No C wire connection in SR 506 zone control

Discussion in 'HVAC Heating & Cooling' started by Bob2701, Sep 29, 2021.

  1. Bob2701

    Bob2701 New Member

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    Sep 29, 2021
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    New Jersey
    Looking to install a Google Nest thermostat and it says I need a power source. I have an older SR 506 from Sweet Controls in CT and there is no connections for a C wire, only T-T for each thermostat.
    Is there any way to get a C wire on this panel?

    TIA, Bob
     

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  2. fitter30

    fitter30 Well-Known Member

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    Retired service tech
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    Peace valley missouri
    With a meter go between r on thermostat wire and chassis ground the nut that hold the transformer down.see if you get 24vac.
     
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  4. Bob2701

    Bob2701 New Member

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    OK, thanks I'll give that a try tomorrow. So since at the panel they are both marked T-T if I get the 24vac that one would be the R? I found a splice where the tech tied into so I just have to determine which side to use. The one circled is from the transformer ans the white wire is from the zone control, other 2 are the thermostat wires.
     

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  5. Bob2701

    Bob2701 New Member

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    @fitter30 I took a measurement as you asked but only read about 3-4vac. From the metal housing I got 37vac but I'm thinking that is from the 120ac rather than the transformer.
    I checked the splice I mentioned and the 24vac from that transformer was feeding the clock on another old thermostat. I have a four wire cable to the new Nest so I can send that 24vac to it but I'm not sure where to connect it. I'm thinking between the C and R but not sure if that will effect the zone control. It seems that the zone control only requires a closure from the thermostat. I have attached a picture of the Google Nest base plate.
     

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  6. DIYorBust

    DIYorBust Active Member

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  7. Bob2701

    Bob2701 New Member

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    This is basically what I have with the 24vac transformer, my problem is that I'm unsure where to connect both wires at the thermostat. It looks like I should be able to just piggyback on the R and the other wire to the C but I would like to be sure as to not damage the thermostat or zone control.
     
  8. fitter30

    fitter30 Well-Known Member

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    Retired service tech
    Location:
    Peace valley missouri
    The left termial on the zone controller T T for that zone is probably the R wire
    The two wires connected at old stat one is R has power all the time and W turns on the heat. A thermostat is only a switch a automatic one.. If old stat didn't have G you don't need one. Don't need to piggyback any wires. If 24 vac between R and chassis ground.
     
  9. Bob2701

    Bob2701 New Member

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    Sep 29, 2021
    Location:
    New Jersey
    I gave up on this Google Nest Thermostat. (Mirrored face)

    I took the board out and traced to the other side of the 24vac transformer and soldered a jumper to it. I sent this up to the thermostat and had 24vac between R & C but the thermostat changed it's error from N260 to N261. Both meaning about the same thing, power problem with the C wire.

    I have another Nest T4000ES working on the first floor which does not require the C wire.

    So I have ordered another T4000ES and will try to return the mirrored face one.

    Thanks to all for the suggestions.

    Bob
     
  10. DIYorBust

    DIYorBust Active Member

    Joined:
    Mar 5, 2019
    Location:
    Long Island, New York
    Normally you would connect the other half to the unused Rc terminal on the thermostat, if it's used there is an adapter to send power from the air handler transformer without adding a wire.

    I see now that this unit lacks an Rc terminal and is not intended by the manufacturer to be used with an external transformer. It might work if you pigtail the transformer to the R terminal, but I'd rather buy different thermostat, as you did. If you buy a thermostat with an Rc, you won't have this problem. And I'd say that applies to most smart thermostats.

    I also learned, you should not buy the Nest thermostat E unless you have a c-wire. They say it will work in 85% of homes, but I doubt 85% of homes have a c-wire.
     
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