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Thread: Parallel wiring for dehumidistat in bathroom

  1. #1
    DIY Junior Member tomtbone's Avatar
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    Default Parallel wiring for dehumidistat in bathroom

    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?

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    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
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    Electrical Contractor/Instructor jwelectric's Avatar
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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.

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    DIY Junior Member tomtbone's Avatar
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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.

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    Moderator & Master Plumber hj's Avatar
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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. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by hj View Post
    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.

    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. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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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 by jadnashua; 07-25-2010 at 03:20 PM.
    Jim DeBruycker
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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by jadnashua View Post
    All switches are in series with their loads...not sure what your point is.
    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

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    Retired Defense Industry Engineer jadnashua's Avatar
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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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by jadnashua View Post
    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.

    precisely .

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by tomtbone View Post
    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.
    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.

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    DIY Junior Member tomtbone's Avatar
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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.

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    Electrical Contractor/Instructor jwelectric's Avatar
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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.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by jwelectric View Post
    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.
    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.

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