Unique Nest Thermostat Help! Non-heatpump B and O wires.

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  #1  
Old 01-19-16, 12:00 PM
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Unique Nest Thermostat Help! Non-heatpump B and O wires.

I need help hooking up a new stat to a RHEEM Oil Fired furnace, model #ROBD-112QBGA and a Trane A/C unit, model #4TTB3042D1000BA controlled by an Ultra Zone control board, model #EWC-ST-2C. I currently have a Honeywell TH4110D1007 thermostat and would like to upgrade to a Nest gen 3. I have 6 wires in my wall.

I have included pictures of all the above as well as of the two dampers that control air flow to zones 1 and 2. I am replacing the Zone 1 thermostat.

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Not sure why all the pictures are sideways. Sorry about that.
 
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Old 01-19-16, 08:22 PM
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Welcome to the forums.

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That's the problem right there. The thermostat needs to have a O and B terminal.
The nest has a combo O/B terminal.

Thermostat 1 sets the system operation.... heating or cooling thru O and B.

There may be a way around it using one or two relays.
Let me think on it and my forum partner, Houston, may have an idea too.
 
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Old 01-19-16, 08:42 PM
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We can add a DPDT relay to make this possible but the Nest thermostat may have an issue with this.


 

Last edited by Houston204; 01-19-16 at 09:20 PM.
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Old 01-20-16, 06:36 AM
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Houston204,

Thanks for the reply. Looking at your diagram, this requires a splice in C and R1. Is that as easy as cutting, stripping and putting a splice connector?

Could I use these?
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Would this be an appropriate DPDT relay?
Packard PR340 DPDT 24 Volt Coil Switching Relay: Amazon.com: Industrial & Scientific

Could I just use a different control board that doesn't need B and O? ...and good catch, I never even saw that!
 
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Old 01-20-16, 10:20 AM
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I would just run the wies from the relay to the zone control board and connect two wires to C and R1 there.
 
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Old 01-20-16, 05:01 PM
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I don't currently have a C wire. I suppose I could utilize the B wire as C and run a short wire from B to the relay as per the diagram?
 
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Old 01-20-16, 05:21 PM
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It looks like you have a brown wire available for use as the C wire.

It might help avoid confusion years down the road if blue didn't have two functions but if brown isn't available at the other end...
 
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Old 01-20-16, 05:29 PM
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http://www.amazon.com/Honeywell-HZ31.../dp/B001GLYC8S

A Honeywell HZ311 zone control would be another option. It would give you greater control over your equipment since zone two is currently a slave stat.
 
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Old 01-20-16, 05:35 PM
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I don't see a brown wire anywhere except the diagram. Am I missing something obvious?
 
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Old 01-20-16, 06:59 PM
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Looks like you have an unused brown wire and a black wire at the control board end of the stat wire. If you have splices between the board and the thermostat they may not be at the thermostat.
 
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Old 01-21-16, 03:47 AM
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That dpdt 24vac relay is fine.
 
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Old 01-21-16, 09:37 AM
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We can add a DPDT relay to make this possible but the Nest thermostat may have an issue with this.
Problem with those relay connections is that the slave can not call for cooling on its own. You also have to set the Nest to think it has a heat pump system.

Simple solution: They sell a manual heat/cool 3-way switch MCS-DXB - EWCEStore that connects to the O, B, and R1 terminals. It does let each thermostat behave normally but it is kind of ugly.
 
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Old 01-22-16, 05:03 AM
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The slave stat cannot control mode now.


50 dollars for a labeled toggle switch?

The slave stat still would not be able to control system mode.
 
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Old 01-22-16, 09:09 AM
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The brown and black wires do not extend to the thermostat.

I'm a little fuzzy on the slave stat and control system mode language. Will the upstairs (zone 2) three wire thermostat not work correctly? I already ordered the relay, so will give it a try anyway.

So it sounds like the only real practical solution is Honeywell HZ311 zone control?

Is this something I could wire based on the current control board? I am relatively handy.
 
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Old 01-22-16, 09:57 AM
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The slave stat cannot control mode now.
Not a mode issue. If there is no O or B then the thermostat inputs are ignored. Right now the slave thermostat can call for cooling on its own as long as the master is set to cool mode. O would be hot as it doesn't have to have an actual cooling call. With that relay wiring the slave cooling call will be ignored unless the Nest was already calling for cooling.

A caveat: I do wonder if the Nest is smart enough to leave O hot even after the cooling call is satisfied? You really would not want the reversing valve cycling every time the compressor was triggered.
 

Last edited by Astuff; 01-22-16 at 11:12 AM.
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Old 01-22-16, 11:44 AM
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It seems like complaints would be posted if Nest cycled the reversing valve with the compressor.

O and B on zone 1 stat controls mode. They must be used on zone 1 stat according to the manual.

Post 2 shows this...
 
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Old 01-24-16, 09:42 AM
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There are complaints on Nest's web site about the compressor banging when the valve is turned off at the same time and the old 'stat didn't do it. A couple of posts say it is now fixed with a menu option to enable seasonal O/B use instead of in sync with a cooling/heating call. That was indeed something big that they missed.
 
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Old 01-24-16, 10:33 AM
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Nest still needs to address the Emergency heat and Aux heat issue.
W2 should be both and Emergency and Aux heat if a jumper is not allowed.

Emergency heat should be visible with the Heat, Cool, Off menu, not a different window.

I think I remember hearing about that cycling of the reversing valve when the first generation was initially released but this would not be an issue today.
 
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