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I belong to a housing cooperative in the arctic and the temperature drops below -40 C. We are having problems with mold on the ceilings in the bathroom. The buildings were built in 1984 with an air ex-changer that is suppose to take air from the bathroom but they do not work well or for some not at all. There are no operable windows in the bathroom. I live along and washed my ceilings done in 2006 with Javex and painted, I leave the door open and the mold has not reappeared. Some of our members are worried it is black mold and demanding an exhaust fan be installed. We are hesitant at putting in a fan to exhaust to the outside as it will draw a lot of warm air outside and cause an escalation in heating costs. My question: can the exhaust fan be installed to draw the steam from the bathroom just to the outside of the bathroom into the hall area? Would this cause any problems? We live in a very dry climate and I thought perhaps the moist air from the bathroom would be good for the rest of the house. Many thanks for your help with this.

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The issue is high moisture, and is why you usually want that exhausted OUTSIDE the building. But in the arctic, I do imagine you have extremely dry air, so perhaps just leaving the door open and/or putting a floor fan near the door would be OK. –  DA01 Oct 11 '12 at 17:26
    
Have you replaced the filter on the air exchange unit? Where does it exhaust and return air in your home? –  BMitch Oct 11 '12 at 17:39
    
Not according to International Residential Codes (IRC) M1507.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building. –  Tester101 Oct 18 '12 at 17:08
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We have dry cold winters as well, -35C, but not arctic. We exhaust bath moisture outside, but our fuel costs are pretty cheap. It would be worth estimating just how much extra heating costs are incurred by exhausting outside, it may not be that bad. Or a good argument why not to go outside. Excessive fan use can be limited by installing timer or occupancy switches.

I suspect the air exchanger doesn't work well because it ices up. It's worth investigating the cause though, in case it is just a clogged filter. A direct exhaust ices up outside around the outlet instead of inside the exchanger. The ice build up can be dramatic after many weeks below freezing and no sun. Another reason against outside exhaust besides being inefficient.

I don't think exhausting from one small space to another is a good idea. If your idea is to work, it would need to exhaust into the largest space. Perhaps above a heat source. I think some will find this objectionable, it's a bit unsanitary sounding. Sort of like using your roommate's damp bath towel. Nothing really wrong with it, yet quite unappealing. Especially if someone used the fan to remove odors instead of moisture! Eww. It's also possible that despite the dryness, heavy moisture load may just be too much to be dispersed within the dwelling.

You should focus on a workable air exchanger solution first. It is the only solution that is both energy efficient and fully sanitary. A direct exhaust may turn out to not be overly expensive, but it's still inefficient and should be avoided. Moving the moisture around inside may work, but should be the last resort, it just sounds a bit unsanitary, and it may not even work in all cases.

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Definitely good call on the timer/occupancy switch. –  gregmac Oct 11 '12 at 22:59
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