Parallel wiring for dehumidistat in bathroom

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

  1. tomtbone

    tomtbone New Member

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    Just want to make sure this is the correct schematic for what I am doing.

    I am installing a dehumidistat in the bathroom for the exhaust fan along with a switch. I want either/or to be able to turn it on.

    i.e. Turn on the fan with the switch when you first get into the bathroom before you start taking a shower. You turn on the shower and the dehumidstat closes its part of the circuit. I'm done with the shower and I turn off the switch, but the fan is still running because the dehumidstat is still closed. Eventually the fan will turn off once the humidity level has gone down. I basically just want them to run independant of each other and not interfere with each other. Is this diagram correct?

    fan.jpg
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Conceptually, yes, it is correct. But, with the electronics in the humidistat controller, it may need neutral to be wired to it, and may not like the by-pass on-off switch. You'd have to read the specifics of that switch. If the humidistat switch was entirely mechanical, then yes, it should work, but it may have electronics which would need power. It would normally say if it needed neutral. You may have to call the manufacturer to verify it wouldn't be damaged by power on both ends when it wants to be off unless it says something in the tech sheet.
  3. jwelectric

    jwelectric Electrical Contractor/Instructor

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    It looks like the humidistat is wired in parallel with the switch and series with the exhaust fan.
    If this is true then neither of the appliances will last very long as one of then will burn up due to low voltage.
  4. tomtbone

    tomtbone New Member

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    Well I have read in multiple places online of people using this with another switch so that they can have manual and automatic operation. I'm just trying to figure out the best way to wire it that way.
  5. hj

    hj Moderator & Master Plumber Staff Member

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    quote; If this is true then neither of the appliances will last very long as one of then will burn up due to low voltage.

    You could have as many switches as you want wired in parallel as long as you remember which one to turn off again. And ALL switches are wired in "series" with the device if you expect it to operate the appliance. NEITHER situation changes the supplied voltage.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    This will probably work. But, as I indicated, some switches (those that have electronics in them) often need a neutral attached for them to operate. It may or may not like having power on both ends of the switch when the parallel path is energized, but it may not care. You may want to look for an integrated humidistat switch that would let you do what you want with one device, rather than guessing if it would work with two. What brand and model are you thinking of? Have you read the spec sheet for it? It may even have a diagram of how to do exactly what you want. There should be no problem with series voltage drops wired the way you have it. The only issue is if the switch would actually function without a neutral, and if it would survive with power applied to the output when it is trying to open to shut the device off.
  7. jwelectric

    jwelectric Electrical Contractor/Instructor

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    But should you wire an electronic switch in series with the bath fan one or the other will let it’s smoke out and we all knows that it ain’t gonna work without its smoke.

    If this switch is a mechanical switch then it is going to be rated in volts and amps.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    All switches are in series with their loads...not sure what your point is. Some electronic devices won't like power applied on both ends, but some don't. Most will need a neutral to function, but not all. The humidisat might even be mechanical, and then it wouldn't matter at all. The best choice would be to find a humidistat that allowed a manual on function as well as acting like a humidistat. Until he can provide the specs of the switch he wants to use, there's no way to tell if it would work reliably or not. We're talking about two single-pole switches wired in parallel connected in series with a load.

    Googled a few switches...some of them are all mechanical and use spst switches to apply power. One like this should probably work fine. Again, though, check the spec sheet and the manufacturer to be sure.
    Last edited: Jul 25, 2010
  9. jwelectric

    jwelectric Electrical Contractor/Instructor

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    electronic switches will need to be wired in parrallel with line voltage while controlling the series circuit. To connect the electronic part of the switch in series with the load being controled will burn the electronics out in the switch
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    We still don't know if the switch is electronic or mechanical. Some of them have mechanical switches in them. Those with a digital readout could have a relay to control the output (mechanical). So, until we know what switch he is interested in, it is all supposition whether it will work or not.
  11. jwelectric

    jwelectric Electrical Contractor/Instructor

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    precisely .
  12. leejosepho

    leejosepho DIY scratch-pad engineer

    As has been said, what you are trying to do will work just fine as long as your switches are mechanical -- two black wires only (plus maybe a ground screw (green)) -- and not electronic. And if your regular fan switch is a timer, it will have to be a spring-loaded twister in order to not be electronic.
  13. tomtbone

    tomtbone New Member

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    I received the dehumidistat in the mail today. It seems to be pretty straightforward with just two black wires in the back to connect inline. I think this should work with that first diagram. I'll probably hook it all up before walls are closed and such. This is the one I got and here are the instructions that came with it.

    Ranco J10

    IMG_0003.jpg
  14. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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.
  15. leejosepho

    leejosepho DIY scratch-pad engineer

    Anybody can at any time walk in and adjust the dehumidistat to "Off" just as surely as can be done with even a portable dehumidifier.
  16. tomtbone

    tomtbone New Member

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    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?
  17. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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.
  18. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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?
  19. hj

    hj Moderator & Master Plumber Staff Member

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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.
  20. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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: Jul 31, 2010
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