both at the same time.

If you're going to do it connect them with a Y rather than a T, and put the dampers on each segment before the Y. Hope that makes sense.

I'd go with separate and definitely take them both out of the attic. Go through attic and roof, or horizontal to a side wall and punch out your siding/brick but don't vent into the attic.

CFM is for how much air you need to move out of the bathroom really. There are formulas for how much you need based on bathroom sq foot size, how long a run you have to make, and how many bends/twists/turns etc the air has to make on its way out.

How many sq feet are the two bathrooms, and how many feet fun does each have to make - straight shot or U turns - would be the info you need to know to find out..
 

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need the watts, not the flowrate (CFM) of the fan. I don't know why fans are marketed with CFM, as we'll need the underlying assumptions. So, the more "checks", twists, turns, Y's and length you'll have, the more watts you'll need. The desired flowrate should stay somewhat constant for the volume of your room, it's the "drag pressure" which changes with each installation.
 

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Check the sites for Fantech and Broan. Both have charts that tell you what fans you need for such distances. I think Fantech also has switching system that allows for single fan that can vent multiple rooms. They probably are expensive to buy and to repair/replace if broken. For only 2 bathrooms, it's not a lot of work to have 2 ducts and 2 outlets.
For 23' the advice probably will be you need a second helper fan. It is called inline helper fan or such. Use steel duct, at least for the straight run, limit the bends and install ducts and fans with accessibility in mind. You do not want to bury these things inside a wall, if possible.
 
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