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How to make window AC interlock for fan Answered

My house is set up like a horseshoe, with the tenants stairway isolating each tip.  I have a large window AC unit that cools the common space, and i want to add a duct fan that circulates the air from tip to tip.  The AC is in the center of the horseshoe.

I want the fan to turn on automatically when the AC unit is on.  How can I accomplish this?

I know that i could open the ac unit up and splice into power wires that are only hot when the unit is running, and then use that as the control voltage on a relay... However, I was hoping to find an external solution.

My next thought was some sort of current switch that plugs between the AC cord and the wall outlet, and a receiver unit on the fan.

So far my search queries haven't turned up anything.

Ideas?

Discussions

A very basic way would be a flap and a little switch, like found for the doors in microwaves.

Mount the flap so it partially blocks the air outlet on the AC.

Something light weight should be used of course like these plastic sheets for signs that look like cardboard.

Mount the switch so the the airflow from the AC pushes the flap up an has enough pressure for the switch.

These switches are available with a little lever as well - extend it to get enough force for proper operation with the flap.

There are extension leads available in the "power saving" section of your hardware store.

Usually for PC and TV use.

One outlet is the master - in your case for the AC.

The others are slaves - AC switches on and they get power too.

But this requires an AV with a plug and a power consumption suitable for the energy saving device.