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Old 09-08-2008, 02:24 PM   #1
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RTH8500 Wiring O and B Terminals


Please help.

I want to replace an old Honeywell CT3600 thermostat with a new touchscreen RTH8500 on a gas hot air furnace w/AC. I think my confusion is what to do about the O and B wires (changeover HEAT and changeover COOL, as I understand it).

Here is manual for new RTH8500: http://www.honeywellcentral.com/ssi/...-69-1902ES.pdf

Here is manual for old CT3600: http://www.instructionsmanuals.com/u...-CT3697-en.pdf

Terminals used on the old thermostat are RWYGOB. The old thermostat is wired to a Trol-A-Temp MM-2 Mastertrol Mini-Zone-2 at the furnace: http://customer.honeywell.com/techli...0s/69-1361.pdf

Here is a quote from the MM-2 manual:
"The Zone1 thermostat requires a changeover subbase with O and B terminals. The O terminal must make with R when subbase is switched to COOL mode. The B terminal must make with R when subbase is switched to HEAT mode."

After some back-and-forth with Honeywell customer service, they tell me that the RTH8500 will not work for me -- but that was at the end of a 20-minute phone call that started with them telling me that I cannot possibly have my current thermostat (CT3600) wired correctly with both an O an B terminal, so I'm not convinced they gave me good information.

I would like to use either an RTH8500 or a TH8320. Honeywell customer service suggested that I could use a TH4110D1007, but I really don't want that. I asked if there was a touchscreen model that will work with my system and the guy said no.

Can anyone tell me if there is perhaps an appropriate way to wire my system either with an RTH8500 or a TH8320?

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Old 09-12-2008, 04:00 AM   #2
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RTH8500 Wiring O and B Terminals


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Originally Posted by dermanj View Post
...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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Old 09-12-2008, 04:22 AM   #3
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RTH8500 Wiring O and B Terminals


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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Old 09-12-2008, 07:43 AM   #4
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RTH8500 Wiring O and B Terminals


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
----------
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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Old 09-12-2008, 09:14 AM   #5
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RTH8500 Wiring O and B Terminals


Quote:
Originally Posted by dermanj View Post
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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Old 09-12-2008, 10:54 AM   #6
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RTH8500 Wiring O and B Terminals


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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Old 09-12-2008, 05:23 PM   #7
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RTH8500 Wiring O and B Terminals


Quote:
Originally Posted by dermanj View Post
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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Old 09-30-2008, 09:17 AM   #8
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RTH8500 Wiring O and B Terminals


Quote:
Originally Posted by dermanj View Post
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
----------
R to R
Y to Y
G to G
B to C
O to O/B
W to Aux (w/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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Old 06-22-2009, 05:59 PM   #9
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RTH8500 Wiring O and B Terminals


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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Old 06-22-2009, 06:52 PM   #10
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RTH8500 Wiring O and B Terminals


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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Old 06-22-2009, 06:59 PM   #11
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RTH8500 Wiring O and B Terminals


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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Old 05-04-2013, 08:00 PM   #12
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RTH8500 Wiring O and B Terminals


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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