Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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)

share|improve this question
Usually there is a C terminal on the furnace controller board to connect it to – Steven Nov 24 '12 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 Nov 24 '12 at 18:35
Possible duplicate: Options for adding a "C" wire to thermostat – BMitch Nov 24 '12 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 Nov 25 '12 at 3:32

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.