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Old 01-27-2011, 02:25 PM   #1
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wiring thermostat controlled attic fan


long time reader-new member. i have a 1400 cfm attic fan controlled by a thermostat in attic-when it gets around 90 degrees in attic the fan automatic turns on-.. would like to be able to by-pass the temp switch from time to time and be able to turn on the fan regardless of attic temp. possible using a on-off-on switch.any problems with send power to fan by passing the thermostat? then switching it back to be controlled by the thermostat. want to put the switch in hallway closet to make it easier to vent attic on demand. thanks again. steve

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Old 01-27-2011, 03:05 PM   #2
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wiring thermostat controlled attic fan


Easy, Run a 2 wire cable from the fan to the switch box, install single pole switch. At fan connect wires across the contacts of the t-stat. Run a wire the same gauge as the existing wiring. 12-2 for 20 amp circuit, 14-2 for 15 amp circuit.

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Old 01-27-2011, 03:14 PM   #3
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wiring thermostat controlled attic fan


thanks, not sure if i asked question correctly. i want the fan to always be "on"(thru the fan mounted thermostat), but i want to be able to remotely bypass the thermostat and turn the fan on by a switch from time to time.and then turn it back to t-stat control. fan is on single 15amp circuit with14 gauge wire. when i go up in the attic to move stored items around, it would be good to get some air moving around , and i dont want to have to reach for the t-stat ( hi up in the rafters)
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Old 01-27-2011, 03:21 PM   #4
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wiring thermostat controlled attic fan


What I said in my post will do what you want. With the switch off the t-stat will control the fan, with the switch on the fan will run regardless of the temp. Run with 14-2 NM cable if it is allowed in your locality.
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Old 01-27-2011, 03:44 PM   #5
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wiring thermostat controlled attic fan


The thermostat operates the fan by making a connection between two wires. When you put a switch parallel to these two wires, closing the switch bypasses the thermostat.
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Old 01-27-2011, 05:33 PM   #6
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wiring thermostat controlled attic fan


think i got it, had to draw myself a diagram. the t-stat is hot, but the switched 14/2 is only used to jump the t-stat? i'm gona upload a diagram, thanks. i was thinking of using a single pole double throw switch
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Old 01-27-2011, 06:13 PM   #7
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wiring thermostat controlled attic fan


Quote:
Originally Posted by mazey View Post
think i got it, had to draw myself a diagram. the t-stat is hot, but the switched 14/2 is only used to jump the t-stat? i'm gona upload a diagram, thanks. i was thinking of using a single pole double throw switch
A single pole double throw switch is a 3 way switch. You do not need that. You need a regular single pole switch. (SPST)
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Old 01-27-2011, 06:18 PM   #8
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wiring thermostat controlled attic fan


3 diagrams, existing, option "a" is what i thought would work with a on-off-on switch that feeds the fan or option"b" which is what i'm interpreting from rjniles, where the fan is fed seperately and i have a single pole switch that closses the t-stat. any way better than the other? i'm replacing the wiring anyway, and the walls open. thanks
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Old 01-27-2011, 06:24 PM   #9
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wiring thermostat controlled attic fan


Your options are both the same but B is more correct. Remark the white wire as black (not on the diagram but on the actual wire).
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Old 01-27-2011, 06:25 PM   #10
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wiring thermostat controlled attic fan


Option B gives you manual on override.

Option A, you would feed the hot to center, then one of the connection to thermostat, other one direct to fan and this gives you forced off, thermostat, and forced on. You'll need one with a center stop though. They're very common in electronic circuit building, but I'm not sure if they have on-off-on type switch for house wiring
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Old 01-27-2011, 06:29 PM   #11
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wiring thermostat controlled attic fan


right,black tape on white wire to indicate its not a neutral,. i had to see it on paper first. good thing because no electrical supply around me stocks the switch i needed for the double throw. am i the only person who wanted to control a fan like this? seems like a good idea to me. thanks again
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Old 01-27-2011, 06:32 PM   #12
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wiring thermostat controlled attic fan


just 1 more question , what if i throw the "extra " switch closed, when the fan is already running because the t-stat was "closed"? would i just have an extra leg of "hot "wire? any reason for something to short out or trip breaker?
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Old 01-27-2011, 06:38 PM   #13
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wiring thermostat controlled attic fan


You can find center off SPDT at Radio Shack with more than adequate capacity for a fan and it will work, but they are for system integration, so if you use it for home wiring, it won't be up to code. To mount it, you'll need to get a blank cover plate, then drill a hole in the middle, or if it's the right size, you can probably use the cover plate meant for CATV hookup.

To do it with home wiring switches, you'll probably have to use a dual ganged SPST switches and use one for on/off, then the second for auto/on. The on/off has to be on for either on or auto to work.

If it's an electronic thermostat without a neutral, the bypass switch will deprive it of power source and cause it to lose settings and/or clcok. If it's one with a neutral or a mechanical one, the thermostat will continue to turn on/off without having any effect on the fan.

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Old 01-27-2011, 06:43 PM   #14
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wiring thermostat controlled attic fan


[QUOTE=HVAC_NW;579167]Option B gives you manual on override.


what is meant by "manual on override"
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Old 01-28-2011, 08:20 AM   #15
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wiring thermostat controlled attic fan


Just realized waht you were trying to do with option A. Yes that will work but you need a 3 wire cable from the fan to the switch box. Remarked white to power the switch and red and black as switched hots back to the fan.

Rather than try to find a single pole, double throw, center off switch as you show (not real common), consider 2 single poles switches in a 2 gang box. Or a 2 single pole combination switch (2 switches on a single yoke that fits in a single gang box.).

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