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Old 03-13-2013, 03:53 PM   #1
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Installing new thermostat


Hello I purchased my first home as of this week and there are a couple things I would like to change. I currently have the Robertshaw 9400 thermostat and Im going to replace it with the Ecobee smart si. I have a 1 stage furnace and 1 stage A/C. I cant figure out why the "Y" has 2 wires running to it? The reason I question this is because there has been some major wiring issue in the house that I have fixed already. I attached a pictures of the thermostat currently installed.

I just want to make sure I install my thermostat correctly without harming HVAC system.
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Old 03-13-2013, 04:55 PM   #2
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Installing new thermostat


Unusual. I would say if both heat and cool are working properly then it's properly wired. Maybe take it off the wall and see if the wiring is spliced behind it, and then check the wiring at the furnace end to see where those wires map to and maybe you can figure out why you have 2 wires on the Y connection.

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Old 03-13-2013, 05:05 PM   #3
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Installing new thermostat


Well I actually haven't tried the A/C in my house since its still pretty chilly outside. When I did a google search on this matter I bumped into the same issue this person has and they have the same thermostat as I.

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I have a feeling they wired the Rc and Y to the "Y" terminal but Im not sure why. I close on the house this Friday so I'll have to take a closer look.

Last edited by beenthere; 03-13-2013 at 06:36 PM. Reason: Removed link to another DIY site
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Old 03-13-2013, 06:04 PM   #4
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Installing new thermostat


Quote:
Originally Posted by word2yamutha View Post
Well I actually haven't tried the A/C in my house since its still pretty chilly outside. When I did a google search on this matter I bumped into the same issue this person has and they have the same thermostat as I.

Links to other DIY sites are not permitted.

I have a feeling they wired the Rc and Y to the "Y" terminal but Im not sure why. I close on the house this Friday so I'll have to take a closer look.
If they did that your A/C would run non stop. Honestly my first instinct is the red wire under Y goes to your A/C unit. That is typically done in the furnace, but would work at the stat like that, except a second wire has to go to common, and I don't see another wire and no common would be available at your stat.

Take a picture of the low voltage connections inside your furnace and that may help.

Last edited by beenthere; 03-13-2013 at 06:36 PM. Reason: Removed link to another DIY site from quote
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Old 03-13-2013, 06:06 PM   #5
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Installing new thermostat


Well as the guy in the link you posted found out, there was a reason they wired it that way. Just because it's unusual doesn't mean there is a problem, additional investigation is warranted before a course of action can be determined.
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Old 03-13-2013, 06:37 PM   #6
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Installing new thermostat


Links to other DIY sites are not permitted.

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Old 03-13-2013, 08:25 PM   #7
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Installing new thermostat


The smart si thermostat requires a C connection at the thermostat.
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Old 03-14-2013, 12:06 AM   #8
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Installing new thermostat


Hopefully these are better pictures for a better clarification. I guess I got lucky knowing that I have a common wire connection. Im not sure what the white wire is that is connected to the common wire.
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Installing new thermostat-20130313_221532.jpg   Installing new thermostat-20130313_220940.jpg  
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Old 03-14-2013, 12:13 AM   #9
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Installing new thermostat


The separate 2 wire loom goes to your outdoor condensing unit. The wire nutted blue and white wires will go to C on the new stat. All other existing connections transfer to the new stat.
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Old 03-14-2013, 02:17 AM   #10
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Will the "Y" terminal be the same with the existing red and yellow wire?
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Old 03-14-2013, 07:15 AM   #11
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Yet it will be the same. Normally the extra wires are connected at the furnace control board. For some reason yours was done at the stat. No big deal.... just unusual
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Old 03-14-2013, 09:49 PM   #12
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Installing new thermostat


Just wanted to say thanks for all the great information. Here is the finished product.
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