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Proper Wiring Help Needed

WiringWiring Member Posts: 3
I purchased a Honeywell RTH8580WF Wi-Fi Thermostat and now I am not sure if I can use it on my system.  The current thermostat has only two (2) wires on it connected to the RH + W terminals.  I spoke with Honeywell and they tell me I need a third wire (24V) to power the display.  I checked the boiler and the thermostat is connected to a TACO SR503 Zone Control.  I only see two terminals where the current thermostat is connected and I am trying to figure out where the third would connect?

Can anyone help me out here please or would I be better off just returning the new thermostat?

Thanks in advance!


  • SWEISWEI Member Posts: 7,356
    Those two terminals

    above the transformer have 24VAC on them.
  • Paul48Paul48 Member Posts: 4,046
    edited April 2014

    I think he needs a Common wire SWEI. Oh...I see what you're talking about now. There's a "C" there.
  • TechmanTechman Member Posts: 2,143

    You have a major miswiring problem here. Is that low volt (24v) wiring  going into the same BX as the line volt (120v) wirers? If I'm seeing that correctly , then you have a big problem looming ahead of you and anyone who adjusts the t'stat.
  • WiringWiring Member Posts: 3
    I thought it strange

    I noticed that when I first opened the cover that the low voltage wires were fed in along with the AC.  I have already disconnected and removed them, feeding them in from the knockout on the opposite side.  I am glad I started this project or I would have never noticed it.  This was installed by a licensed contractor 2-3 years ago and has been that was since.  I have to pull a new wire from the thermostat down in order to get the third wire and just have not had the time as yet.  Thanks all for the info!  If I understand everything correctly, all I need to do is add the wire from the Common terminal of the 24VAC in order to get the new thermostat functioning.
  • BusterBuster Member Posts: 28
    Proper wiring

    The 24vac terminals on the Taco control will go to the RC & C terminals in the thermostat, the existing wires from the Taco control (old thermostat wires) will go to the R & W terminals of the thermostat and remove the metal jumper between R & RC.
  • WiringWiring Member Posts: 3

    Thank you Buster!  Clear and concise!
  • STEVEusaPASTEVEusaPA Member Posts: 1,259
    edited April 2014
    End switch

    @ techman..Looks like those 2 wires are going to the end switch
  • TechmanTechman Member Posts: 2,143
    edited April 2014

    Hi Steve, the low volt wirers being for the end switch makes good sense. The problem is the insulation value for the lo volt is 300V and 600v for the line volt wirers. So the line volt can "jump " thru any cracks , wear, or worn spots in the insulation , therefore "line volt wire " should have been used for the lo volt circuit if both are run thru the same conduit. We do that all the time. Plus if done on electronic control circuits there can be "interference" in the lo volt wiring due to magnetism.
  • STEVEusaPASTEVEusaPA Member Posts: 1,259
    ahhhhh.....good point

    I didnt even look at that because i've never put low and line voltage in the same conduit.
  • heatpro02920heatpro02920 Member Posts: 991
    Everyone does things their own way...

    I see the lv and bx ran like that all the time, I don't do it myself because I do it the way I always did it, kind of stuck in my ways. I get on a job and it seems to go super fast when I do it the way I always did it...

    And I as well as a lot of other guys will look at a different than our own practice and think its wrong or not as rite, but if its up to code, performs as intended, and the customer is happy with it, then its good...

    I had people comment about how I wire in a zone valve control on a retrofit, since the wries are never long enough to reach the control I mount my control, then run 18-5 up to the zone valves, tie red and white into the t-stat wires and tire the blue, green, and yellow into 1, 2, and 3 on the zone valve... I strip the 18-5 back around 8 inches and put a couple screwdriver loops in the r,w and then the b,g,y it looks good when its done, IMO, its fast, convenient, and works well for me... Now a tech that works for me and a friend of mine in the field like to put a handy box in the ceiling and connect all the t-stat wires in there with new longer extensions that go to the control, then run 18-2 for that and 18-3 to each zone valve.... To me it takes too long, doesn't look as good, costs more in materials, and gets confusing when servicing...

    I know a contractor that runs all his t-stats in one cable up to the ceiling {no box} then connects them all to a single cable and then runs 18-3 to each zone valve, so if he has 5 zones he runs 18-10 {10 different wires} for all the tstats and then wires each one, the wire he uses has double colors, so it for 18-10 it will be blue and then blue with a white stripe, then orange and orange with a white stripe, ect so each color is its own zone, then he puts a piece of that color electric tape on the zone that corresponds to it so the zone valve that he used for the orange wires gets some orange tape....

    To me it all works, but I like my way...
  • TechmanTechman Member Posts: 2,143
    edited April 2014
    Lo volt vs line volt

    But it is against code to run both wirers in the same conduit. Put the 24v into "line volt wirers " and it is legal. Look inside control panels, relays boxes , zone panels and there is a "divider" (usually gray colored cardboard) between the two voltages separating them from each other. Look at the RIB relays, they use all "line volt" wirers.Look at AC cond units, lo volt wirers(24v) connect to line volt wirers(24v) then the lo volt circuit goes thru hi/lo press switches, the Copeland "staging control" or "time delay relays" or any other controls.
  • HeatBudyHeatBudy Member Posts: 1
    I'm a new bee to all this but this is what I found out after research. If you have a Taco SR503 switching relay and your current system is a 2-wire, and you are looking to add a smart thermostat Honeywell. Then you will have to do the following,
    If your only have two wires you will need to upgrade, if you have 3 wires or more you are ok..
    Let me say this first.. I am only talking about a heat only system, if you have A/C then you will need more than 3 wires anyway.
    1st. upgrade your wiring to 3 wires or you can get one of those add a wire devices, but I am not speaking to that option.
    2nd Once the wire is in place or you already had more than two wires. you will need to connect a C or common wire to the Taco SR503. remove the cover on the RS503 to the right of the zones I have 3 zones you will see a connection labeled 24vac on one side and com on the other side.
    3rd. take the blue wire and connect it to the ( com ) side - this is the hot side that has the 24volts need to power your thermostat. If you have only one zone let's say then you will have a white wire to the (W) a red wire the the (R). for zone 1.
    Now take the blue wire and connect it to the com port.. If you don't have blue pick another color. So now lets go to the thermostat,
    I just did a Honeywell smart thermostat and I think the layout is the same for the Honeywell smart thermostats.. remove the old thermostat make sure you mark your wires or at lease know it's letter i.e. W or R or Y or what ever.
    Heat only system will have two wires most likely W and R. put the new mounting plate up for the thermostat, connect the wires as followed.
    The White wire to the terminal marked W,
    The Red wire to the terminal marked R
    and the new wire to the terminal marked C.
    put the cover on power up your unit and you should be good to go.
  • vaporvacvaporvac Member Posts: 1,512
    @HeatBudy‌ , you should write their manuals! I 'm using your system to look over my wiring at the Tstat tomorrow. Thank you.
    Two-pipe Trane vaporvacuum system; 1466 edr
    Twinned, staged Slantfin TR50s piped into 4" header with Riello G400 burners; 240K lead, 200K lag Btus. Controlled by Taco Relay and Honeywell RTH6580WF
  • NeedAdviceNeedAdvice Member Posts: 1

    How is this done when you have two or more zones with a taco relay and only one com port. Thanks
  • ChamkilaChamkila Member Posts: 8


    How is this done when you have two or more zones with a taco relay and only one com port. Thanks

    Same question, i have 2 zones and 2 different thermostates, can i run these two on the same 24vac terminals? with 2 wires each in each terminal? Thanks
  • ChamkilaChamkila Member Posts: 8
    Buster said:

    Proper wiring

    The 24vac terminals on the Taco control will go to the RC & C terminals in the thermostat, the existing wires from the Taco control (old thermostat wires) will go to the R & W terminals of the thermostat and remove the metal jumper between R & RC.

    Can you help with the question above please. Thanks
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