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I have a fan (hot, neutral, ground) that requires 120V and uses 1amp max. I'd like to be able to control this with a thermostat so that the fan turns on when the temperature is above a certain set point. The thermostat I'm looking at, Nest, requires 3 wires ... 24VAC, a neutral, and a switch wire. When the thermostat activates the circuit, it connects the neutral to the switch wire.

How can I control the 120V circuit with this low-voltage thermostat? And is there a way to pull 24VAC power at the same time to run the thermostat?

I was thinking I'd need a 120V to 24V transformer, but I'm not sure how the switch circuit would look.

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Just don't short the 120v circuit like it shows in that schematic to the left of the transformer haha – user49261 Jul 13 '14 at 3:22
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That's not a short, that's two wires and the text "120VAC". As in "Connect 120VAC to these two wires". – immibis Jul 13 '14 at 3:26

You need a 24 VAC transformer to power the thermostat, and a 24VAC (coil) 120VAC (or more) (contact) 1amp (or more) (contact) relay.

The 24VAC supply goes to the thermostat and one side of the relay coil. The other side of the relay coil goes to the switched terminal, and the (24V supply) neutral goes to the thermostat.

The hot supply of the 120VAC goes to one side of the relay contact, the fan goes to the other, the (120V supply) neutral and ground go to the fan.

When the thermostat switches, the relay coil energizes and your 120V load turns on.

schematic

simulate this circuit – Schematic created using CircuitLab

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4  
Reader keep in mind that in this description that "neutral" of the 24VAC transformer/relay coil/thermostat is a separate circuit from "neutral" of the relay contact/AC source/fan. These two "neutrals" are not meant to be tied together. – Michael Karas Jul 13 '14 at 0:21

Ecnerwal had it right, just providing a little more clarity and created my own diagram. I live in a condo building where each unit has a blower unit that blows across water filled coils to heat/cool (hot water in the winter, cold water in the summer). So its essentially a Fan that should run to heat (when set to heat in the winter with hot water circulating) and cool (when set to cool and cold water circulating).

I originally just had a regular Honeywell thermostat that worked great before, but I ended up having to run a common wire to the Nest. Nest powers its self off of the Rh or Rc wire, in addition to using this to complete the circuit to run heating or cooling. Without the common the nest had difficulties running and would click off as soon as it started when on heat (but for some reason worked fine on cool). By adding the common, next is able to power its self (off of the Rc/Rh) and the common, and just opperates like a regular thermostat for the relay connection now.

Circuit Diagram

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