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Replace Honeywell T8034C1481 Mercury Thermostat with Honeywell RTH8580WF Wifi

Replace Honeywell T8034C1481 Mercury Thermostat with Honeywell RTH8580WF Wifi

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  #1  
Old 04-04-13, 10:50 AM
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Replace Honeywell T8034C1481 Mercury Thermostat with Honeywell RTH8580WF Wifi

I have been conducting fairly extensive google searching already but was not able to find a definitive answer for my questions regarding the terminal designations of my old Honeywell T8034C1481.

The thermostat has the 'usual' B, G, R, Y, W terminals but in addition 2 more red wires on 1 and 2. So there are a total of 3 red wires and I wasn't able to determine what they are used for and how to connect them to my brand new RTH8580WF. There is no AC installed, just a furnace and fan.

Any explanation of the terminal designations and maybe even terminal mapping to my new thermostat would be worth gold for me right now.

I attached 2 images of the wiring as it is currently. Thanks much!
 
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  #2  
Old 04-04-13, 12:24 PM
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You only have five wires in your thermostat cable, red, white, yellow, green and blue. The "two additional red wires" are a factory applied jumper between terminals 1 and 2.

Standard colors/terminations are"
Red is power to the thermostat.
White is heating signal back to furnace.
Yellow is cooling signal back to furnace
Green is manual fan control signal back to furnace.
Blue can be for several different purposes as determined in the field.

I have no idea what the red jumper between terminals 1 and 2 may be for. Current Honeywell literature does not even show these terminals.

Best thing you can do is to look in the thermostat wiring section of your furnace to see what terminals are used with what color. More likely as not the blue wire will be on a C terminal or not connected. All other colors should be on the terminal corresponding to the wire color. If the blue wire is NOT connected you need to connect it to the C terminal.

At the thermostat connect the wires color for color and connect the blue wire to the C terminal.

Really, the instructions are well written for this model.
 
  #3  
Old 04-04-13, 04:43 PM
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Thank you so much for the answer. I feel embarrassed I did not even notice that 1 and 2 were simply jumpered.
I will open up the furnace tonight to see if B is connected to the C terminal and connect it if not. Do you have a link for good instructions how to hook up the wire to the C terminal? It will be my first try doing something on the furnace.

Again, thanks a bunch for your response.
 
  #4  
Old 04-04-13, 08:54 PM
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Blue wire is not connected

I opened up my furnace and unfortunately the blue wire is not connected. I attached a picture of the wiring.
The Honeywell instructions suggest to connect the G wire to the C terminal at the thermostat, move the G wire to the C terminal at the furnace (thats a problem for me because I don't see a C terminal) and jumper Y to G.
Since all they are trying to achieve is getting 24V AC onto the wire I was thinking I could use the disconnected B wire and hook it up to the unused Y1 terminal or connect it to the hopefully appropriately labeled Com 24V terminal after making sure it carries 24V.
Again, any hint here is highly appreciated. The lack of a C terminal is frustrating but maybe C and Com24V were used interchangeably?
 
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Last edited by horstkotte; 04-04-13 at 09:23 PM.
  #5  
Old 04-04-13, 09:46 PM
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Com 24 V is the correct terminal. The wording about moving the green wire was only for a four-conductor cable, since you have a five-conductor cable leave the green as it is and use the blue.
 
  #6  
Old 04-05-13, 07:51 AM
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Awesome, will have at it right after work. Thanks much for the confirmation!
 
  #7  
Old 04-07-13, 09:36 PM
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The C wire was super easy to hook up to the 24V COM terminal although I did blow the fuse in the process. After a little fiddling with my router I finally got the thermostat connect to the internet and have since been able to change my temperature from my laptop, tablet and phone. The best invention since string cheese. Again, thanks a lot for your help Furd.
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