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First time poster....short time lurker.

Thanks in advance for taking the time to read my post. I am hopeful that this board can provide me the proper guidance required to properly wire my new thermostat.

I recently purchased a Zen Thermostat to install as part of my connected home setup. I chose the device due to the ability for it to connect to my Zigbee network and act as a Zigbee repeater as well. When I went to install the thermostat, I noticed that I had only two wires connected to my existing thermostat. This kicked off several days of research where I learned more about HVAC systems that I have ever expected (but clearly I still know little). The two wires were the Rc and C wires. I found out later that they only provide power to my thermostat. I realized I had a wireless thermostat setup with the base (Honeywell EIM) located next to my furnace. Fast forward a day later, here is where I am:

My Objective: Connect the Zen Thermostat to my current system

The Problem

  • The Zen Thermostat supports the following connections: Rc, Rh, C, G, Y1, Y2, W1/O/B, W2/AUX
  • I do not have a clear understanding of what connections to make to my furnace
  • I need to run new wire to my thermostat since the current connection is only two wires (this is easy, so no concerns here)


My Equipment Setup (I created a diagram that walks through all of this, but I can't post links. If you can take the time, try going to http[/][/][:]ss01[dot]photobucket[dot]com/user/ktcred/media

  • Furnace: Trane XV80
  • Thermostat: Honeywell THM5421R EIM and Head Unit
  • Compressor and Heat Pump: Trane XR17 (4TWR7024A)


Current Wiring Configuration (Furnace --> EIM/Thermostat)

  • O --> O/B (W if conventional, not heat pump)
  • W --> AUX 1 (W2 if conventional, not heat pump)
  • W2 --> AUX 2 (W3 if conventional, not heat pump)
  • Y --> Y
  • YLO --> Y2
  • G --> G
  • R --> R
  • C --> C


What I can't figure out is how to wire up the furnace to the new thermostat. Here are some assumptions I am making:

  • O drives the heat pump and completes the circuit to the outside unit
  • G goes to the furnace only and runs the fan
  • Y & YLO complete the circuit back out to the outside for cooling
  • W goes to the furnace only to run the gas heat


If I was to go on those assumptions, I would run the following to my Zen Thermostat (Furnace --> Thermostat):

  • O --> W/O/B
  • G --> G
  • Y --> Y1
  • YLO --> Y2
  • W --> W2/AUX
  • R --> Rc (has jumper for Rh)
  • C --> C


If those assumptions are true (are they?), what is W2? I've exhausted all of my Zen Thermostat port connections.

Is it making the run outside for my temperature sensor? If so, why does it need to make a connection to the furnace as part of the run? Logically, I would think it would bypass the furnace, so my guess is that it is not the outside sensor.

With all of that as the background, here are my questions (thanks for making it this far):


  1. Can I completely remove the EIM from the picture?
  2. If yes to #1, am I correct that I can then move the wires on the EIM (going to the outside unit) to the furnace and then double up the wires for the thermostat? Meaning, is it OK to double up the furnace board with wires. For example, one O wire out to the thermostat and one O wire out to the heat pump, but both on the same screw.
  3. I have an outside temperature sensor. How is that connected? Could it be wireless or is it one of the aforementioned connections? As a note, I can care less about the outside temperature, so I am more than happy to remove from my configuration. I will be getting weather data through the internet instead.
  4. Are my assumptions above correct? Am I thinking correctly about the connections?
  5. What is the W2 out on the furnace and why is it connected to AUX 2 on the EIM and out of my house?
  6. What else am I missing? Am I going about this all wrong and everything I wrote above is just way off?


Thanks again for your help. I will not start kicking my refresh key in anticipation<br>
 

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You have two wires only connected to your stat for power (24volts) because it's a wireless thermostat. It connects wirelessly to the EIM.

All the connections that go to your EIM will have to be run to the Zen thermostat as it is NOT wireless.

Yes. The wires to your outside unit can go directly to the air handler.

The outdoor sensor can either be hard wires or wireless depending on which one you have. The outside temp sensor is important in order to lock out the heat pump at low temperatures. If you do not have it then you should add an external lock out on the outdoor unit itself (Not sure if the Zen has low temp lock out). The sensor is also important if you have a humidifier.

W1 is first stage back up heat

W2 is second stage back up heat.

O is the heat pump reversing valve
 

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In thinking about this further.... not sure if your trane has its own external lock out temp sensor on the condenser, but if it doesn't then you would be advised to install one regardless of whether or not Zen has it.

Zen gets its whether info from the internet, so if you lose internet for any reason (power failure for example) you will also lose your lockout ability.
 

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Your furnace is 2 stage. But your Zen thermostat won't be able to control 2 stages.
 

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Your furnace is 2 stage. But your Zen thermostat won't be able to control 2 stages.
I think it's kind of weird switching out the iaq for a zen in the first place... kind of like a downgrade. But I guess if you need to work it into a home system then you really have no choice.
 

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Discussion Starter #6
Thanks, everyone, with the quick and helpful replies. Here are some quick thoughts:

I think it's kind of weird switching out the iaq for a zen in the first place... kind of like a downgrade. But I guess if you need to work it into a home system then you really have no choice.
I'm not sure I see a lot of value from the current thermostat. With the Zen, I can remotely configure it and place it into my home automation setup. That means I can set the temp automatically when I'm away, back, on vacation, when the temp outside changes, etc... Much more flexibility, IMO. Maybe I'm missing out on these capabilities with the current setup

Your furnace is 2 stage. But your Zen thermostat won't be able to control 2 stages.
The Zen does support 2 sage furnaces. Here is the info from the website:

HVAC SYSTEMS

24VAC control compatible with forced air, heat pump, gas, electric, millivolt, hydronic.
1 or 2 heating stage plus emergency heat
1 or 2 stage cooling
1 or 2 stage reversible heat pump with auxillary/emergency heat

The outdoor sensor can either be hard wires or wireless depending on which one you have. The outside temp sensor is important in order to lock out the heat pump at low temperatures
How can I tell if my system has a lock out and if it is setup?

W1 is first stage back up heat

W2 is second stage back up heat.

O is the heat pump reversing valve
So is the issue that I have two stage *and* a heat pump reversing valve? I ask because the Zen says it supports both, but is it a mutually exclusive setup?


Thanks again for the help.
 

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If the W1 terminal is also the O terminal, then it can't do dual fuel with a 2 stage gas furnace. You will have to set the furnace up to time to second stage.
 

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That means I can set the temp automatically when I'm away, back, on vacation, when the temp outside changes, etc... Much more flexibility, IMO. Maybe I'm missing out on these capabilities with the current setup
You have honeywell redlink I believe which means you can do the same. I can control mine over the cell phone. Some screen shots:

Screenshot_2016-01-03-10-55-15.jpg
Screenshot_2016-01-03-10-54-43.jpg
Screenshot_2016-01-03-10-54-32.jpg
Screenshot_2016-01-03-10-53-29.jpg
Screenshot_2016-01-03-10-55-36.jpg





All you have to do is purchase/install the gateway, setup a (free) account with Honeywell and you should have wifi ability.

Not sure which Honeywell you have but if it's the prestige 2 then it is FAR better than the zen and it may be that you are simply not aware if its abilities.
 

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Okay. You have a 2 stage heat pump, and a 2 stage furnace. So 4 stages of heat, and 2 stages of cooling. You can cripple your gas furnace and let it time to second stage. A lost of comfort, but can be done.
 
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