Adding a C wire to my 2 wire thermotat

Discussion in 'HVAC Heating & Cooling' started by rockycmt, Mar 20, 2017.

  1. rockycmt

    rockycmt Member

    Nov 13, 2007
    New York Metro Area
    I have a 2 wire thermostat (heat only). Simply connects the circuit to the zone valve. When it closes it turns triggers the end switch of the valve which kicks on the circulator. Sounds straightforward.

    Well I want to add a Wifi Thermostat. All the ones I looked at need a C wire. (For power) What is the best way to go? Can I just add a single wire from the 24v transformer to the thermostat? will it use a ground from the other 2? (assuming I locate the one that is ground)
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Personally, I'd prefer to run a new 4, 5 or 6 wire cable (so I'd have a spare or to potentially control an a/c unit later, especially if it had its own 24vac control circuit), but running a single wire, since it's low-voltage will work. The secondary, 24vac output of the transformer may not have a grounded side - you'd get 24vac (probably higher) when measuring across the secondary, but may not get a stable or reliable voltage from one of them to ground. You'd need to find the transformer (or maybe the terminal board) and determine where the other side of the 24vac transformer is connected, and run the new wire from there to your new thermostat. If you have a zone controller, the transformer might be in that box verses your boiler's controller.

    There are other ways to do this, but that's probably the most straightforward.
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    No matter how you cut it you'll need at least four wires. Honeywell, in my opinion, has the best WiFi thermostat and this is the one I have. It is less costly than NEST and the same features with a lot less fuss. Access from a smartphone or a PC is very fast via Honeywell's servers. If you buy one on line be sure it has the R in the model number for "retail". With this model you can get customer support from Honeywell. Without the R it is a model for their dealer network even though they'll identical. (RTH9580WF)

    Look up the installation manual for "heat only". If you cannot run a new thermostat wire to your circulator you can buy a 24 volt AC plug in transformer to be near the thermostat but it wont look good. The two wires would be connected to the R and C terminals. If you can run a new wire then at the circulator or furnace you'll need to tap into the 24 v transformer or installed a new one just for this thermostat.

    A transformer at the furnace.

    A transformer at the thermostat.
    Last edited: Mar 22, 2017
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