Adding a Common to existing furnace wiring

Discussion in 'HVAC Heating & Cooling' started by fracai, Nov 22, 2012.

  1. fracai

    fracai New Member

    Feb 21, 2008
    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.


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

    wjcandee Wise One

    Apr 27, 2012
    New York, NY
    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.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    It sounds like the common is the white wire, but the only way to tell for sure is with a meter and some knowledge.
  5. kyleanderson

    kyleanderson New Member

    Jan 19, 2013
    manchester, kentucky
    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
  6. Failure2Comply

    Failure2Comply New Member

    Sep 3, 2012
    Commercial HVAC
    Virginia, USA
    "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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