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What To Do If You Don't Have a C Wire For Your Thermostat

HeatingHelp.com Member, Moderator, Administrator Posts: 7
The common wire is the neutral wire from the 24V transformer that provides the power that WiFi thermostats require in order for their features to work. Most existing homes do not have a C wire since non-connected thermostats don’t require them. Read on.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,775
    Simple, easy. Every since @Robert O'Brien wrote this article, I've used it probably a dozen times.
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 1,901
    Thank you, @Robert O'Brien, for sharing this article with us.
  • jmulla
    jmulla Member Posts: 12
    Thank you.
    I want to replace my old, 2-wire thermostat with a Nest that requires the C-wire. I have a 1-zone Taco SR501 controller. I am able to run a new 3 conductor cable from the thermostat to the boiler room.

    Question: Is there any preference whether I run the C-wire to the neutral on the thermostat, or the "COM" terminal on the Taco? They are both located in close proximity.
  • Steamhead
    Steamhead Member Posts: 15,501
    Use the C terminal of whichever transformer actually powers the thermostat.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • HomerJSmith
    HomerJSmith Member Posts: 1,677
    Question: Is there any preference whether I run the C-wire to the neutral on the thermostat

    I don't understand that statement. The third wire you going to add should be connected to the "C" terminal on the Nest thermostat to the 24 VAC/COM terminal which is next to the "RW/TT" terminals on the Taco. That would add 24Vac charging to the Nest with the Taco transformer providing the power. I think that the Taco transformer can handle the 2 pole relay on the Taco board and the Nest, too.
    HVACNUT Member Posts: 4,654
    @jmulla, I believe the SR501 only has a 15 or 20 va transformer, so it should be wired using a separate 40 va transformer and an isolation relay. 
  • Hollywood111
    Hollywood111 Member Posts: 6
    On the general topic on the "c" common wire feeding your new Nest and or other power consuming stats, most contractors will run in a low voltage cable, with multiple (extra) wires in case an issue arises with shorts, broken wire etc. 18/5-18/6 -18/8 etc. Depends on the flavor of the day or what they had on the truck. Most new power consumption stats require constant hard wired lo volt (24 VAC) to the t stat. Battery backup is just that, if you lose power the batt. can usually run the unit as long as sufficient battery backup is available, but only as back up for a certain period of time. Most internet connected devices need an interrupted power source to not only stay connected, but to do their job. Not like the older stats mentioned above. Common wire if needed, check your cable and or have a qualified contractor help you with this issue.
  • Jells
    Jells Member Posts: 462
    I had this problem. I stripped the relay and 24v transformer from an junked Minitherm boiler and popped them into a 4.5 x 4.5 x 1.5 steel box attached to the main control box of the burner. Works like a charm and cost me nearly nothing.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,541
    edited September 17
    HVACNUT said:

    @jmulla, I believe the SR501 only has a 15 or 20 va transformer, so it should be wired using a separate 40 va transformer and an isolation relay. 

    Nest does not need 40 VA for powering the Wifi features. The 15 VA transformer is more than adequate. But don't believe me, Ask one of the TACO reps that frequent this site. I seem to remember that the only time you need to add a transformer is when you use the multi zone relays and are powering up more than 2 thermostats. Then you may need to use a more powerful transformer than the one in the relay. The SR501 can only handle one thermostat because there is only one relay to operate one zone.

    @HVACNUT is correct when you apply this to the multi zone relay and you are powering 3 or more thermostat from that small transformer. There have been many discussions on this topic at HH and other forums.
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • ChrisJ
    ChrisJ Member Posts: 13,207
    edited September 17
    Honeywell prestige doesn't need a C wire from the equipment as you can use a separate 24v xformer and the thermostat head only needs two wires as the equipment controller can be installed by the equipment.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment