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

ArmDon

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

Should work just fine.

ArmDon

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

Correct. Well, 24 VAC more or less!

HVACNUT

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

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.

Steamhead

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.

ArmDon

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!!!