My current furnace has Rh and W1 to the thermostat. I'd like to add a C for my new networked thermostat, 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 have this unfounded feeling that it might be more appropriate to connect to the zone valve where the transformer, aquastat, and thermostat come together currently (terminal 1 on the valve 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 and the valve is Taco 555-102.

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

  • 1
    Usually there is a C terminal on the furnace controller board to connect it to
    – Steven
    Commented Nov 24, 2012 at 17:30
  • I'll have to go back and look more closely, but I don't think there is such a terminal, but maybe I just haven't been looking in the right place.
    – fracai
    Commented Nov 24, 2012 at 18:35
  • Possible duplicate: Options for adding a "C" wire to thermostat
    – BMitch
    Commented Nov 24, 2012 at 23:02
  • Maybe a duplicate, I looked at that answer while researching and maybe this is just down to terminology. It sounds like the transformer supplies '24v ac' and 'common', so I can just add a parallel wire to the transformer common.
    – fracai
    Commented Nov 25, 2012 at 3:32


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