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  1. #1
    Hi Folks,

    I'm noticing a little mildew on the ceiling in our master bathroom and some of the wallpaper is starting to peel in a couple of places. The problem being that there is no fan in the bathroom to remove the excessive humidity from showering and bathing. There is a fan in the separate little toilet room which I try to run to reduce this, but I think it's useless and I'm not sure that it's even blowing any air at all as it's not connected to a vent, but just open to the attic, and not even really open as there is a sheet of plywood over it...

    I was getting ready to have a power vent installed in there that I can run for a short time during and after a shower to remove that humidity but another idea occurs to me

    The master bath is directly over the attic where the HVAC unit is that services that part of the house. I write software for the home automation industry so my system knows when the AC is running or not, and even can guess when someone is taking a shower. I was imagining a system where the powervent ran if the heat was on, or if the AC wasn't running, but a system of dampers to instead connect the vent with the return air duct if the AC was on. Letting it get the warm humid air directly and not pulling in hot humid air from outside under doors or wherever by just running the vent. (there is no real makeup air in this house yet, I'm going to address that someday too)

    Would it be better to basically have a return air duct in the bathroom to de-humidify in there, keeping in mind that it would only be connected to the return air if the AC was actually running, or would it be better to just run the vent normally. I know that the air in there is likely to be much higher humidity than the outside, at least sometimes, and don't know if the AC can keep up with it. But then, I do live in coastal Georgia, and in another month or so the outside is going to be a lot hotter and more humid than my shower ever gets!

    What do you think, interesting idea? or just a hack to be avoided

  2. #2
    Join Date
    Jul 2004
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    322
    Interesting idea. Generally you don't want returns in bathroom, not because of humidity issue but because of smells. Also the general rule is that bathroom fans should vent outside to prevent mold and condensation in the attic.

    I also have a bathroom vent in the little room for the toilet. It is vented outside but I still get mold over the shower stall. I have one less exotic suggestion that seems to help. If you have a ceiling fan in the master bedroom, run it on high with the door to the bathroom and the door to the bedroom open. This creates a circular air movement in the bathroom and bedroom and moves some of the humid air out of both rooms.

  3. #3
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
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    1,209
    A power vented fan should be run in the bathroom area, not a return. Smells and moisture will contaminate the system. You can run a timer in the furnace that brings on the bathroom fart fan and install an outside air duct to the return with a powered damper that will change over fresh air and exhaust stale air. This system was used in super good cents homes with fairley good effect. So, install 4" duct to the return with a powered damper before the filter, install powered fart fan in bathroom then a timer in the furnace which brings on the furnace fan and the fart fan for approximately 6 hours per day, this will reduce the mold issue in your bathroom and provide fresh air for the rest of the house.
    Proud supporter of Springfield Millers and Oregon Ducks.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,376
    Install a bath fan adjacent to the shower area that's rated to move enough CFM's for the size of the room. Vent the bath fan and the fan for the commode compartment up to the roof (don't vent them together, you'll get backfeed if one isn't running but the other is plus other problems) and call it a day.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    For a quiet ,effective fan: http://www.fantech.net

  6. #6
    Join Date
    Jun 2004
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    Florida
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    556
    I doubt that your plan would pass code. Bathrooms are required to vent outside and you are not supposed to return through a bathroom. You would probably be better off installing a larger fan. In my experience the fans that are usually installed in residences are inadequate. You could automate this fan with a humidistat with a override for those stinky situations.

  7. #7
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    What might work

    JFS, I am a homeowner in S.Texas (climate not too different from yours) who also experienced mold in the bathroom shower. What solved that problem was a strong exhaust fan, I forget whether 150 or 300 CFM. We put a timer on it and only ran it 10-20 minutes when we felt the need. Makeup air would be fine in theory but our house was not very tight and we did not even know about it then.

    That Fantech solution would probably move air pretty strongly and solve your shower mold problem. I really suspect your current exhaust fan is a cheapie and not moving much air at all -- much like mine in a newer house. Check out Panasonic models if you want a high end bathroom fan which claims to still be very quiet.

    Everything I have ever heard says that returns in a bathroom are verboten. One other naive idea I would wonder if is a hack or not, might be -- a small powered duct from the drier part of the house *into* the bathroom. Smaller and more focused than running the whole central fan system. My thinking is the exhaust fan still intermittently removes stink, and the humid bathroom air will be diluted with drier air from the main house. Innovation or folly?

    I would be interested in knowing what relative humidity readings you are seeing in that bathroom. As always I tout the fact that $15 buys a humidity meter with thermometer, at Wal-Mart.

    Hope this helps -- P.Student

  8. #8
    Join Date
    Apr 2002
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    11,808
    Self edit, did not carefully read the OP
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  9. #9
    Join Date
    Aug 2003
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    Fort Worth, TX
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    One other naive idea I would wonder if is a hack or not, might be -- a small powered duct from the drier part of the house *into* the bathroom. Smaller and more focused than running the whole central fan system. My thinking is the exhaust fan still intermittently removes stink, and the humid bathroom air will be diluted with drier air from the main house. Innovation or folly?
    Such a rig, if allowed by building codes, would pose the possibility of pressurizing the bathroom space if the amount of air being forced into the space was not equally exhausted outdoors.
    A pressurized bathroom, with air laden with moisture from showering or bathing, would have air being forced into the walls. Not a good thing to have going into any wall where the moisture in the air might come into contact with materials at or below dew point (during cold weather). Boom...you got condensation going on in the wall cavity, which can lead to mold and compromised insulation.

    Should such a problem be overcome by balancing incoming air with exhaust air, I'm not sure there's much benefit from ducting air from one part of a house to a bathroom, especially if the bath is served by the a/c system in warm weather. The bath fan will draw air from the space it is in, with makeup air entering from adjoining spaces, such as a bedroom or hallway (under the closed door if it is sufficiently undercut), which will be "drier" because it is not in the same location as the shower.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    A few more details

    Shophound, thanks very much for giving serious thought to my naive idea. Namely transferring air via separate duct from the main part of the house, to the bathroom in an effort to reduce humidity there.

    >>Such a rig, if allowed by building codes, would pose the possibility of pressurizing the bathroom space if the amount of air being forced into the space was not equally exhausted outdoors.

    The problem of excess pressurization is one I would take very seriously, and would never want to cause. My guess is based on the notion that low airflow would equal insignificant pressure differences.

    I should have been more specific about the size of fan I envisioned. Perhaps one of those Panasonics rated for continuous duty and about 100 cfm. It is my thinking the air would return to the main part of the house via the gaps under the doors.

    The code part -- it seems to me inherently no more controversial than having central HVAC supply ducts in that room.

    Does that make it seem any more reasonable? Any other ill effects you imagine? Certainly it is not anything I have seen installed elsewhere, or written about. Keeping in mind that talk is cheaper than doing, I am wondering if that might be somewhat effective at homogenizing the air between the bathroom and rest of the house.

    Thanks very much -- P.Student

  11. #11
    Join Date
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    Ducting air from another room to the bath,might overpresurize it,remember there's no return in there,so supply air is already going under the door,and you are adding more.

    As far as no returns in the bathroom,consider a typical large master bath,lots of glass (facing west) around the tub.Load requires 200 cfm supply air ,exhaust fan required is 120 cfm.


    So how does the "stinky" air know to go out the exhaust ,instead of out under the door with the extra 80 cfms??

  12. #12
    Join Date
    Aug 2003
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    should have been more specific about the size of fan I envisioned. Perhaps one of those Panasonics rated for continuous duty and about 100 cfm. It is my thinking the air would return to the main part of the house via the gaps under the doors.
    That would be forcing moist air, possibly mixed with foul odors, back into the house. Seems self defeating to me.

    Most bathrooms, especially in our climate, are served by the HVAC system (unless you bought a 60's era econobox like I have. I had to add a duct to make the hall bath usable in summer). So, when the homeowner runs the standard exhaust fans found in the average tract house, if it moves any air at all, the air exhausted from the space will be replaced by air in adjacent rooms, and air entering the bath from the a/c supply register when the a/c system is operating. You could even say that when the a/c system is off, makeup air can enter the bath from the supply ducting when the exhaust fan(s) are running. That would be about equivalent to ducting air from another area of the house.

    I would think that in any case, you do not want air in a bath to needlessly creep into adjoining spaces. My thinking is to confine the excess moisture where it originates by exhausting it free of the structure. The makeup air from the house into the bath is already conditioned and cooled by the a/c system, plus the air that enters the bath from the supply register in the bathroom when the system is running, should in most cases be enough to keep the moisture levels in the bathroom controlled.

    Dash also makes a very good point about a CFM imbalance caused by a possible high heat load for a/c.

    [Edited by shophound on 03-29-2005 at 02:55 PM]
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    Considerations

    I highly appreciate your taking my proposal seriously enough to dissect it. Certainly if the bathroom exhaust does not effectively remove the stink, any supply at all will tend to spread it through the house. As a general wisdom it seems a rule to first make sure the bathroom exhaust is as good as it can be. Hopefully after that there will be no stink problem.

    My own instance has a few special qualities which may have caused me to not see some of the good points others have made. I happen not to have significant West-facing glass and so did not think about high heat load, but other houses may well have that in spades. As originally designed there was a ton of airflow that had no obvious return path, so now there has been added a 12-by-12 transfer vent through the wall. With this particular design the air passes through a *second* bathroom before entering the main house area. So far there has not been a noticeable stink problem (the toilet is in its own small room with a closable door). But I absolutely agree odor is something to always avoid.

    The main reason I burden you with these details is, that new return path apparently lowered average bathroom humidity 5-10% -- a result I did not anticipate and would not have predicted. So that put the idea firmly in my mind that further mixing might further lower humidity. Installing something like the AirCycler in this situation might be a help (homogenizing air is one of the original AirCycler goals) that is within conventional wisdom. It would seem that a specific duct "solution" would be a subset of the AirCycler idea, and provided I could avoid the pitfalls of stink and pressure imbalance... well I haven't quite put this heresy to rest.

    A rhetorical question... is it really superior to keep that humidity inside the bathroom or shower space? Will that increase or decrease the likelihood of problems vs. homogenizing the air through the whole house?

    Very much appreciate the comments -- P.Student

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