I want to install a fan in my attic. I want the fan to be controlled by a thermostat in the attic, but I want the ability to bypass or override the fan from within my house. Ideally I would like to have a two-state switch in the house:

  1. Fan is controlled by thermostat
  2. Fan is on

I'm trying to figure out what type of thermostat (do they make them with bypasses built in?) and switch (numbers of poles and throws?) to buy. A wiring diagram would be helpful, but I haven't been able to find one.

(I know I'm not supposed to want an attic fan, but I have an uncommon situation and do want it.)


Nothing complicated about it.

Power to switch. Power from switch to fan.

Power to thermostat. Power from thermostat to fan; or, more likely if you cannot source a thermostat rated to switch the motor load directly:

Power to thermostat. Power to relay. Power from thermostat to relay coil. Power from relay contacts to fan. [in this case, the switch could also switch the relay coil, rather than the fan itself, so only the relay would need to be rated for the fan motor load - may or may not matter depending on the size of the motor.]

If either one is on, the fan runs. If both are on, the fan runs.

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  • Thank you. Regarding option 1, I was concerned that having the line between the fan and the thermostat go hot might damage the thermostat. I'm glad I don't need to worry about that. Regarding option 2, I need to learn how to use a relay---I don't know this yet. – Michael H Jun 6 '15 at 19:21
  • This doesn't let you override the thermostat to turn off the fan, only to turn on the fan. – Tester101 Jun 6 '15 at 20:08
  • @Tester101 - read the question. That mode is not requested. – Ecnerwal Jun 6 '15 at 21:40

OH! That's easy.

  1. Get a 3-way switch. Common goes to HOT.

  2. Traveler 1 goes straight to the fan.

  3. Traveler 2 goes via the thermostat to the fan, so that line is interrupted by the thermostat.

  4. Traveler 1 and the switched line from the thermostat merge with the fan's HOT input.

Neutral needs to be part of the wiring bundle, so use 12/3 if you're not using conduit.

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Run a 12-3 w/ground from switch box to fan thermostat box. Make one of your wires from the 12-2 (either red or black) a direct power to your thermostat. Make the other wire from your 12-2 run off your switch to your fan and splice it in with the wire going from the thermostat to the fan motor; in other words that switch will go straight to the fan motor. So when the switch is off you still have direct power to the thermostat controlling the fan, but when you turn your switch on it bypasses the thermostat and gives the fan motor power.

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  • I'm actually doing this in my house right now – Benjamin Polley Jan 7 '17 at 19:54

this was helpful, and I got er done, but I was really looking for a diagram. So built a diagram (see attached) showing how I used a two switches (single pole as a cut-off) and a 3 way to change between using the thermostat and just "always on" mode. I want to use my fan in the winter after we get snow on the roof which blocks our ridge vents (warm air escapes, melts the snow and refreezes cutting off the air flow entirely in the attic). I've now mounted a 15" fan in the roof with a 4ft tall 16" stack over the top to ventilate the attic (gable vents were not an option for me). I used a 15" GAF fan from Home Depot and the wiring couldn't have been easier on the thermostat...you can easily see which black wire goes into the thermostat and which one bypasses it. Hope this diagram helps anyone else. One last thing, if you put a digital multi-meter on your lines after the 3-way, don't be surprised to see a few volts--the meters pick up really small amounts, an analog meter would read zero. Also, I was pulling power from a GFIC outlet (not required...just my setup in the garage)

enter image description here

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