Relay vs transformer and relay

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Old 08-15-19, 02:52 PM
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Relay vs transformer and relay

I'm trying to install a nest gen 3 with separate heat and AC units. The AC already has 24 v wires connected to the nest with a common to power the thermostat. My heating unit only has 2 120v wires going to the old thermostat. I hastily ordered the aube 840t-120 without doing much research. I'd like to install the aube near the thermostat so I won't need to run new wire thought the house. Since I already have a common to the thermostat, can I just use a relay? If so, can I use the 840t (diagram says I need 3 120v wires) or should I get the aube 840(relay only).
 
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Old 08-15-19, 04:31 PM
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Welcome to the forums.

The aube 840t-120 requires a continuous 120v so it would not be the best choice to be at the thermostat. Any 24vac SPNO relay will work. Since you are installing the relay near the thermostat it will need to be in some type of enclosure.
 
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Old 08-15-19, 05:04 PM
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Thanks. Are there any 24v spno relays you'd recommend? Is the rc 840 sufficient?
 
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Old 08-15-19, 05:11 PM
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Do you have an idea how you're going to contain the relay ?
Just letting hang in the wall wouldn't be the best choice.

The RC840 is fine.
You could install it in a deep cut-in gem box.... plastic or metal..... and put a blank plate on it.

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Old 08-15-19, 05:38 PM
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Yeah,that much I do know lol. Thank you.
 
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Old 08-16-19, 06:56 AM
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Just to confirm my set up would be:

​​​​​​120 wires to relay
24 v wires from relay to nest (rh and w1)?

​​​

 
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Old 08-16-19, 09:50 AM
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Yes..... 120v wires to the relay.
The low voltage 24v wires go to C and W1.

You'll have to splice the C wires together so that only one connects to the nest.
 
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Old 08-16-19, 10:03 AM
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Hi, it sounds like you had a line voltage Tstat for the heat originally, correct? If so you will need to rework the connections at the boiler so as to have a neutral for the relay.
Geo
 
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Old 08-16-19, 10:06 AM
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He did have a line voltage stat and now he's using a 24v relay only to handle the switching.
 
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Old 08-16-19, 10:13 AM
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Oh, got it!,,,,,,,,,,,,,,,,,
Geo
 
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Old 08-20-19, 03:59 PM
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Pjmax,

I was able to set it up at my home with the Nest 3rd gen. Everything works. I'm wondering if you think this set up would also work with a Nest E?
 
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