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Wiring a thermostat without a C wire

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Brianfrench
Brianfrench Member Posts: 4
in Thermostats and Controls

I have a Taco SR506 relay connectedto a HTP Elite gas boiler. I have 3 zones of heating with hot water baseboard. I have 3 Lennox cooling air handlers in the attic. There are only 5 wires, RH and W go to the Taco relay. It is next to impossible to run an extra C wire through the walls and ceilings. I see there is a SA11 module, however with my system Taco ST506 for heating and Lennox CBX25UVH-024, 2 units and Lennox CBX27UH-030, 1 unit. 
how do I wire the SA11? Is it possible to add a 1,000 ohm 1/2 watt resistor in the Taco SR506, and move the W wire to the C terminal in the, Emerson Sensi Touch Smart Thermostat SKU: R5000.45, thermostats?

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  • HVACNUT
    HVACNUT Member Posts: 5,834
    edited August 2020
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    Are you sure theres not another wire that was cut back to the jacket? 
    You can't use white as a Common. It's needed to complete the circuit through the thermostat. The resistor goes between C and W in the SR506.
    The SA11 looks like it was tailor made for that thermostat. 
    Brianfrench
  • GW
    GW Member Posts: 4,692
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    You sure you need the G? Swapping g to c is common. We often jump y and g and the ahu if we need to grab a C 

    also, if your air handler and boiler are close (can run a wire between the two), you can use the rH and w for ahu wiring now. In this case you set up a small RIB relay to operate the boiler. Small pain but now you have two more wires 

    also, Honeywell has their 2 wire with interface module is none of the above suite your fancy 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
    BrianfrenchSolid_Fuel_Man
  • Brianfrench
    Brianfrench Member Posts: 4
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    HVACNUT said:
    Are you sure theres not another wire that was cut back to the jacket? 
    You can't use white as a Common. It's needed to complete the circuit through the thermostat. The resistor goes between C and W in the SR506.
    The SA11 looks like it was tailor made for that thermostat. 
    I pulled out the wires and, yes - only 5 inside the jacket. 
    Inside the Taco relay, the R and W terminals connect to the red and white wires respectively. 
    There is a 24 volt set of auxiliary terminals in the Taco relay. 
    The instructions for the SA11, do not show the Taco application, only gas furnace. 
    I did see the Fast-Stat FSCM00, which shows my type of application. 
    I am wondering if the SA11 could be wired the same way?
     Thank You Very Much ; Brian 
  • Brianfrench
    Brianfrench Member Posts: 4
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    Brianfrench said:
    HVACNUT said:
    Are you sure theres not another wire that was cut back to the jacket? 
    You can't use white as a Common. It's needed to complete the circuit through the thermostat. The resistor goes between C and W in the SR506.
    The SA11 looks like it was tailor made for that thermostat. 
    I pulled out the wires and, yes - only 5 inside the jacket. 
    Inside the Taco relay, the R and W terminals connect to the red and white wires respectively. 
    There is a 24 volt set of auxiliary terminals in the Taco relay. 
    The instructions for the SA11, do not show the Taco application, only gas furnace. 
    I did see the Fast-Stat FSCM00, which shows my type of application. 
    I am wondering if the SA11 could be wired the same way?
     Thank You Very Much ; Brian 
    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Fast-Stat-FSCM00-Install-Instructions.pdf
  • Brianfrench
    Brianfrench Member Posts: 4
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    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Fast-Stat-FSCM00-Install-Instructions.pdf

    The Fast-Stat installation instructions 

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