Thermostat conversion from 120v to 24v for NEST


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Old 01-07-15, 11:58 AM
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Thermostat conversion from 120v to 24v for NEST

I have an Oil Furnace with water baseboard radiators.

I recently installed a NEST Thermostat downstairs in my mothers home and she loves it (big numbers, easy to use) and it was easy as it was already low voltage wires.
I wanted to do the upstairs also so I purchased another NEST and when I took off the cover of the thermostat I found this:

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It has 2 wires and is 120v.

After much searching around the internet I realize (correct me if I am wrong) that I need to install a transformer 120v to 24v.

I was looking at the Honeywell (AUBE) RC840T-120 (Relay and transformer) since it is so compact.
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But it has three wires coming into it (LOAD, LINE 1, LINE2) (RED, BLK, BLUE respectively)

How do I connect this? do I connect both L1(BLK) & L2(BLUE) to the live wire? and LOAD (RED) to the load side?

Or should I go the route of getting a transformer without the relay and connect it to the NEST? Do I need the relay? if so I would use the RC840-120 (w/o transformer)
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From what I can gather the current thermostat is basically a switch and when it gets cold it makes contact and turns on the furnace/pump). which in theory should not be a big problem.

I have read a lot of the forums and could not find a clear example of my issue.
Thanks
 
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Old 01-07-15, 12:26 PM
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Welcome to the forums.

Don't rush into this. You need to confirm that the thermostat is actually switching 120vac. It could also be switching 24vac.

You will need a 24vac source to run the Nest.
If it is 120vac then you will need a relay.
You will need at least a three wire thermostat cable from t'stat location to furnace. I'd run a few spares.... maybe a five wire cable.

The 840T120 may work but you'd need to find a wiring diagram for your furnace.
 
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Old 01-08-15, 12:13 PM
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Thx PJmax,

Wouldn't the transformer provide the 24vac the nest needs?

From what I can see the current thermostat is basically a 120vac switch.
If I us the RC840T-120 wired as shown below wouldn't that work?
DIAGRAM 1
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If I wired it this way (RED & BLUE wires both connected to the LOAD side & BLK-LINE) being I only have 2 wires coming out of the wall (LOAD?LINE) and 3 wires on the RC840T? Does this make sense or am I totally off?

Or this way...
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The wiring diagram from the RC840T instructions are:
DIAGRAM 3
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Where I would say the LOAD is the Furnace not the Electric Baseboard.

The problem I have is I only have 2 wires poking thru the wall where the current thermostat is.
I tried to trace the wires but have been unable so far.
I also took the cover off the Furnace but I have not had a chance to get really in there to see the connections and/or wiring diagram if there is one.

My gut tells me I should be able to just wire it as shown in D1 & when I look at D3 & the diagram on the actual RC840 I want to wire it as shown in D1 (BLUE + RED ---> LOAD).
 
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Old 01-08-15, 04:22 PM
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Where I would say the LOAD is the Furnace not the Electric Baseboard.
Is your existing thermostat controlling a furnace or electric baseboard heat?
 
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Old 01-08-15, 04:29 PM
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He's controlling and old oil fired furnace.

You can't just arbitrarily connect to the two thermostat wires. You need to connect all three wires from that relay so we need to know the furnace wiring !
 
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Old 01-18-15, 03:35 PM
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How to know which is load and which is line?

So question: how do you know which wire is line and which is load?

I have the exact same situation as the OP. I have two wires coming out of my wall where the thermostat is: one white and one black. The two wires connect to a 120v dumb/mechanical thermostat.

The Aube 840T-120 has three wires coming out of it. In my case they are white, red, and black. (The manual often refers to the white one as blue.)

I tried connecting things this way:


Aube Wall
Black Black
White White
Red White


But when the Nest called for heat, the Aube transformer made a clicking and buzzing sound that definitely sounded wrong. And the red LED light on the Aube that signals when it is activating the 120v side would flicker on and off. So that seemed wrong.

Then I tried this:


Aube Wall
Black Black
White White
Red Capped off, disconnected


In this configuration, the Aube would not buzz or make any weird noises when the Nest called for heat. But the device the thermostat was controlling (in my case a pump) would not come on.

About the only thing else I could try is to connect it this way....


Aube Wall
Black Black
White White
Red Black

But I guess I'm a bit scared of doing something wrong, wrecking my pump or something.

Any advice?


By the way, to the OP, here are a few other things that I ran into as issues while doing this process but was able to overcome.

#1 use thick 12v wire between the Aube and the Nest. I originally used telephone wire, and the Nest would randomly stop detecting which wires were connected to the backplate. Then I took a power cord from some old device I wasn't using that had a wall wart, so i.e. the cord itself was meant to carry 12v DC from the wall wart to the device, and I ripped that up. It was much thicker wire. And presto - the Nest successfully and consistently detected what was plugged in.

#2 there is no need to plug in the C wire. Just plug in W and Rh. For some reason when I plugged in the C wire the Nest did not detect the right setup.


So with that said....would love some advice on the 120v side of things, which way to connect the Aube (3 wires - red, white, black) to the house wiring (2 wires - white and black).

Thanks!
 
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Old 01-18-15, 04:02 PM
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It makes things extremely confusing to have two posters in one thread. I'd say you'd be lucky if you hadn't burned up your Aube unit. You need to ID wiring before connecting... not just guess at it. We have no idea what you have for a heating system so we can't give you exact help.

The relay MUST install at the furnace. It needs a neutral connection AT the furnace.
Go to your furnace and ID the two thermostat wires there.

Here's one way to try to figure out the wiring.... of the two thermostat wires.....
One wire should be always hot. That would be the black wire.
The other wire should be the switched hot and connected to red.
Now you have to locate the neutral to connect to the blue/white wire.

(on the 240v model it's blue and on the 120v model it's white)
 
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Old 01-18-15, 05:31 PM
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Apologies; didn't mean to highjack the thread.
 
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Old 07-25-16, 03:52 PM
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This is a highly specialized and possibly dangerous conversion. If after reading this thread you still don't have a solution.... please start a new thread.
 
 

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