C wire from ~external transformer with Ecobee Smart Si


  #1  
Old 10-26-14, 03:43 PM
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C wire from ~external transformer with Ecobee Smart Si

Hello,

I have a boiler and a central AC and an old thermostat without the C wire. Ecobee Smart Si was ordered and delivered. Installation was based on page 17 of the installation manual (https://www.ecobee.com/wp-content/up...tall_Guide.pdf, single stage boiler with single stage air handler) and I used the boiler's (~external) transformer to get a 24V to the C wire in addition to the Rh and W.

I checked the voltage before connecting the thermostat and the difference between C and G was 48V so I powered things down and swapped the wires from the same external transformer. G-C voltage difference is now ~28V but G-Rc voltage difference is only 10V.

Do I need to remove the G wire (that comes from the cooling unit) or can I leave it in place? G apparently runs the fan.

Old setup: http://imgur.com/U4u1wI9

New Setup: http://imgur.com/zUtZa5v

I think I need to either replace a wire with the yellow (stand alone, not connected) or tie that with another wire (Rh?). My guess is something is off with the phase... any better suggestions?

Thank you!
 
  #2  
Old 10-26-14, 05:14 PM
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Two separate entities. Don't worry about voltage between them. Be sure Rc-Rh jumber is NOT in place.

FIVE WIRE TO BOILER

C = common = blue to boiler transformer
Rh = red = 24v boiler red
W = boiler white



THREE WIRE A/C CABLE
Rc = cooling 24v
W = Y or compressor
G = fan
 
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Old 10-27-14, 10:48 AM
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Yes, I removed the jumper between Rc-Rh per the manual.
You said before "FIVE WIRE TO BOILER" but you listed three, so three wires to the boiler, right?

Will the thermostat get power from the difference between C and Rh? Should I check that voltage difference? Depending on how I connect the C wires I get either 21.3V or 2V.

The good news is that there are a couple of wires free from the boiler area to the thermostat but the boiler is ancient so it does not have a C wire or anything modern.

Update: I just was on the phone with the company's support, the are saying that I can connect the hot side of the external transformer to the Rh and connect the common wire of that transformer to the C and it should work.

I am not sure the terminals were marked on the external transformer but I will check when I go home.
 

Last edited by kinezi; 10-27-14 at 10:58 AM. Reason: Update
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Old 10-27-14, 10:37 PM
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A five wire cable.... three are used and two are not !

I thought you knew what C was.... you have it connected in picture.

Depending on how I connect the C wires I get either 21.3V or 2V.
You'd want the 21.3vac connection.
 
  #5  
Old 10-28-14, 12:18 PM
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Solved!

Hello-
Yes, a 5-6 wire cable was already run from the boiler area to the thermostat and I connected the blue C wire to the thermostat but it was not connected to the rest of the system on the other end, at least not until ALL of the HVAC google results were investigated until 11:30PM yesterday.

After removing some redundancy and shortening some cables I got the voltage difference to ~24V and ~1V so per ecobee's suggestion I merged together the Rh and the hot side of the external transformer and used the other side of the external transformer as the C. So far so good, I have not yet tried heat but I will do it as soon as I can. Fan worked fine and I am able to see live data remotely.

Hint for people who might be in a similar situation. Old transformers are not marked as R and C for hot and common. Mine had V and T. Nothing, I mean nothing online had information for this, the transformer might be from 1960.
The way how I found out is that the white side is usually the neutral/common and the red/black side are the R/hot. The multimeter pointed to this direction.

Now the next project/emergency is the repair of the leaking sink drain pipe IN the wall
 
 

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