Nest E Wiring Dilemma (Searched before posting)

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Old 09-28-20, 05:21 PM
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Nest E Wiring Dilemma (Searched before posting)

I have a house from the 1930's with a fairly new boiler system controlled by a battery-powered Honeywell digital 2 wire thermostat. I picked up a Nest E seeing (to my surprise) that I had 5 wires total with 2 wires labeled 'C' (3 wires tucked in into the wall cavity). The wires themselves look to be original to the house (fabric insulation). So far so good.

I know the Nest E won't charge without close to 24VAC as is well document and they even sell those standalone plug-in styled adapters. So before installing the Nest I went to verify the voltage. Here is where my headache started. Transferring the wires over as-is, Nest E would not receive sufficient power to stay charged.

The only way I am able to get 24VAC is by measuring between two 'C1 and C2' labeled wires that were tucked into the wall. I verified at the transformer and it's outputting it 24VAC.

So my question is do I forgo the existing R wire, and use C1 and C2 instead? Placing them in the R and C terminals in the Nest respectively?

Here are other voltage measurements (VAC)
R-W = ~1.15 (basically open circuit, makes sense)

I was expecting/hoping one of these combinations to measure 24VAC
R-C1 = ~12.95
R-C1 = ~10.9
R-C3 = ~3.25

*C1-C2* = ~25 (this gives me hope but not sure what to do)

Measured these just for the hell of it...
W-C1 = ~14
W-C2=~10
W-C2=~4.2

Any help would be much appreciated. Thank You!

C1 and C2 above were labeled 'C' when i pulled them out of the wall.

Nest E Guts

The disconnected wires were to separate the circuit, normally it's all connected
!
 
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Old 09-28-20, 05:28 PM
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I see the 24vAC transformers.
Line voltage and low voltage wires should not be in the same junction box.

You're asking us to guess.
I need to know what exactly does the thermostat operate..... an aquastat, a zone valve, a relay ?
I need to see that wiring.
 
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Old 09-28-20, 06:19 PM
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Hi, thanks for pointing the mixed voltage issue. That is on the to-do list of things to fix. Here are some additional pics. Far as I can tell from tracing the wires, the thermostat turns on the Honeywell Pressurtrol PA404A. My boiler is Slantfin system. If you need more details or pics, happy to provide. Greatly appreciate your help.



 
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Old 09-28-20, 06:43 PM
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I modified your diagram. You have 24v 3va transformer. Is that correct ?
3va is too small. It needs to be 20va minimum to charge a stat.

I just used standard wiring color codes. The transformer and the pressurtrol don't have a polarity.
 
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Old 09-29-20, 05:12 AM
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Hello, thanks again for the response. Your diagram of what the future state conceptually makes sense. However, this am I realized that the 24VAC/3VA transformer has no real purpose, it's hooked up in the basement and doing nothing on the T-stat end as the wires C1 and C1 (1st post) are not connected to anything.

The thermostat is part of the 24AC circuit that is part of the Slantfin boiler. The wiring schematic is shown here (link to google photos so you can zoom in if you need).

Based on this wiring diagram, does your recommend wiring change? Am I adding a C wire from the 24VAC transformer in the boiler?


Slantfin boiler internals
 
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Old 09-29-20, 09:51 AM
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Then you wiring diagram is wrong as you do show that transformer as connected.
No problem....

I illustrated connecting the thermostat in the diagram. Previously since you only were using two wires.... there was no polarity. Now with three wires.... polarity connection to the stat is important. The blue line signifies common. I don't know where the best place to connect to it is.


 
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Old 09-29-20, 05:52 PM
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So after countless hours of research think I found a the ideal solution for the Nest E which less terminals then the Nest 3rd gen (if i had this more expensive version adding external auxiliary power is easy--gazillion YouTube videos). It's this product on Amazon. Since I have 5 wires at the thermostat, I should able to connect it fairly easily (fingers crossed). One reviewer drew a nice wiring diagram.

If I may ask for one final wiring review from you . My old Honeywell thermostat operated the boiler perfectly fine. However, looking at the circuit diagram from your previous response, I see that wire connections inside the pressure cut out are essentially reversed.

Can you please confirm what I'm seeing? I'm assuming since it's basically a single series circuit with the thermostat in the middle it ends up not mattering (although technically incorrect)?

Basically I have:
  • Blue wire from LWCO wire nutted to white wire
  • (other blue wire) at one end of the P Cut-Out terminal and T-stat red wire on the other.
If the wires are indeed wired 'reversed', is this something I should fix or leave as is? Thanks again for your help.
 
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Old 09-29-20, 06:09 PM
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The blue wire that goes to pin 4 of the venter plug is the red wire. So it looks like your wires are now correctly connected. You just need to attach one of the spare wires to the C line for C to your stat.
 
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