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

HeatingHelp.com
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.
EdTheHeaterMan

Comments

  • STEVEusaPA
    STEVEusaPA Member Posts: 6,050
    Simple, easy. Every since @Robert O'Brien wrote this article, I've used it probably a dozen times.
    steve
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,041
    Thank you, @Robert O'Brien, for sharing this article with us.
    President
    HeatingHelp.com
  • 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: 15,901
    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
    Consulting
  • HomerJSmith
    HomerJSmith Member Posts: 2,059
    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
    HVACNUT Member Posts: 4,955
    @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. 
    EdTheHeaterMan
  • 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: 517
    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.
    jmullacbprov
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,377
    edited September 2022
    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 F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    jmulla
  • ChrisJ
    ChrisJ Member Posts: 14,130
    edited September 2022
    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
  • ngp155
    ngp155 Member Posts: 4
    I have a Burnham gas fired steam boiler that runs with a 40V transformer. I'm trying to replace a Honeywell programmable thermostat, connected with 2 wires (W & R) & runs on 2 AA batteries, with a Nest thermostat. I have additional wires to the thermostat and a separate 24v transformer available. The transformer has 2 terminals, Load & C, but the Nest only has a C connector. Where would the Load wire get connected to close the circuit? The remaining connectors are: W (connected), R (connected), C (available), Y (open), G (open) and OB (open). Thank you.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,377
    edited January 24
    There is sometimes a problem with this. Can you post a picture of your wiring diagram or the model number of the boiler? The age and model number will make a difference.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,377
    edited January 24
    There is sometimes a problem with this. Can you post a picture of your wiring diagram or the model number of the boiler? The age and model number will make a difference.

    If you have a newer boiler the wiring is fairly easy.

    If I guess correctly the R from the thermostat gets connected to the R on the transformer. There are some other wired connected to the R on the transformer.

    The C on the thermostat get connected to the C on the transformer

    The W on the thermostat get connected to the other thermostat wire nut. But I must see the actual wires on your boiler to be sure. Unlike a 2 wire thermostat, will not work if the R and the W from the thermostat are reversed.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • ngp155
    ngp155 Member Posts: 4
    Thanks for the quick response Ed. I don't remember, exactly, but the boiler is over 15 yrs. old. I tried to attach 2 pix to the initial post, but they don't seem to appear on the thread. I am attaching 2 pix of the schematic for the boiler and the ones of the transformer & Nest backplate and hope they transfer along with this post. Thanks again.
  • ngp155
    ngp155 Member Posts: 4
    @EdTheHeaterMan
    I have changed the files to jpeg, don't know how they saved with a strange extension, and attached them again along with a word document containing the pix as well. Also, I was looking at the current transformer directly attached to the boiler and even though the diagram on the back of the cover says the unit runs on a 40 v transformer, the transformer itself has a tiny label that says it is a 24v transformer. Thanks again.



  • Jamie Hall
    Jamie Hall Member Posts: 20,866
    That's a 40VA transformer, putting out 24 volts. VA is a measure of the power the transformer can provide, not the voltabe.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ChrisJ
    ChrisJ Member Posts: 14,130
    That's a 40VA transformer, putting out 24 volts. VA is a measure of the power the transformer can provide, not the voltabe.
    Volt amps is a measure of apparent power.


    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
  • Jamie Hall
    Jamie Hall Member Posts: 20,866
    fussy party. Watt's all this about power factors?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterMan
  • ChrisJ
    ChrisJ Member Posts: 14,130
    fussy party. Watt's all this about power factors?
    No no, no watts there, just volt amps
    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
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,377
    You do not need the additional transformer. You can use the transformer that is inside the boiler. The transformer that is on the wiring diagram. Here is a close up of the wires you need to connect to inside the junction box on the heater.

    This is essentially the diagram I posted in my first response The Ladder Diagram is easy to understand. The wiring diagram on the left illustration is easier to see where the components are actually connected.

    If you look closely the power wire on the boiler transformer has a BLUE wire connected to it. The Common wire has a YELLOW wire connected to it. Make sure the Nest R is connected to the BLUE on the boiler transformer. Make sure the Nest C is connected to the YELLOW wire on the boiler transformer. the Nest W wire is connected to the BROWN wire that operates the relay.

    That should be simple.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • ngp155
    ngp155 Member Posts: 4
    THANK YOU ALL ! Very appreciative of all of your expertise and advice. I just finished the connections and tested it and BY GEORGE your directions were perfect. Thanks again so much
    EdTheHeaterManMikeAmann
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,377
    edited January 27
    ChrisJ said:



    fussy party. Watt's all this about power factors?

    No no, no watts there, just volt amps


    I'm sorry, Who is on first... Watt's on second!
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics