Adding C wire to Furnace

Discussion in 'HVAC Heating & Cooling' started by rockycmt, Nov 25, 2015.

  1. rockycmt

    rockycmt Member

    Joined:
    Nov 13, 2007
    Location:
    New York Metro Area
    So I am looking to add a wifi Honeywell thermostat to my first zone. Currently it is a simple 2 wire thermostat for a Heat call only. It is a hot water hydronic monoflow loop controlled by a zone valve. Can I simply add a 3rd wire from the 24v transformer to provide the C wire that the thermostat requires?

    So use the two existing to control the relay on the zone valve and the new C wire will give the unit the power it needs to do the computer stuff.
     
  2. Stuff

    Stuff Well-Known Member

    Joined:
    Mar 7, 2013
    Location:
    Pennsylvania
    That is normally how it is done. Make sure to pick the correct wire from the transformer as one of them is already going to your thermostat. So in the end you have R and C coming from the transformer and W going to the zone controller board. Some boards have T and T for the thermostat connections and you have to figure out which one of them is really W.
    What zone controller are you using?
     
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  4. rockycmt

    rockycmt Member

    Joined:
    Nov 13, 2007
    Location:
    New York Metro Area
    So I have 3 zone valves no zone controller. So the transformer powers the 3 valves. I have 2 wires from one zone to the thermostat. Where would the 3rd C wire come from? The transformer directly? If so which wire? the 24v or the neutral?
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    To make any complete circuit, you need to have a connection from both sides of the secondary on the transformer. All a typical thermostat does is act like a switch that connects the 'hot' (both sides are actually 'hot', but one side might be grounded) to the device in question, and it passes through it, with the other side to the other terminal of the transformer. So, you might need to go back to the transformer, but there may also be a common terminal at the terminal board of the system. In any case, you should be able to measure 24vac between the two leads all of the time - if you can, you've made a complete circuit across the secondary of the transformer.
     
  6. hj

    hj Master Plumber

    Joined:
    Aug 31, 2004
    Occupation:
    Plumber
    Location:
    Cave Creek, Arizona
    One wire from the transformer goes directly to the controlled device and the other one goes to the thermostat and then to the device. You want your "C" terminal to connect to the side that does NOT go the thermostat.
     
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