Nest E and Aube RC840T-120 Wiring Help

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Old 09-21-20, 12:42 PM
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Nest E and Aube RC840T-120 Wiring Help

I need help replacing a thermostat. I currently have a Lux ELV4 and want to upgrade it to a Nest E. I have 2 wires in my wall (line voltage) and want to use an Aube RC840T-120 to connect the two together. Thank you.
 
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Old 09-21-20, 01:26 PM
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Hi, what are you controlling with the existing Tstat? You will need a 3 wire thermostat cable between the Nest E and the RC 840T 120 , the existing 2 wires will need to be connected to the relay, so ,the Nest E will now controlling the relay.
Geo🇺🇸
 

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Old 09-21-20, 02:21 PM
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Welcome to the forums.

Two wires in the wall will not help you. You need line, load and neutral to run the Aube unit.
I'm trying to remember if the Aube will power the nest. It will not power all the wifi stats as it doesn't have a large enough 24v transformer in it.

Is this electric heat ?
There is a line voltage wifi stat. I'm not sure if it will run on two wires but I can check if you want.
 
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Old 09-21-20, 02:25 PM
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Thank you for the responses. Nothing on the oil burner to distinguish make/model. But it's controlled by a Honeywell R8184G4009 and it's hydronic baseboard heat.
 
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Old 09-21-20, 02:28 PM
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You would need to install the Aube unit near the boiler but you'd need three wires from the stat location to the boiler. There is no easy way to do what you want.
 
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Old 09-21-20, 02:33 PM
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Thank you pjmax for the response. Let's say I run a new 3 wire to the thermostat location. How does the Aube get wired to the boiler?
 
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Old 09-21-20, 02:41 PM
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Hi, do you know where the current thermostat wires are connected?
Geo🇺🇸
 
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Old 09-21-20, 03:32 PM
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So there are 3 cables coming into the burner. One from the fuse box and one each from the downstairs and upstairs thermostats. They come into a junction box. Some wires come out of that into a Honeywell aquastat and then into the oil burner controller. The rest come out to the two Taco 007-F5 circularing pumps.

 
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Old 09-21-20, 03:49 PM
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Under further review.... that aquastat has low voltage or high voltage switching. Your TT terminals must be shorted and it's using high voltage switching.

The easiest way to convert your system is to open the TT terminals. Disconnect the two line voltage thermostat wires and connect where they were back together.

Then all you'd need is a basic 24v 20-40w transformer that you could mount to the side of the safety switch. You could leave the two heavy wires in place to the thermostat, add smaller gauge tails at the ends. Connect those two heavy wires to TT at one end and Rh and W1 at the nest.

Run a two conductor wire from the transformer to the nest. It will connect to Rc and C.

You could also run a four wire cable and abandon the high voltage wiring.

 
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Old 09-21-20, 04:11 PM
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You are correct. The TT terminals are jumpered. The Nest E only has the following connections: Y1, G, R, W1, C, and *OB. Any transformer you recommend?

 
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Old 09-21-20, 04:11 PM
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Nest E has a single R and you need separate Rh and Rc connections to use the transformer trick properly.

Think about if you will be swapping the other thermostats to low voltage units. If so you would be better served installing a zone relay board. Taco SR502 or similar.
 
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Old 09-21-20, 04:35 PM
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Thanks A for catching that..... nest E.

You need two thermostats. Removing the TT jumper would work if you only have one controlling stat.
 
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Old 09-21-20, 05:22 PM
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I plan on switching out both line voltage thermostats with Nest E low voltage models.
 
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Old 09-24-20, 12:08 PM
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Thank you everyone for your responses. So if I go with the Taco SR502-4 it looks pretty straightforward as far as wiring goes. I would then have an R, W, and C for the Nest E thermostats. Anything to look out for?



Th
 
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Old 09-24-20, 06:21 PM
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That looks like an effective and easy unit to wire.
 
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