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Help installing new thermostat

BeastSword's Avatar

Join Date: Sep 2017
Posts: 2

09-11-17, 06:41 PM   #1  
Help installing new thermostat

I got a new Honeywell thermostat RTH8500D and having some issues connecting it up. I have a Gas Furnace as well and need help with wiring new thermostat, thanks for any assistance,
Image of original Wiring from Emerson Thermostat

The issue I'm having is the O/B or Black wire, there is no where to put that wire on the new thermostat which has the wiring diagram here:

Also here is a picture of the block on my furnace if that would help

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Houston204's Avatar
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Join Date: Oct 2009
Posts: 7,014

09-11-17, 07:58 PM   #2  
Do you have a model number for your thermostat, gas furnace or heat pump?

PJmax's Avatar
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Join Date: Oct 2012
Posts: 41,936

09-11-17, 08:20 PM   #3  
Welcome to the forums.

I think we need confirmation of wiring at furnace too.
No idea what black is doing there.

Can you confirm these colors on listed screw terminals ?
C = blue
G = green and white
R= black and red
W= white
Y = yellow

In looking further.... I see the black on R and the white on G coming in on a two wire thermostat cable. That means the black wire at the thermostat is not connected at the furnace.

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~ Pete ~

BeastSword's Avatar

Join Date: Sep 2017
Posts: 2

09-12-17, 09:45 PM   #4  
I ended up omitting the black O/B Wiring and everything seems to work perfectly, I think it may be used for a Air Conditioning but being in Canada I don't have A/C. Thx for the help

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