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I would like to power a 120V attic fan every time my AC comes on. The low voltage wire connecting the thermostat with the compressor (to control the compressor) runs through the attic (as the compressor is on the roof) and I was thinking to put some sort of junction in the attic so the same wire that turns the compressor on also controls the attic fan. I can easily branch off a ceiling light box to install a box to power this, outlet or direct wiring.

My question is: which electrical device (switch?) can I use to input 24V and release or stop 120V outlet or direct wiring (on a separate circuit than the low voltage)? I imagine something like a power transformer that hangs on the outlet (input), and an input from the low voltage wire which controls another outlet (out) depending on whether it is charged or not.

I am asking this because the fan that I have is not smart and does not take thermostat input, only 120V on/off. So I am trying to control the release of the 120V every time the AC is on, as the control wire for it runs near.

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A relay, just like the other stuff your thermostat controls uses for this purpose.

Not surprisingly, you'll find that 24VAC coil 120/240VAC contact relays are very, very common, and available in a wide range of ratings. You'll need one rated to operate the horsepower of your fan motor (or more, but not less) at 120V. It may also be rated for more horsepower at 240V, but you need HP at 120V rating if the fan motor is wired for 120V.

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  • do relays usually come with e plug outlet?
    – amphibient
    Commented Jan 17, 2021 at 5:01
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    No, they are usually wired into a device, or junction box. However, such things are made, taking a quick look. As you know, product recommendations are off topic, but the search term 24VAC relay controlled recptacle got some results like that, among others.
    – Ecnerwal
    Commented Jan 17, 2021 at 5:02
  • But the plentiful ones readily available from electronic parts suppliers are not UL-listed. They are RU-Recognized but that's not the same thing at all because it fails to address a bunch of stuff about packaging, protection and fitment. Further, you can't mix 24V and 120V in the same box, so how are you going to hook it up? Right answer is something like an Aube, which is UL listed and has its wires going to appropriate places.
    – Harper - Reinstate Monica
    Commented Jan 17, 2021 at 20:34
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Connect a relay that has a 24V AC coil between the C wire and the Y wire

use the relay contacts to switch your extra fan.

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If your goal is saving energy by pulling hot air from the attic when the air conditioning cycles on, your efforts may be counter productive. Research has demonstrated you may be pulling conditioned air out of your home by introducing a negative pressure in the attic. The ceiling contains many penetrations for wiring, lighting and plumbing. These penetrations provide a path for indoor air to be pulled by the attic fan out of the home. Your home is probably pressurized by the furnace or air handler and the attic fan makes the problem worse. Your best bet if possible, is using large static ventilation at the peak of the roof or expanding the area of gable end venting or other alternatives. The fan you install or have existing adds to your utility bill and is a point of repair or maintenance.

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  • Interesting thought, but I'd think going to a hot roof or, barring that, properly air sealing the top ceiling plane would be a better bet yet?
    – ThreePhaseEel
    Commented Jan 17, 2021 at 5:53

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