Electrical – How to override an attic thermostat with a wall switch

That thermostat is not meant to switch high voltage. You need to use a 240V thermostat. Or wire that one to switch a relay to switch the fan.

A 240V t-stat would be easier. It simply wires like a light switch.

To use a relay you need something like this.

Insteon does make "2-wire" dimmer switch modules. But not relays which you would need for motor control. I presume this is because a closed relay would leave no current for the circuitry in the Insteon module itself to run, and the dimmers basically only go up to 99% intensity using the power of that last % to run the module.

Having deployed a significant number of insteon modules myself,

Here's what I would do:

1. Buy an additional Insteon switch module or remote fob for the new remote switch location
2. Buy a 'micro on-off module' to be placed in the attic in the fan's jbox.
3. Take the old switch out of line of the fan.
4. Wire the switch module you already have up where you want it, and ignore its red load wire. (Cap it safely with a wire nut.)
5. In the fan's j-box:
   • Wire together in parallel:
     • the mains in.
     • the line out to your replacement switch.
     • the input to the mini on/off.
   • Connect the fan to the mini on/off's output.
   • Cap off the micro on/off's purple and yellow sense wires.
6. At this point you should be able to control the fan from the buttons on the mini on/off module
7. Use programming to make either switch module control the mini-relay, and all other switches. (Making a switch control another switch might seem useless, but it is what will keep your indicator lights in sync.)

You are probably going to want to have one of their pc link modules and the pc software to do the programming. But it can be done following tedius steps with just your fingers. If you want to do it without the pc software, PROTIP, wire everything up temporarily on a table top and program it there before taking modules to their final install locations.

THE CHEAPSKATE OPTION:

If you want to save the cost of one of the Insteon switch modules (they aren't cheap) would involve taking advantage of the relay module's "sense wires".

You could for instance connect a switch between sense wire 1 and line. That switch would then be able to actuate the relay directly without sending Insteon signaling. That switch could be a 50 cent on/off switch in the wall where the old timer switch used to be.

Programming micro relays that use sense wires is difficult and I have ran into firmware bugs that flat out render their micro relays bricked in the on position in some circumstances until you factory reset them. You won't want to go up into the attic to do this all the time, so consider skipping the cheapskate option.