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Thread: Adding a Common to existing furnace wiring

  1. #1

    Default Adding a Common to existing furnace wiring

    My current furnace has Rh and W1 to the thermostat. I'd like to add a C for my new networked stat, but I'm not sure where to connect it.

    From my research it sounds like I should be able to just connect a new wire to one of the sides of the transformer, but I feel like it might be more appropriate to connect to the Taco zone valve where the transformer, aquastat, and thermostat come together currently (terminal 1 is W1, terminal 2 is white from the transformer and white from the aquastat, terminal 3 is aquastat red, and Rh is joined to transformer red).

    So, can I add a Common wire for the new stat by connecting directly to one side of the transformer? Or joining one of the terminals of the zone valve?

    For reference, the aquastat is a Honeywell L8124 C 1102.

    Thanks.

    (I can post pictures of the valve, transformer, aquastat if it helps)

  2. #2
    DIY Senior Member wjcandee's Avatar
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    If your new networked stat is a Filtrete, you can also just get a dinky transformer at Radio Shack and power it locally. They even have a little video on how to fish the wire through the wall.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    It sounds like the common is the white wire, but the only way to tell for sure is with a meter and some knowledge.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member kyleanderson's Avatar
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    hello there should be an C if trans has screw connection. if push wire then red hot, and here blue is c. if you have air conditioning then the y off trans, because state feeds red hot to contractor on outside unit, and the the y from trans feed C side this is if you have air with your system. the C from trans leads stright to parts, relays and such. the stat only feed hot to parts, relays and such. so the w1 on stat is feed hot to parts, relays and such. about the only reason we need C to stat now is for power the new stats. hope this helps

  5. #5
    DIY Junior Member Failure2Comply's Avatar
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    Quote Originally Posted by kyleanderson View Post
    hello there should be an C if trans has screw connection. if push wire then red hot, and here blue is c. if you have air conditioning then the y off trans, because state feeds red hot to contractor on outside unit, and the the y from trans feed C side this is if you have air with your system. the C from trans leads stright to parts, relays and such. the stat only feed hot to parts, relays and such. so the w1 on stat is feed hot to parts, relays and such. about the only reason we need C to stat now is for power the new stats. hope this helps
    "R" is HOT from the transformer, "Y" = cooling, W1/W2 = first & second stage heat ( or W2 = auxiliary heat if heat pump), "G" = fan, "O" or "B" (if Rheem/RUUD) = reversing valve, and "Common" most often designated as "C" from other side of transformer. A thermostat is nothing more than an automatic on/off switch that connects "R" to the other terminals to feed the corresponding controls. "Common" is fed outside to one side of the controls and "R" through "Y" starts the compressor, "R" through "G" starts the fan, etc.

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