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Thread: Parallel wiring for dehumidistat in bathroom

  1. #16
    DIY Junior Member tomtbone's Avatar
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    Quote Originally Posted by jwelectric View Post
    I am trying my best to figure out why someone would want to go to the expense of wiring something like this up unless this bath is in a moldy basement.

    Once the shower is over and you exit the bathroom leaving the door open the humidity is gone is just a couple of minutes.

    Could you explain why you want to make this illegal installation?

    The bath fan must be wired with an absolute off. An automatic means of starting the fan is a very bad idea unless there is means for an absolute off.
    First off, I dont think $25 more for the dehumidistat is really all that expensive and if you think it is, maybe you shouldn't even be doing a renovation in the first place.

    Its simply not true that all of the moist air will be gone in a few minutes, are you just trying to stir things up?

    How is having two parallel switches illegal? Anyone else care to chime in on that?

  2. #17
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by tomtbone View Post
    First off, I dont think $25 more for the dehumidistat is really all that expensive and if you think it is, maybe you shouldn't even be doing a renovation in the first place.

    Its simply not true that all of the moist air will be gone in a few minutes, are you just trying to stir things up?

    How is having two parallel switches illegal? Anyone else care to chime in on that?
    No I am just trying to figure out why anyone would go to all the cost and trouble doing something as worthless as what this set up is going to be.

    Last night I turned on the steam generator in our master bath and let it run for twenty minutes with the shower door standing wide open. Then I opened the door to the bathroom and the mirror was clear in less than three minutes. The entire 120 square foot bathroom was completely clear of steam in less than three minutes without the bath fan ever being turned on.

    So why would someone want to go to all this trouble?

    Automatically starting of an appliance in mentioned throughout the electrical code. It doesn’t take an engineer to understand that the auto starting of an appliance while being worked on is dangerous. As long as the switch has an absolute off then there would be no problem but if it did have an absolute off then what is the need of the toggle switch.

    Now if you have a situation where the moisture is a problem I would approach it with more than a bath fan.

  3. #18
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by tomtbone View Post
    Its simply not true that all of the moist air will be gone in a few minutes, are you just trying to stir things up?

    How is having two parallel switches illegal? Anyone else care to chime in on that?
    In backwards order;

    No one has chimed in because they can’t disagree with the automatic starting of the exhaust fan.

    Explain to me why you think it is so important to get all this moisture out of the bathroom when boiling water will cause as much or more moisture in the kitchen and you are not installing one of these setups in there. Is the place that you take a sh** more important than the place where you cook your food?

  4. #19
    Moderator & Master Plumber hj's Avatar
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    Most bath fans have a plug on the motor with a receptacle on the housing. THAT is the "positive" off on the motor. Just unplug the fan if you have to work on it.

  5. #20
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by hj View Post
    Most bath fans have a plug on the motor with a receptacle on the housing. THAT is the "positive" off on the motor. Just unplug the fan if you have to work on it.
    A good reply but not good enough. In order for the plug on the fan to fulfill the requirement that plug would have to disconnect the entire appliance. with the motor unplugged the junction box is still energized.

    Now would at least one person try to explain the benefit of such a set up.

    Simply by opening the door would increase the amount of air flow by 125% over what a 50 CFM bath fan is removing.
    Why would anyone want to waste all that energy by leaving a bath fan running? This is something I have never understood and would appreciate if someone could explain the reasoning.


    I have a 120 sq ft master bath and a 220 CFM bath fan but simply opening the bathroom door will clear the mirror almost four times as fast as letting the bath fan run. Which is more economical? Which is better for our planet?
    Last edited by jwelectric; 07-31-2010 at 02:22 PM.

  6. #21
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by jwelectric View Post
    Now would at least one person try to explain the benefit of such a set up.
    Must be one powerful hard question as no one has tried to answer in the past week.

  7. #22
    Retired Defense Industry Engineer jadnashua's Avatar
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    Small family, lots of people serially taking a shower in the same room, no windows, the door is normally closed, it's in a basement that is normally somewhat damper than the upstairs; any of those could justify a fan that can run when it needs to. None of them apply to me, but you are not there. It may also be that they don't have central air, and live in a humid area. Trying to exhaust as much moisture as possible is a good idea. This setup ensures it occurs.
    Jim DeBruycker
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  8. #23
    Electrical Contractor/Instructor jwelectric's Avatar
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    Okay I can understand how some of those situations would be cause for concern but with either of these would not a dehumidifier do a much better job than a small exhaust fan?
    Would it not be more economical to use a dehumidifier than to let a suction fan pull out the moisture? Remember for every cubit foot of air that is removed a cubit foot of air must replace that air or we will not be removing that much air in the first place. This is one of the biggest mistakes I see being made with exhaust air. We are trying to take more out than we are replacing which means we are only taking out what is being replaced. A 100 CFM fan that is only removing 40 to 50 CFM due to the replacement area wouldn’t be doing all it could.

    Maybe if we cut a foot off the bottom of the door it might work.

    404.14(B) must disconnect all ungrounded conductors to the load served would be what I would use to turn down the installation.

  9. #24
    DIY Junior Member mrd's Avatar
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    Quote Originally Posted by jwelectric View Post
    Would it not be more economical to use a dehumidifier than to let a suction fan pull out the moisture?
    I'm fairly sure an exhaust fan would be more efficient than a dehumidifier, even including heating/cooling of replacement air, maintenance and repair of equipment, etc.

    Quote Originally Posted by jwelectric View Post
    Maybe if we cut a foot off the bottom of the door it might work.
    Also, a passive intake to the rest of the house or outside air would work.

    Quote Originally Posted by jwelectric View Post
    404.14(B) must disconnect all ungrounded conductors to the load served would be what I would use to turn down the installation.
    The exception to 404.6(C) in 05 NEC appears to be applicable here. A permanent sign adjacent to the open switch with an equivalent message "WARNING - LOAD SIDE TERMINALS MAY BE ENERGIZED BY BACKFEED" would make it code-compliant. ..Though seemingly overkill with a humidistat present and breaker available for safe servicing.

  10. #25
    Retired Defense Industry Engineer jadnashua's Avatar
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    This is just an observation...we don't call a thermostat that is set to cooling a dethermostat or a de-heater. Why is it a dehumidistat? It is measuring humidity, not dehumidty...the fact that you have it set to turn something on to decrease the value is irrelevant. My two cents...
    Jim DeBruycker
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  11. #26
    Electrical Contractor/Instructor jwelectric's Avatar
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    It is 9;20 AM Wednesday August the fourth in the year of our Lord two thousand ten and a humidity of 94 percent in downtown Asheboro NC.
    Should I decide to supply outside air to this bathroom how would the weather affect the set-up outlined in this thread if the humidistat is set at 50 percent?

    EDITED TO ADD:

    Sorry but the following information is badly incorrect. The ICC building code says that the smallest bath can be 25 square feet and this will be with a pedestal sink and a cubic feet of 200.

    The room with 5000 cubic feet was copied and pasted from another site. Sorry for my mistake



    If this is the smallest bathroom allowed by the ICC building codes then it will have 5000 cubic feet. A 50 CFM exhaust fan would have to run for 100 minutes to replace the air in that bathroom.
    If The supply air was supplied from the rest of the house the opening at the bottom of a 30inch door would need to be about 5 inches in order to replace the air in the room in 100 minutes with a 50 CFM fan.

    Using the limited knowledge that I have I just can’t see where using an exhaust fan controlled by a humidistat could be more cost effective than a dehumidifier.
    It is my personal opinion that this is some half baked idea of a do-it-yourselfer that has caught on and is slowly becoming a fad and a costly one to our children and grandchildren.
    Last edited by jwelectric; 08-04-2010 at 12:12 PM.

  12. #27
    DIY Junior Member tomtbone's Avatar
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    Glad to see that the thread got hijacked.... oh well. Well my original thought was to have a switch in addition to the dehumidistat so I could simply flip the switch when going to the bathroom instead of turning the dial in order to turn the fan on. My wife and I have decided to go to the simple timer route instead which will make simpler for guests and for us.

  13. #28
    Nuclear Engineer nukeman's Avatar
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    jw: I think you have a typo there. For 5000 ft^3, that would be a 500ft^2 bathroom with 10' ceilings. A 5x8 standard bath with 8' ceilings would be: 5x8x8 = 320 ft^3

    I get your point. It is similar to an attic fan that has a humidistat on it. It might make sense when it is humid in the attic (from showers, etc.) and dry outside, but the humidistat does not know where the humidity is coming from. So, you get a humid day and the fan just runs all day pulling humid outside air into the attic and then pushes it back out.

  14. #29
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by tomtbone View Post
    Glad to see that the thread got hijacked.... oh well.
    Don't mean to hijack your thread it is simply I don't understand why someone would even want to leave the fan running in the first place. I was hoping to have you the original poster to explain this to me as you were the one doing the installation. Show me using math how this is doing one ounce of benefit. Show me one once of profit gain.

    In reality what you are doing is wasting our natural resources as the fan left running dose absolutely nothing to benefit you or your house. This means that off spring is slowly dying so you can blow air around.

  15. #30
    DIY Junior Member mrd's Avatar
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    Quote Originally Posted by jwelectric View Post
    Show me using math how this is doing one ounce of benefit.
    t is the duration of the shower in minutes.
    bw(t) is the amount of water added to the bathroom air during the shower
    bw(t) = aerated_water_per_minute * t

    iw(t) is the amount of water added to the indoor air during the shower, or after the shower when you open the door..
    iw(t) = bw(t)

    now if we have 3 occupants, and assume they exhibit patterns in their shower use, then we can empirically find an average daily shower duration for each occupant. we could call these a1, a2, and a3, each in minutes.

    the amount of water added to the indoor air daily, daily_water, can be calculated as such:
    daily_water = iw(a1) + iw(a2) + iw(a3)

    if we add an exhaust fan during the shower, and assume it removes 100% of the aerated water during the shower, then we can redefine the function iw(t) as such:
    iw(t) = 0

    then we can see daily_water also goes to zero.

    the benefit of not introducing bath water in the indoor environment is that it leads to a more stable amount of indoor humidity. if the indoor environment is humidity controlled, it can be designed with less dynamic constraints, which can lead to less expensive equipment investment and operation. if it is cold outside, and the bathroom shares an exterior wall, the chance of significant amounts of water vapor condensing within the wall cavity and leading to rot is decreased. the exhaust fan also doubles as a way of removing odious odors from the bathroom when the water closet is put to use.

    a fan uses less electricity to operate than a dehumidifier, and it doesn't require plumbing in a drain. it's also less prone to mechanical problems. if the outdoor humidity is higher than the bathroom humidity, it would still be beneficial to remove the concentration of humid air from the bathroom, be it with an exhaust fan coupled with a passive intake located in another open area of the house, or even blowing the air out to another part of the home. although simply opening the door right after you shower would probably do a well enough job of preserving your wall cavities.

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