My attic gable fan (http://www.homedepot.com/buy/building-materials/
ceilings-attics/master-flow/power-gable-vent-pg3-1600-cfm-21356.html)
has a thermostat built in that kicks on when it gets too hot up
there. The fan is powered by a dedicated BX cable that runs up to the
attic, and this cable has a regular wall switch that I can shut on/off
from the 2nd floor hallway. I want the option to run the fan manually
too, bypassing the thermostat. I can replace the BX calbe with a
4wire cable (two hot/one neutral/one ground), and have one of the
wires bypass the thermostat.

But, what kind of wall switch would I need to buy that has, basically,
three settings: off, on (for automatic fan operation via thermostat),
and on (for manual operation of fan, bypassing thermostat)?


*You could use a combination device with one single pole switch and one
three way switch. The single pole would serve as the on-off and the three
way will change it from thermostat to always on.

http://www.legrand.us/passandseymour...hes/696wg.aspx

I think this combination is also available in Decora style. Go to an
electrical supply company to get one.