Wiring – How to make a 110V fan turn on when the thermostat turns the AC on


I have installed a new a/c with thermostat…easy enough, but thought to add a fan to the mix. Basically, I'd like to add a fan to increase ventilation into a second room that would turn on whenever the A/C turns on. I know you can run to multiple low voltage devices off a single thermostat, but what would be required to add a 110 fan so that it turns on exactly when the A/C turns on?


Best Answer

All you'd need is a relay. You'll need one with a coil rated for 24VAC (or whatever the thermostat voltage is), and a contact rated to handle the current draw of the fan at line voltage.

  • Connect the ungrounded (hot) feed to the Common terminal of the relay.
  • Connect the ungrounded (hot) wire for the fan, to the Normally Open (NO) terminal of the relay.
  • Connect the grounded (neutral) from the feed, to the grounded (neutral) of the fan.
  • Connect all grounding conductors.
  • Connect a wire from the G terminal on the thermostat, to one of the coil contacts on the relay.
  • Connect a wire from the C (common) terminal of the A/C transformer.

When the thermostat calls for COOL, it should turn on the fan as well.

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Didn't add it to the diagram, but it's probably a good idea to have a fuse on the R wire.