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How to Power Unused C Wire

Hi!

I recently moved into a house with steam radiators run off an oil burner. I'd love to upgrade to a wifi thermostat that would require a C wire for power. When I removed the old thermostat, I was pleasantly surprised to see an unused C wire end wrapped up behind the drywall (see photo with exposed copper connection). The other end of the wire is not connected to anything in the basement. The oil burner uses a Carlin 70200 primary control, which only offers two t terminals - an RH and a W connection (see photo of the Carlin). So unfortunately the solution isn't as easy as just connecting the C wire at the same place as the RH and W wires on the burner.

I'm trying to figure out the best way to connect up the C wire and would love any advice! One idea that I had was using a 24V step down transformer off a nearby box. According to a video I found online, it said that I could run a wire off each side of the 24V transformer up the wall to the t-stat and have one wire go to the C terminal and the other would connect at RH (so RH terminal would have both the wire from the burner and the wire from the transformer). Is that right? The downside of this approach is that I don't use the existing C wire and have to run the wire up the wall.

Instead of running new wire up the wall, could I run one wire off of the 24V transformer to the existing C wire and connect the second wire off the 24V transformer to the existing RH wire in the basement? Would that be functionally the same as running the new wire up the wall and crossing with the RH at the t-stat (see photo where t-stat wires from furnace connect to wires running up the wall to t-stat)? What I'm worried about is accidentally doubling the voltage to 48V by adding two circuits.

I see that the Carlin 70200 also has two normally open dry 24 VAC alarm terminals. Is there any way to use those to simplify this set up?

Thanks so much for the help!

Thermostat wiring:
thermostat wiring

Wiring at the burner on the Carlin:
primary control wiring

Wire off the burner connecting to wire to the t-stat:

Comments

  • Zman
    Zman Member Posts: 7,328
    Where is the transformer powering the burner "T1-T2"?
    That is where the common will tie in.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    SWEIdelta T
  • ryanb
    ryanb Member Posts: 3
    Zman said:

    Where is the transformer powering the burner "T1-T2"?
    That is where the common will tie in.

    I'm not an expert, so I could be wrong, but I think the Carlin 70200 (the primary control where the T1-T2 are) is the transformer itself. According to the documentation (image below) the 70200 takes 120V AC in, and outputs low voltage to T1-T2 for the control. And I'm not sure how the common could tie into the 70200.

    Am I misunderstanding your question?


  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,449
    edited November 2016
    I would do a separate 24v transformer, and have the thermostat operate a relay such as a pam-1 which would in turn tie to the burner primary control TT. This will isolate the transformer from the primary control, you don't want to be connecting wires from a separate transformer to the burner control, just the dry contacts of the relay.

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Robert O'Brien
  • JUGHNE
    JUGHNE Member Posts: 9,750
    You mean the dry contacts of the relay he has to add would connect to T-T........not the dry contacts (alarm circuit) of oil burner control.

    Ryanb, you must be sure your understand the Warning #2 on the page you posted or you could let the smoke out of your burner control, (could cost a lot more than your wifi setup).
    ryanb
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,450
    RIB relay and a 24v transformer
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • SWEI
    SWEI Member Posts: 7,356
    Cleaner, yes -- and far easier for a novice to install.

    Caleffi ZSR101 or a Honeywell R8845U would provide equivalent functionality.
    cablemanryanb
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,449
    "Cleaner" but far more expensive. The Scottish in me always takes into account the $$ aspect as well as the simplicity and reliability of the installed system.

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • JUGHNE
    JUGHNE Member Posts: 9,750
    Then there is the old standard fan control center of 20 years ago.
    Transformer, plug in fan relay, screw terminals and being able to mount it all on a 4" sq box give it a clean look. The only issue of not being completely correct is having the T-T connections in the back of the box with the 120 VAC supplying the transformer. But a pretty simple wiring scenario if you follow the diagrams.
    Solid_Fuel_Man
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,449
    I actually just was on a no heat call that had a newly installed (by HO)Honeywell WiFi thermostat. Ran an air handler in a dentists office, heat and a/c. Thinking it would be miswired, I actually found the g wire which controlled the fan to not be working. The fan relay at the air handler shorted and burned up the small relay in the expensive thermostat. Some isolation with relays would have saved it. But alas, my rant is over.

    The engineer in me, is always looking for the simplest, most reliable, solution.

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    MikeL_2
  • Zman
    Zman Member Posts: 7,328

    The cleanest setup you can have uses the Taco SR501-4 switching relay.

    http://s3.supplyhouse.com/manuals/1351090952835/85160_PROD_FILE.pdf

    The 'stat goes to R and W. The third wire (C) goes to 24VAC Common (right next to R and W)

    The relay needs line voltage on N and H (neutral and hot).

    TT on the Carlin goes to 5 and 6 (N/O).

    Done.

    Since the burner manufacture does not give you clean access to the internal common, I agree that this is your best option
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ryanb
    ryanb Member Posts: 3
    JUGHNE said:


    Ryanb, you must be sure your understand the Warning #2 on the page you posted or you could let the smoke out of your burner control, (could cost a lot more than your wifi setup).

    Indeed! Burning down the house is not part of the plan! :smiley:

    The cleanest setup you can have uses the Taco SR501-4 switching relay.

    http://s3.supplyhouse.com/manuals/1351090952835/85160_PROD_FILE.pdf

    The 'stat goes to R and W. The third wire (C) goes to 24VAC Common (right next to R and W)

    The relay needs line voltage on N and H (neutral and hot).

    TT on the Carlin goes to 5 and 6 (N/O).

    Done.

    SWEI said:

    Cleaner, yes -- and far easier for a novice to install.

    Caleffi ZSR101 or a Honeywell R8845U would provide equivalent functionality.

    These sound like excellent recommendations. I'll look into these relay options. It sounds like with any of these options I'll be able to have 120V and the TT connections off the burner going in one side, and have RH, W, and 24V C coming off the other side, which is exactly what I need. Are there any substantial differences between the Taco SR501-4, Caleffi ZSR101, and Honeywell R8845U that I should be aware of? As I'm definitely a novice, these all-in-one options sound a lot better (even if slightly more expensive) than something more customized.

    Thanks again for all the tremendous advice!
  • Steam
    Steam Member Posts: 45
    edited November 2016
    You will need what's called an isolation relay.

    I've had perfect performance from the ARGO IR882 isolation relay. Also you will need a 120v to 24v transformer. The 24v output wires from the transformer go into the applicable isolation relay terminals, the two TT wires go into the input side (x1, x1) of the isolation relay. On the output side you will get your R,W & C wires that you would run to the thermostat. Very simple install. I used it for my ecobee setup. I mounted my relay inside an old control box.

    The TACO mentioned seems like a good choice too as the 24v transfomer is already within.

    Best of luck to you.
  • cableman
    cableman Member Posts: 69
    I have 2 zone valves, a 24v transformer and a l8124 aquastat. Do i need to add a seperate switching relay also?
    My setup was high voltage thermastats years ago so i only have 2 wire going to 2 separate thermastats.
  • Jamie Hall
    Jamie Hall Member Posts: 18,903
    @cableman -- that depends. Do you have a reason to want to power thermostats from the transformer? If you do, then you do. However, there are a lot of thermostats available, including many very good, flexible programmables, which are self powered (batteries).

    Where you certainly would need the C wire -- and a transformer and relays -- is if for some reason you wanted a WiFi thermostat. They guzzle power, and so require a grid connection.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,449
    All these new WiFi thermostats need that c wire to power the hungry WiFi capabilities. The old fashioned digital programmable thermostats you could get away with just 2 wires and have the batteries run through thermostat.

    Bottom line is that any WiFi thermostat needs a minimum of 3 wires for a single stage heat only application.

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • cableman
    cableman Member Posts: 69
    edited November 2016
    Jamie Hall
    Id like to upgrade mine to yes wifi thermastats! By any chance no one makes one that controls 2 seperate zones correct? Id rather not by 2, just use a sensor or existing wires for upstairs zone.
    Right now they are battery operated and pretty basic.

    Hatterasguy
    So i can rob 24v to feed the c terminal, just need to run a 3rd wire?
  • cableman
    cableman Member Posts: 69
    Hatterasguy
    Ok so i would just need a c wire ran to both stats?
    I actually ran 2-2 pair to the 1st floor stat, one pair for that stat and tied into the existing pair which feeds uptairs stat. Im thinking i only need 1 more wire to feed upstairs stat being that 1st floor has 4 wires. Not sure if that makes sence
  • cableman
    cableman Member Posts: 69
    Hatterasguy
    Thankyou!
    Ill check the rating on the transformer.
    Lemme see if i understand this, 24v red wire can be linked from thermastat to thermastat but the commons have to be separate correct?
    If thats the case being that i ran 2 pair to 1st floor thermastat i should only need to get another wire to second floor from 1st floor thermastat (second floors fed from 1st floors location)
  • cableman
    cableman Member Posts: 69
    Thankyou again
    Is there any problem linking the 24v or the c as far as load issues?
  • cableman
    cableman Member Posts: 69
    But is linking the wire ok meaning 1 16awg wire feeding 2 thermastats?
  • cableman
    cableman Member Posts: 69
    I changed up my wiring and now have a c wire to 1st floor thermastat. My transformer is a honeywell 24/40va, think it would power 2 honeywell v8043e zone valves and 1 or possibly 2 wifi thermastats?
    If it cant do i look for a 50 or 60va model?
  • Zman
    Zman Member Posts: 7,328
    cableman said:

    I changed up my wiring and now have a c wire to 1st floor thermastat. My transformer is a honeywell 24/40va, think it would power 2 honeywell v8043e zone valves and 1 or possibly 2 wifi thermastats?

    If it cant do i look for a 50 or 60va model?

    A 40 Va transformer should handle that easily.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • cableman
    cableman Member Posts: 69
    Great! Thanks
  • hot_rod
    hot_rod Member Posts: 16,272
    The zone valves don't draw that much, volts x amps = va

    Some of the new wi fi stats can pull high current when they go into battery recharge. One relay brand suggest no more that two Nest on their 40Va boxes.

    The t-stat you plan on using should have a power spec like this valve shows. most common wall stats are very low current draw, under 1 amp.


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • cableman
    cableman Member Posts: 69
    Im gonna go for the honywell 6530, just a wifi stat not a learning stat. 1 for now then add the other when i can get another c wire to its location.

    https://www.amazon.com/gp/aw/d/B00Y6M2OUC/ref=aw_wl_ov_dp_1_3?colid=1PRPJK1GS7C5T&coliid=I35FTJ62T8YE12&vs=1
  • rick in Alaska
    rick in Alaska Member Posts: 1,362
    Back to RYANB's problem. If I remember correctly, at least on the old 8184g's, you could pick up the "c" wire from one leg of the f-f terminal. Some furnaces use that scenario to power up a timer to start the fan early. Not sure if the Carlin is powered the same, but is sure the cheapest and easiest way to go.
    Rick