Parallel wiring for dehumidistat in bathroom

Discussion in 'Electrical Forum discussion & Blog' started by tomtbone, Jul 24, 2010.

  1. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Must be one powerful hard question as no one has tried to answer in the past week.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

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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.
  4. mrd

    mrd New Member

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    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.

    Also, a passive intake to the rest of the house or outside air would work.

    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.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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...
  6. jwelectric

    jwelectric Electrical Contractor/Instructor

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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: Aug 4, 2010
  7. tomtbone

    tomtbone New Member

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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.
  8. nukeman

    nukeman Nuclear Engineer

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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.
  9. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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.
  10. mrd

    mrd New Member

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    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.
  11. tomtbone

    tomtbone New Member

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    tomtbone in Boston
    Now we just need a mathematical equation to show that my bathroom fan is going to kill our children. Lol
  12. jwelectric

    jwelectric Electrical Contractor/Instructor

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    To address a couple of the issues you mention in such an impressive yet tentative post let me start with the issue of moisture trapped on the interior of your bath walls.
    This would lead some to believe that someone is spending way too much time in the shower. How big is your water heater?

    Then again if we are talking about moisture on the interior of the outside wall due to bad aim then the exhaust fan won’t help remove much of this moisture but having a mop handy will help a lot.

    I hope that you understand the term efficiency or Efficiency = (output / input) * 100

    The efficiency of a dehumidifier over an exhaust fan is at least 300%. This means that the dehumidifier will cost about 1/3 that of the fan over a length of time.

    You can go to this link and enter the correct numbers to see what we are doing to our off spring and the air that they will have to breath. This is not including the natural resources we are using up that they won’t have. But I guess it will be alright just because I am trying to fix something that ain’t broke or I just don’t want to close the door till the order goes away.

    http://www.greenprogress.com/carbon_footprint_calculator.php
  13. nukeman

    nukeman Nuclear Engineer

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    Basically it boils down to what the conditions are as far as what is best to use:

    - Say it is winter. Typically, the indoor RH is low during this time and often people will run a hunidifier to increase humidity. In this case, it is much better just to open the door. You increase the humidity in the rest of the house and not pump the conditioned/humid air out that will just need to be replaced with heating and adding humidity to the air that replaces the air that you just pumped out.

    - Now say it is summer. What option is better depends on your location. In the east, it will tend to be humid outside, you'll probably have AC running to control heat as well as indoor humidity. Turning the fan on will pump out conditioned air (replaced with warmer/humid outside air). If you open the door, you won't lose the conditioned air, but the AC will have to work to remove the mositure from the bathroom in order to bring the temperature down further (humid air holds more heat than dry air).

    -In the spring/summer where your indoor RH is okay and you are not using AC/heat, it might be better to use the fan. It really depends on what the RH is in the rest of the house.


    One thing that is interesting is looking at what happens during the shower. Say you start out at 50% RH in the bathroom. You turn on the shower and the RH quickly approaches 100% RH. As you approach 100% RH, the rate that the shower produces the vapor decreases. Nature tries to balance things by going from a higher concentration to a lower concentration. At the water droplets coming from the shower head, the RH is 100%. At first, when the shower just starts, the water vapor transfers the quickest. As the RH of the room goes to 100%, the vapor generation goes to zero (that is why you have a hard time staying cool in humid weather..evaporation pretty much stops). Once the room hits 100% RH, the vapor that is replaced from the shower is only to replace what is lost out of the air (condensation on walls/mirror, air lost under the door, air lost with fan). So in fact, running the fan may help decrease the RH in the bathroom (depends on water temperature, droplet surface area, air RH, bathfan real CFM, RH of makeup air coming in from under the door, etc.), but the fan will also increase the rate of evaporation (more water being put into the air).

    Unless your fan or dehumidifier can keep RH below 70% with the shower running (doubtful). Trying to extract moisture during the shower is pretty much a lost cause. Best bet would be to open the door or turn on the fan after the shower is done. Best way to control bathroom mositure is to take shorter showers and/or run the shower at a lower temperature. Drying off shower walls and drying off walls/mirrors that have condensation after each shower will help a bunch in controlling mold/mildew/moisture damage.

    Personally, I like timers for fans. With a switch, you may forget it and leave it running all day (especially with newer/quiet fans). Running it automatically may cause it to run when it isn't really needed.

    Technically, once the room hits 100%RH you will still be able to evaporate water, but that is because the air temperature is also increasing (warmer air can hold more water).
  14. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Well put Nukeman

    The idea of having the exhaust fan blow for any extended amount of time is a waste of resources. It does absolutely nothing to improve our existence as a human race. It does nothing to help preserve our framing members. It does nothing to prevent mold and if it stinks that bad then we might think of joining our friend the bear and using his bathroom. How about doing math like adding a little water to subtract a little she it.

    The idea that this extra blow helps preserve our home goes into the same category as dentures for chickens, I think that sun glasses for bats would be a better idea. Why not open a shop selling scuba gear to fish.
  15. chris8796

    chris8796 New Member

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    It sounds like you better turn your AC off so you don't cost your grandchildren any more grief.

    On a side note, I'm not a fan of the humidty controlled fan idea. Although I do use an exhaust fan in the summer. The best solution I have found to the problem is to run the HVAC with the fan on continously during periods of likely shower use (6-8 am, with programable thermostat with independent fan control). It also moderates the temperature with all the different activities going on. Although the solution to your situation is dependent on the local variables and not something that can be generalized.

    Another issue I've had with humidity sensors is they are less than reliable and have a poor response time.
  16. Christopher51

    Christopher51 Tri-County Home Inspection Service, LLC

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    Columbia, SC
    My goodness. This has all been made so complex. Just put in a timer switch. Easy, yes? :)
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