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What's the purpose of a bathroom fan with no outside venting? One of my bathrooms is set up this way. I'm about to redo the ceiling, and I'd like to replace the fan unit with something better looking, but confused why a fan would not vent to the outside?

Is it ok to replace with another no-vent unit? Should I look into other solutions?

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IMO, and IME, bathroom fans should always be vented outside. Bathrooms are one of the biggest (if not the biggest) producer or water vapor in your home. When water vapor is trapped is can cause things like mold, mildew, damage to furniture, added difficulty in conditioning the air, and many more.

Just because a bathroom vent is not currently vented to the outside, does not mean it can't ever be. The house I'm currently living in had no ceiling exhaust fan or light (just a vanity light).

In this situation I was able to wire in and install a new unit, with manufacturer recommended vent line and exhaust this line through the closest soffit.

Depending on the layout and positioning of your bathroom, you could run through above joist spaces until you reach a soffit, run into the attic and then out a vent, or simply through the nearest exterior wall. Many times the shortest route is the simplest.

If this is something you aren't comfortable with, I highly suggest finding someone who can like a general contractor, trusted handiman, etc.

Keeping excess moisture out of your home will pay large dividends in the problems you're preventing.

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If you live in an area covered by International Residential Code; or similar building codes, it's required that the fan exhaust to the outdoors.

See this answer, for exact code verbiage.

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40 years with no problem and bathroom fan only vents to attic. I have a ridge vent and attic fan which I turn on periodically in winter and almost continuously in summer.

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I recently installed a vent fan in a bathroom that my kids use. I hooked it up to the electrical line used for the lights in the bathroom so that the fan would go on when someone turned on the lights. I was able to cut out a 4" hole in the roof right above the bathroom for the vent hose and installed a vent cap on the roof. It is best to vent to a location to keep the hose as short as possible, minimal turns and use an insulated hose if you live in a cold climate. Venting through the soffit is not recommended if you have soffit vents and ridge vents as the steam is likely to get sucked right back into the attic via the soffit vents.

One of my bathrooms is vented directly into the attic which is not good practice. But in 30 years, there has been no sign of any steam related damage in the attic or mold. I am guessing this is due to the steam going out via the ridge vent in the roof.

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