# Attic Fan Bypass & Kill Switch



## Nealtw (Jun 22, 2017)

I would use two regular switches. The bypass switch to put power to the fan.

Use a 3 wire to run another line to the controller and back to the second switch and from the second switch to the line side of the first switch.
So the controller will still have power but will be disconnected from the motor.
A thrid switch could be added to kill everything.


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## rjniles (Feb 5, 2007)

1 switch: on, center off, on.


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## sonnaps (Jul 21, 2015)

rjniles said:


> 1 switch: on, center off, on.


Yes I thought about this too but it still would be the same situation back by the thermostat with the power feeding back into it, no? The only reason I'm not going this route is because I plan on using some z-wave smart switches in this setup. I can't find a z-wave center-off double throw.


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## sonnaps (Jul 21, 2015)

Nealtw said:


> I would use two regular switches. The bypass switch to put power to the fan.
> 
> Use a 3 wire to run another line to the controller and back to the second switch and from the second switch to the line side of the first switch.
> So the controller will still have power but will be disconnected from the motor.
> A thrid switch could be added to kill everything.


I think I get it but I would definitely need that third switch, right? Idk if I want to do that.


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## Fishbulb28 (Jul 8, 2016)

sonnaps said:


> I would be bypassing the thermostat and powering the fan directly, but wouldn't power feed back to the thermostat as well? What I read was that the power will follow the path of least resistance, which is the running motor on the fan, and there wouldn't be a problem. Nothing bad would happen to the thermostat. Is this true?


These thermostats are dumb mechanical devices. You can treat it as just another SPST switch. Voltage on both terminals simultaneously will not damage it.

As for power following the path of least resistance, that's an often repeated statement that is not true in the slightest. Current will follow ALL paths available to it in a complete circuit.

Consider a simple circuit consisting of two incandescent lamps wired in parallel. One lamp is 25 Watts, the other is 150. The impedance of the 25W lamp is six times that of the 150W.










The circuit path through the 150W lamp is the path of least resistance. Yet both lamps light fully. How is this possible?


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## sonnaps (Jul 21, 2015)

Fishbulb28 said:


> These thermostats are dumb mechanical devices. You can treat it as just another SPST switch. Voltage on both terminals simultaneously will not damage it.


Ok, this makes sense to me and now I feel confident that this setup will work just fine now.

I am curious, though... isn't there some type of device (I don't know what it is or what you would call it) that I can put between the 14-3 cable and the thermostat to prevent power from feeding back to the thermostat? Like some type of relay maybe? When not energized, it would stay normally closed and complete the circuit through the thermostat letting power flow through one set of terminals. But when energized by 120V though, it would open up and allow power to flow through a different set of terminals.

I attached a revised sketch. I'm sure the mystery box I have there is incorrect and not laid out properly... but just for a visual.


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## Nealtw (Jun 22, 2017)

sonnaps said:


> Ok, this makes sense to me and now I feel confident that this setup will work just fine now.
> 
> I am curious, though... isn't there some type of device (I don't know what it is or what you would call it) that I can put between the 14-3 cable and the thermostat to prevent power from feeding back to the thermostat? Like some type of relay maybe? When not energized, it would stay normally closed and complete the circuit through the thermostat letting power flow through one set of terminals. But when energized by 120V though, it would open up and allow power to flow through a different set of terminals.
> 
> I attached a revised sketch. I'm sure the mystery box I have there is incorrect and not laid out properly... but just for a visual.


Diodes do that in DC, I don't think you will find anything in AC


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## FrodoOne (Mar 4, 2016)

sonnaps said:


> Ok, this makes sense to me and now I feel confident that this setup will work just fine now.
> 
> I am curious, though... isn't there some type of device (I don't know what it is or what you would call it) that I can put between the 14-3 cable and the thermostat to prevent power from feeding back to the thermostat? Like some type of relay maybe? When not energized, it would stay normally closed and complete the circuit through the thermostat letting power flow through one set of terminals. But when energized by 120V though, it would open up and allow power to flow through a different set of terminals.
> 
> I attached a revised sketch. I'm sure the mystery box I have there is incorrect and not laid out properly... but just for a visual.


The diagram indicates that a DuoStat Adjustable Dual Thermostat/Humidistat Control is being used and the wiring diagram for this is at http://bvc.com/media/wysiwyg/Installation Instructions/DuoStat Installation Instructions.pdf

Within this device the Line wire is connected via the Thermostat and Humidistat, in parallel, so that if either "operates" the fan will be turned ON.
While this diagram appears to show the Neutral "feeding through" the device, the photo at https://www.amazon.com/s/?ie=UTF8&k...id=kwd-33264517179&ref=pd_sl_5ih0cj2n5n_e_p19 and *sonnaps*'s drawing shows that this is *not* so.
Placing the same Line voltage at both the Line and Switched Line terminals (leads) of the DuoStat will do no damage whatsoever.

Hence either of the following circuit configurations could be used without any risk of damaging the DuoStat control unit.







The first arrangement will connect the Line voltage to the Fan (Motor) via Switch 2 no matter whether Switch 1 is ON or OFF. 
(If either of the contacts in the DuoStat are closed, it will also connect the Line voltage to the outgoing contact of Switch 1 via either of these contacts.)

The second arrangement will connect the Line voltage to the fan *only* if Switch 1 is ON.

(An SPDT switch is unnecessary. Only two SPST switches are required.)


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## sonnaps (Jul 21, 2015)

FrodoOne said:


> The diagram indicates that a DuoStat Adjustable Dual Thermostat/Humidistat Control is being used and the wiring diagram for this is at http://bvc.com/media/wysiwyg/Installation Instructions/DuoStat Installation Instructions.pdf
> 
> Within this device the Line wire is connected via the Thermostat and Humidistat, in parallel, so that if either "operates" the fan will be turned ON.
> While this diagram appears to show the Neutral "feeding through" the device, the photo at https://www.amazon.com/s/?ie=UTF8&k...id=kwd-33264517179&ref=pd_sl_5ih0cj2n5n_e_p19 and *sonnaps*'s drawing shows that this is *not* so.
> ...


That's wonderful, thanks so much FrodoOne for the clear diagram and the confirmation I won't damage the DUOSTAT. I almost gave up on my plans for controlling this with some type of smart switches only because I really wanted to get this all in a 1-gang box and not have to replace the box with a 2-gang. I already purchased this : Leviton 1285-L 20-Amp 120/277-Volt Toggle Single-Pole AC Quiet Switch, Black based on rjniles suggestion for a ON-CENTER_OFF-ON switch. Since I only need two SPST switches, now I can also look into something like this Koogeek Smart WiFi Light Switch later on if I really want to be able to control this via my home automation setup.

It seems like the only one I can find that fits in a 1-gang box. Liviton did make something similar but it seems to be discontinued. I'll set everything up the normal way with the Leviton 1285 and see how that all goes and functions for a while. Later on I can always swap it out with the Koogeek if I really want to include the attic fan in my smart home setup.

The real reason I am doing all this and trying to integrate it into my smart home setup is because I also plan on installing a whole house fan. I would like to automate things where if it's cool outside at nighttime, I would trigger a routine that will turn the whole house fan on, as well as the attic fan, to quickly draw the hot air being pulled into the attic back outside. I'm also looking into a motorized control for a window on the first floor. It would be a quite automated and convenient setup. When the routine is triggered both fans will turn on as well as open that motorized window downstairs. Once finished running for a predetermined time, the fans will shut off and the window will close. This home automation stuff is still kinda in its infancy and things can get a little quirky. It sounds great in theory but we'll see how it goes. :biggrin2:

Getting a window motorized is a whole other story. I'm still searching for a good solution, I keep coming across some Chain Window Actuators that have motors running on 24V DC. That'll prob be another post on here somewhere I'm sure.

Thanks everyone.


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## fastsvo (May 31, 2017)

sonnaps said:


> The real reason I am doing all this and trying to integrate it into my smart home setup is because I also plan on installing a whole house fan. I would like to automate things where if it's cool outside at nighttime, I would trigger a routine that will turn the whole house fan on, as well as the attic fan, to quickly draw the hot air being pulled into the attic back outside. I'm also looking into a motorized control for a window on the first floor. It would be a quite automated and convenient setup. When the routine is triggered both fans will turn on as well as open that motorized window downstairs. Once finished running for a predetermined time, the fans will shut off and the window will close.



OP, did you ever get this running? How is life with both a whole house and attic fans?


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## chandler48 (Jun 5, 2017)

The thread is over 5 years old and the OP hasn't been back in a while. You may try private messaging the OP for information.


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