# RTH8500 Wiring O and B Terminals



## kbsparky (Sep 11, 2008)

dermanj said:


> ...The old thermostat is wired to a Trol-A-Temp MM-2 Mastertrol Mini-Zone-2 at the furnace...


Do you have a 2nd thermostat somewhere else for the other zone?


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## kbsparky (Sep 11, 2008)

From studying your manuals, it would appear that you have to use "heat pump" settings on your new stat. This is to ensure your "O" and "B" wires get the signal in each respective mode.

From the new manual on page 12:

Connect the O wire to the O/B terminal.
Connect the B wire to the C terminal.

When programming the new thermostat, select option 2 for system type (Page 18 of your manual). This programming should make the O and B wires operate the same as your old stat.


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## dermanj (Sep 8, 2008)

kbsparky:

Thanks so much for the reply. So you think I need to follow the wiring instructions on page 12 -- "Alternate wiring (for heat pumps only)" -- and what about the old W wire? Would it go to Aux, and a jumper to E.

Honeywell phone support told me that I can't use this thermostat, period -- because I need separate O/B terminals, but looking at what you are saying and what it says on page 12, I'm thinking maybe those Honeywell phone support guys are mistaken.

Would it go like this?

old to new
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R to R
Y to Y
G to G
B to C
O to O/B
W to Aux (w/jumper to E) ???


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## kbsparky (Sep 11, 2008)

dermanj said:


> kbsparky:
> 
> Thanks so much for the reply. So you think I need to follow the wiring instructions on page 12 -- "Alternate wiring (for heat pumps only)" -- and what about the old W wire? Would it go to Aux, and a jumper to E....


I would try that scenario and see if it works.

Those phone tech guys are not always up to such tasks, when it comes to custom designed systems with things like zone controls, etc.


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## dermanj (Sep 8, 2008)

kbsparky:

Part of your recommendation is to set the system type to 2, as shown on page 18 of the manual. Note that the manual says system type 2 is "no backup or auxillary heat." Given that detail, would you still think Aux terminal (w/jumper to E) would work for the old W wire?

BTW, responding to your previous question: Yes, I do have a 2nd thermostat for zone 2. That one was easy. It only has 3 wires, RYW. I recently hooked up a TH8320 in that location (replacing a CT3600) and it works great.


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## kbsparky (Sep 11, 2008)

dermanj said:


> kbsparky:
> 
> Part of your recommendation is to set the system type to 2, as shown on page 18 of the manual. Note that the manual says system type 2 is "no backup or auxillary heat." Given that detail, would you still think Aux terminal (w/jumper to E) would work for the old W wire?...


Yes, when you program the thermostat for type 2, it reassigns those terminals for the proper operation.


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## DBH2008 (Sep 30, 2008)

dermanj said:


> kbsparky:
> 
> Thanks so much for the reply. So you think I need to follow the wiring instructions on page 12 -- "Alternate wiring (for heat pumps only)" -- and what about the old W wire? Would it go to Aux, and a jumper to E.
> 
> ...


I have the same issue as you with the RTH8500. Have you tried this work-around? Does it solve the problem?


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## rogik (Jun 22, 2009)

*Have you had success? I have same exact problem as you*

I hope somebody can help me since Honeywell customer support was useless in my case as well. 
I'm upgrading old RobertShaw thermostat to a new HoneyWell RTH8500 one. 
My old RobertShaw connection looked like this:
Connected:
G
O
Y1
W1
C
R
Not Connected: B

I installed the RTH8500 and connected it the following way:
R into RC (jumpered with R)
O into O/B
Y into Y
G into G
C into C
W into Aux (jumpered with E)

The unit works perfectly for cooling. No problems there. 

I experience the problem with heating. 
My heat-pump is not designated to heat, it is ONLY for cooling. 
When I turn on heating, thermostat turns on both heat-pump (which it should not) and my electric baseboard heating (which should be the only one turned on). 

I called Honeywell and they told me its impossible to turn on just electric baseboard heating without heatpump on this thermostat, which I find very hard to believe. 

The problem as I understand it is the following:
I have only 1 stage of heating (baseboard). The way I wired is for 2-stage heating system (where my heat pump is assumed to provide heat as well). 

How do I wire it to make Thermostat think my baseboard is the only heating source and not turn on heat-pump while in heating mode?

I though just plugging W1 wire into O/B (where O is right now) could help?

Thanks a lot!

Oleg


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## bervin306 (Oct 21, 2007)

I don't quite understand what you have. It is a Straight A/C only system and electric base board heat? Or do you have a heat pump that you do not use for heating and if it is a heat pump do you have electric heaters in the air handler?


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## rogik (Jun 22, 2009)

Thanks for your reply!

I just figured it out. 
I connected W wire to O/B input (which in conventional system mode is W input) and made the system think it has generic AC + baseboard heat system (no heat pump). 
now it works fine.

Thank you for your help though!


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## lrodrigues (May 5, 2013)

*Tricking the Thermostat*

Although it is an old post, I hope it can help someone else since I did not find any solution elsewhere.

I had the same issue with the Trol-A-Temp MM-2 when trying to replace my mechanical thermostat with RTH7500Ds. I tried the wiring scheme described by ‘dermanj’ with different system types and none worked properly. In the end, I came out with the following solution:. 

The Trol-A-Temp MM-2 uses 6 wires for the 1st zone (Y R W B G O) and 3 wires for the 2nd zone (W R Y). On the 1st zone, the following wires need to be connected in order to activate the cooling/heating/fan (if one connects them manually it will also work):

R+O+Y = Cooling
R+B+W=Heating
R+G=Fan

Having it said, the RTH7500D was definitely failing on closing these circuits. Thus, I made RTH7500D to think that I have a 4 wires system by connecting some wires together as follows:

Old to new:
G to G
O+Y to Y
B+W to W
R to R (w/ jumper to RC)

Followed by configuring 0170 to 1 (Heat/Cool)

In this manner, the thermostat properly activated (O+Y)+R for cooling, (B+W)+R for heating and G+R for fan.

On the 2nd stage I just connected Y to Y, W to W and R to R. 

ps: I believe it only works with battery devices since B is already connected (with W) to W.


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## Mooring10 (Nov 21, 2014)

*Question related for RTH8580WF*

I have a natural gas and electric A/C system. The system is split into two zones. I have the same battery powered White Rodgers for both thermostats These have 7 terminals: W, RC, G, Y, O, B, and RH. There is a jumper wire connecting the RC and the RH terminals. There is a wire coming up from the basement on each terminal except for RC. On the second thermostat there are wires connect only to RH, W, and Y with a jumper connecting RC and RH.

The zone controller board has these instructions:
Zone 1:
 SMZ Terminal “R” goes to “R” on Zone 1 Thermostat.​ SMZ Terminal “B” goes to “B” on Zone 1 Thermostat.​ SMZ Terminal “O” goes to “O” on Zone 1 Thermostat.​ SMZ Terminal “W” goes to “W” on Zone 1 Thermostat.​ SMZ Terminal “Y” goes to “Y” on Zone 1 Thermostat.​SMZ Terminal “G” goes to “G” on Zone 1 Thermostat.

Zone 2:
 SMZ Terminal W goes to Terminal W of the Thermostat.​ SMZ Terminal R goes to Terminal R of the Thermostat.​SMZ Terminal Y goes to Terminal Y of the Thermostat.

I am trying to use RTH8580WF (or any of the Honeywell WiFi stats). There is a C terminal on the zone board, but not going to the thermostats. They show a work around with the G wire that you give up the fan control. If I have to give up fan control by using the G wire; guess I will have to but would prefer not to.

Still don't know what to do with the O and B wires--it's not a heat pump system.

Honeywell has been non responsive and the local heating contractor isn't calling me back--not sure he knows how to fix this.

I really need/want to use a WiFi thermostat setup for our use patterns.

Any advice would be appreciated. Thanks.


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## Bullmastiff (Jun 5, 2016)

Older thread, but I'm hopeful someone can lend a hand in finding a resolution.

Purchased the RTH8580WF from Home Depot to replace a faulty thermostat.

After following the solution lrodrigues proposed I was able to get my Zone 1 installation to turn on all systems. Prior to that, on my initial try it would not turn on outside compressor, just fan. One problem resolved thanks to lrodrigues.

Much to my dismay, my other two zones will now only turn on if my Zone 1 installed RTH8580WF is on (set temp lower than ambient).

Photo Details Attached:

1) Original Wiring of faulty thermostat
2) Control Panel Wiring Diagram
3) Control Panel Wiring

I'm very hopeful I can get this to work properly with your help.

Can anyone help me troubleshoot this?

Thank you!


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## beenthere (Oct 11, 2008)

If your thermostat doesn't energize O and or B when its not calling for cooling or heating, then the panel will not work for any other zone to function until zone 1 calls for cooling or heating.

The remote control panel, it was the fix for this. But I not sure if EWC makes them anymore.


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## Bullmastiff (Jun 5, 2016)

Thanks for the quick response beenthere.

I am including a photo of my newly installed RHT8580WF.

If I move both O and B wires to the other side marked 'Heat Pump' O/B do you think this will be a solution?


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## supers05 (May 23, 2015)

Your solution might work. Tieing them to R would allow the other stats to work without this one calling. But It might allow for heating and cooling at the same time. 

If that doesn't work, at the zone board, you'll have to add a 3 way toggle switch. Common on the switch wired to R. The the other terminals to B and O separately. It'll switch from cooling to heating mode. 

Cheers!


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## beenthere (Oct 11, 2008)

Your zone panel needs a constant 24 volt to either O or B for the other zones to work in either cooling or heating mode. But both O and B can't be energized at the same time.

A toggle switch like Super suggest would work though.


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## Bullmastiff (Jun 5, 2016)

Thank you for the responses supers05 and beenthere.

Questions:

1) Since I currently only need A/C, which one do I energize, O or B? And how? At the thermostat or control panel?

2) Can anyone direct me to a toggle switch that would work with my system? Link 

Thank you!


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## beenthere (Oct 11, 2008)

O is for cooling.

https://www.zoro.com/carling-techno...9nWM5xVuMVx-zDSNuVC0DhoC_Cvw_wcB&gclsrc=aw.ds


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## stevejm35 (Nov 13, 2017)

*Re: Question related for RTH8580WF*



Mooring10 said:


> I have a natural gas and electric A/C system. The system is split into two zones. I have the same battery powered White Rodgers for both thermostats These have 7 terminals: W, RC, G, Y, O, B, and RH. There is a jumper wire connecting the RC and the RH terminals. There is a wire coming up from the basement on each terminal except for RC. On the second thermostat there are wires connect only to RH, W, and Y with a jumper connecting RC and RH.
> 
> The zone controller board has these instructions:
> Zone 1:
> ...


Hi, I have the exact same issue as you (same system, etc, same old thermostat same wiring). Did you find a fix? I was going to a NEST and I learned that I need to change my MM2 control board at the furnace, and then wiring will be a breeze. Waiting on the control board to come in mail tomorrow, and we'll see how it goes!


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## nnhhuu (Feb 9, 2018)

Hello guys, 
i have a similar problem, i want to use a wifi programmable thermostat , actual wiring use the O, B & C wires (picture attached). I bought a honeywell T5, but honeywell support team were not able to answer me how to connect it, they told me that T5 is compatible but they don't know how to wire it... :vs_laugh: I returned the T5, my specific question is what thermostat do you recommend me to install, i just want to have a programmable wifi thermostat.

attached the actual thermostat wiring and zone control board. pictuares. 

THANK YOU FOR YOUR ADVICES.

Nelson


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## stevejm35 (Nov 13, 2017)

nnhhuu said:


> Hello guys,
> i have a similar problem, i want to use a wifi programmable thermostat , actual wiring use the O, B & C wires (picture attached). I bought a honeywell T5, but honeywell support team were not able to answer me how to connect it, they told me that T5 is compatible but they don't know how to wire it... :vs_laugh: I returned the T5, my specific question is what thermostat do you recommend me to install, i just want to have a programmable wifi thermostat.
> 
> attached the actual thermostat wiring and zone control board. pictuares.
> ...


I had the dreaded O B and C wire problem, and I did exhaustive research so I could set up two different WIFI thermostats (NEST, and Honeywell), and I finally figured out that the easiest way was to change my zone control panel. It solved all my problems. I got the Honeywell TrueZONE: HZ311 Zoning Control Panel For Up To 3 Zones (1H/1C). It is very simple plug and play. And FYI, I have a gas furnace , single stage, and an A/C unit, single stage. I have one main zone, and one slave. 
Hope this helps


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## nnhhuu (Feb 9, 2018)

thank you for your soon answer, it seems that that is the direction, i don't want to spend to much money changing almos the whole system. 

is there any other option? i hope soo

thank you
Nelson


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## Houston204 (Oct 18, 2009)

While adding a 24 volt DPDT relay would help most of the posters on this thread it will not work on your Carrier / Bryant 2 zone Nelson.

Carrier does something that Honeywell doesn't that you may miss if you decide to replace the zone control board and both thermostats.

If the Carrier Leaving Air temperature detects the the temperature is too cold or too hot it will start to modulate the zone dampers that were shut so that they open a little. 

Carrier uses a timed signal to the dampers so that it knows the damper position. it can use this information to open the dampers a little as opposed to shutting down the system if a single zone is open and the LAT sensor senses that possible freezing up or overheating conditions exist.

Those Carrier dampers do not have an end switch and will suffer a shorter life when you install a Honeywell zone control with them.

( I would verify R and C if using this diagram, Configure O/B for O)


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## nnhhuu (Feb 9, 2018)

Houston204 said:


> While adding a 24 volt DPDT relay would help most of the posters on this thread it will not work on your Carrier / Bryant 2 zone Nelson.
> 
> Carrier does something that Honeywell doesn't that you may miss if you decide to replace the zone control board and both thermostats.
> 
> ...




Thank you @Houston204, very clear answer... let me ask some basic questions in order to be sure that i'm sentenced to live with my old thermostat.

1. i'm not looking for something "fancy" and "genius" i just want to have at least programmable thermostat, if it has WiFi would be nice. Is there any option ? I don't want to go to the thermostat every morning and adjust ir before leave and then adjust it again when i'm back at home.

2. there was a recommendation to eliminate the zones, it does not make sense for me. What do you think? 

once again thank you....

Nelson


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## Houston204 (Oct 18, 2009)

http://s7d2.scene7.com/is/content/W...hp01_article_1410348560497_en_ii1.pdf?fmt=pdf



> 1. i'm not looking for something "fancy" and "genius" i just want to have at least programmable thermostat, if it has WiFi would be nice. Is there any option ? I don't want to go to the thermostat every morning and adjust ir before leave and then adjust it again when i'm back at home.


It is a programmable thermostat. You may need to reprogram in the fall and spring but maybe not daily.



> 2. there was a recommendation to eliminate the zones, it does not make sense for me. What do you think?


I'm not sure that I would eliminate the zone control but that is your decision.

If you decide to replace the zone control board I recommend getting the Carrier 3 zone control. It will support your existing motorized dampers and allow you to use any thermostat.

How many conductors do you currently have to zone 1 and zone 2 thermostats?


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## nnhhuu (Feb 9, 2018)

Houston204 said:


> http://s7d2.scene7.com/is/content/W...hp01_article_1410348560497_en_ii1.pdf?fmt=pdf
> 
> It is a programmable thermostat. You may need to reprogram in the fall and spring but maybe not daily.
> 
> ...



Thank you Houston2004, 

at the end i found a programmable thermostat with separate O and B wires, th4210d1005, it is not wifi and can't be manage by my phone but i can set up the thermostat for different times at day every day. 
i keep evaluating ($$) the option to change the zone controller as you recommended me. 
thank you
Nelson


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