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?

Thanks, Pradhan

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    Is that air conditioner of the commodity in-the-window variety? Or is it part of a central system? – wallyk Aug 31 '16 at 4:20

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.

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

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  • Any chance you could include a diagram of this? :) – DarthCaniac Aug 31 '16 at 0:14
  • Thanks for your prompt reply. I've seen the RIB2401B relay mentioned. Will that do the trick for the relay? – user2082331 Aug 31 '16 at 0:42
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    @DarthCaniac How's that? – Tester101 Aug 31 '16 at 1:11
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    @DarthCaniac any time the thermostat energizes the G terminal. Which is usually if you turn the fan switch to ON, or during a call for COOL. If you wanted it to work for heat too, you'd need to work in the W terminal somehow as well. – Tester101 Aug 31 '16 at 10:11
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    @ Tester someone needs to get you a copy of InkScape :-) – Carl Witthoft Aug 31 '16 at 14:41

The relay will work great, your other option is to use one of these https://www.amazon.com/BCTINT-10031-010-Automated-Vacuum-Switch/dp/B0035YGLZG

They are used to turn on a vacuum when a tool starts running, most of them also have a delay built in so the vacuum would run for a few seconds after the tool shuts off.

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