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Is it acceptable to route a bathroom exhaust fan to the return of a central air system? I'm afraid this will cause mold to grow in the air return ducts. There is currently a fan in our bathroom and I can't seem to find any separate outlet or vent for it, so I assume that it must be connected our central air system somehow. What is the best practice for routing bathroom fans if mold is a concern?

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It's entirely possible it's not vented at all. That would be a fantastic way to grow mold. – Steve Jackson Oct 13 '10 at 14:59
I'll get in there this weekend and see exactly where it goes. If that's the case I'm going to be mad! – Joel B Oct 13 '10 at 15:18
It could be a ductless fan too, we had some in a rental in Florida. Blows through a charcoal filter to remove smell but does nothing for moisture. example: kitchensource.com/bathroom-fans/br-682.htm – zk. Jun 1 '12 at 18:54
up vote 22 down vote accepted

A bathroom exhaust fan should vent directly outside with its own dedicated duct, for exactly the reasons you are stating.

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+1. Sometimes they'll just open into the attic; also bad. – Niall C. Oct 13 '10 at 14:27
Unfortunately it seems that it's standard practice to vent to attic. The only houses I've seen that were vented properly were ones where the homeowners themselves added the vent later... :-( – Brian Knoblauch Oct 19 '10 at 13:46

I have seen where a bathroom fan is run to a charcoal filter and then vented back into the interior of the house. A terrible idea, as you noted, it is a recipe for mold!

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While it may be acceptable to vent into the central heater during winter when humidity is low, it is a terrible idea year round. The humidity load of the bathroom vent air is just too high. You want an external vent. And you very much want to trace that vent... if it goes to the attic that's quite bad.

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