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"C"wire needed - convert from battery to wi-fi T-stat

I am hopeful to move and transition from a battery operated T-stat to a wi-fi enabled (Honeywell RTH-6580WF) T-stat. I have learned that the new T-stat's now require a "C" wire and I could use your help. Our home is a 2-story contemporary that we; (1) heat using an independent hot water (baseboard) Weill-McLain GV Gold boiler (6-zones) with TACO zone controller and; (2) cool using a separate A/C unit with air handler. I will need to run a wire to serve as "C" to supply constant 24vac to the new T-stat and the best place will be at the T-stat presently controlling heat and AC on the first floor, and that T-stat is directly above the boiler and easiest to achieve. Behind the the existing T-stat (see pic) I located two cables, one (2-conductor) with RH and W running from the boiler TACO zone controller to the T-stat and one (3-conductor) with RC, Y and G running from the A/C air handler to the T-stat. I looked closely in the the air handler for wire markings but everything at that end are wire nutted together, a rat nest of wires and unmarked. At the TACO zone controller (see pic and diagram)

I can identify zone numbers (zone #1 in this case) and wire terminations on the controller board, I also located an open set of contacts marked with 24VAC/common on far left (see pic). So, my help question is: Can I run a single conductor between the COM terminal on the circuit board "common" to the T-stat "C" terminal and that will power the new T-stat requiring "C"? If so, coming from the control board (zone 1 with RH and W) to the T-stat plus the additional new run wire (from the COM terminal) would be the easiest to achieve. Thank you for your anticipated help and assistance.


  • Jamie Hall
    Jamie Hall Member Posts: 22,085
    Should work just fine.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ArmDon
    ArmDon Member Posts: 3
    Good morning and Thank Jamie... Question - If I use a volt meter, I should read 24VAC between the COM terminal I found on the left side of the board below the transformer and the RED RH wire leading up to the T-stat on zone 1 on the board? Correct?
  • Jamie Hall
    Jamie Hall Member Posts: 22,085
    Correct. Well, 24 VAC more or less!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,452
    Run a new 3 conductor wire instead of 1. If the existing wire isn't stapled to the studs, you can use it as a snake to pull your new 3 conductor.
  • hot_rod
    hot_rod Member Posts: 20,725
    Or 4 wire, I like to have a spare conductor always. I've never seen a single conductor thermostat wire? Use wire listed for the application.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Steamhead
    Steamhead Member Posts: 16,451
    edited January 2020
    On some thermostats, the C wire must go to the A/C unit rather than the boiler, because of the way the unit is wired internally. The instructions for your unit that I found online do not specify this- you might want to call Honeywell tech support to be sure.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • ArmDon
    ArmDon Member Posts: 3
    Just off the telephone with Honeywell T-stat support. The tech I spoke with believes that the this particular T-stat will function whether the "C" comes from the A/C air handler or from the boiler TACO valve controller board. Said "C" is "C". After checking their knowledge base, he found no contradiction noted that would indicate the T-stat would not function properly picking up "C" from the boiler side. I will run a brand new 5-wire run from the boiler zone valve control board, giving me 2 spare conductors for the future if need be. Am hopeful I can use the existing 2-conductor T-stat wire as my pull fish. Thank you to everyone for weighing in on this issue!!! :)