Need Help Converting Old Thermostat to Nest


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Old 01-12-19, 11:36 AM
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Need Help Converting Old Thermostat to Nest

Hi Everyone,

I'm not sure if this has been done or not here but I'm still searching. Here is what I'm trying to do.

My old thermostat has 2 wires and the voltage are reading a bit high (I hope I'm doing it right). We have 2 thermostats for 2 floors of the house. Want to change to a Nest or similar Wifi thermostat. Please direct me on what needs to be done.

Here are some pictures of what I have.
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Last edited by PJmax; 01-12-19 at 04:23 PM. Reason: cropped/resized pictures
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Old 01-12-19, 11:40 AM
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Old 01-12-19, 12:22 PM
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You need to find where the thermostat wires connect to at the boiler end. To a valve or controller or pump?
I assume you are able to run a new cable/wires to the thermostat locations? One option is to connect the old heat wires to the W1 and Rh connectors while using a transformer like you pictured to connect to Rc and C.

Measure across heat and com at the thermostat - if really 120vac then this is line voltage and you need a relay to isolate most WiFi thermostats. Line voltage can destroy thermostats not designed for it.
 
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Old 01-12-19, 12:25 PM
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All I can see is what I posted in the pictures. What does this controller look like?

Also tried to connect to nest but it won't work with the 120v system.
 
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Old 01-12-19, 12:42 PM
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A controller is usually has a gray or green metal cover and has lots of wires coming in and out. Follow the cables coming out of the two Taco pumps to see where they go. The back side of the boiler must have some wiring.

Also, what does that transformer connect to?
 
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Old 01-12-19, 12:46 PM
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The aquastat or control is on the right side of the boiler.
Looks to be stuffed in between the boiler and the wall.

If you have already tried to connect the nest to those wires..... it will more than likely be fried.
 
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Old 01-12-19, 01:39 PM
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Luckily nest is not fried plus I have another honey well thermostat that I can test with.

The other side of the boiler has this box with no labels

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Last edited by PJmax; 01-12-19 at 04:27 PM. Reason: added pic from link
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Old 01-12-19, 03:07 PM
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That's a junction box and doesn't look like the wires going to the thermostat. There should be an Aquastat nearby.
I would recommend getting in a pro and have them install a zone controller relay box. Taco SR502 or similar would convert the wiring to the thermostats to low voltage (24vac).
 
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Old 01-12-19, 04:00 PM
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Your pictures were massive and had used up your entire 10 megs of storage. I cropped, resized and reposted them for you.

Every boiler has an aquastat. It's a control attached directly to the boiler. If yours is not on the outside.... you will need to look behind the removable service doors.
 

Last edited by PJmax; 01-12-19 at 04:28 PM.
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Old 01-12-19, 08:55 PM
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I found this control box. Can this be changed to the 24v system?
 
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Old 01-13-19, 09:42 AM
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You would need what's called a fan center. It's a combination transformer and relay on a 4" square plate. It may mount directly to your junction box.

An example........ Fan center
 
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Old 01-13-19, 09:49 AM
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Thanks PJmax,

A few questions.
  • If I install this can I use the same wires that are going to the thermostat or need to run new wires?
  • After installing a fan center can one thermostat still run on old wires or both need to be upgraded (I have 2 thermostats)
  • Is there a tutorial that you know that I can read on to do this or should I get someone to install it (trying to save money)?
Again thanks everyone for their help.
 
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Old 01-13-19, 10:01 AM
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The fan center will supply the needed C connection to run the wifi stat. A nest MAY run ok with only two wires but no guaranty. I personally don't install any wifi stat without a C connection. That means your install would be more reliable with three wires to the stat.
 
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Old 01-13-19, 10:04 AM
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I'm a bit lost here. Since I have two thermostats, do I need to install two fan controls?
 
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Old 01-13-19, 10:43 AM
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Each thermostat controls its own circulator pump. Therefore there needs to be two relays in the mix. You could use one fan center and an additional relay or two fan centers.
 
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Old 01-29-19, 07:43 AM
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Hi Team,

Ok finally got the part now need a bit of help wiring it.

Here is a diagram of how our system and the image of the fan control part.

Where do I connect the red, black and brown on one side and the white, black on the other side of this new part?
 
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Old 01-29-19, 08:11 AM
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The white and black need 120v.
The black (C) and red (NO) go to the two wires that are now going to the high voltage stat.
Cap off the extra wire.

On the screw terminals....
R to R of stat.
C to C of stat.
G to W of stat.
 
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Old 01-29-19, 08:34 AM
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Thanks PJmax,

There are 2 black wires connected to C and G terminals already (view pic above) leave them as is and connect on top of them for the new thermostat?
 
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Old 01-29-19, 09:03 AM
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Yes..... those stay as they are.
 
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Old 01-29-19, 01:48 PM
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Still not luck... After doing what was suggested. I get Nest e102 error and heat is not working.
 
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Old 01-29-19, 01:53 PM
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Did you run three wires or are you using two ?
If you're using two only it would be R on the FC to Rh on the nest and G on the FC to W1. If that doesn't work for two wire..... move the Rh wire to Rc.
 
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Old 01-29-19, 02:05 PM
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Ran 3 new wires from FC to NEST. G ->W1, R->RH or RC, C->C

I am getting 27V at the RH or RC
 
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Old 01-29-19, 04:12 PM
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27vAC is ok. It's nominal 24vAC.

You should not be getting a problem with excessive current draw.

Try this for me...... Take the nest off the base. Remove the W1 wire and touch it directly to the R terminal. That should activate the relay in the fan center and start the heat.
 
 

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