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Thermostat C wire to "COM" on Taco SR502?

Hi all,

I'm replacing a thermostat in one of our zones with one that can use the "C" wire for power. All I have is heating (hot water, 2 zones). The old thermostat there are 3 wires in the wall behind the existing thermostat, but only 2 were in use. One connected to "W" and one connected to "Rh" (which was also jumpered to "Rc" (See picture below). I tracked the wiring down to the Taco SR502 switch relay and found that "W" goes to the left side "T" in zone 1, and "Rh" goes to the right side "T" in zone 1 on the Taco (see Taco wiring picture below). The third wire was not connected at the switch relay either (colored red with black tape on the end in the Taco picture).

For the new Stat. can someone please confirm that W and Rh wiring would be the same and the only additional step to supply constant power to the thermostat is to use the third wire to connect "C" on the stat to "COM" (upper left hand corner of the Taco picture) on the switch relay?

Your help is much appreciated.



  • Paul48
    Paul48 Member Posts: 4,469
    edited April 2013

    I don't see 3 wires at the thermostat, and I also see new( modern) wire at the switching relay and old cloth covered at the thermostat.If there is 3 wires at the thermostat, make sure whereever they spliced the old and new , they connected all 3.
  • daniel_31
    daniel_31 Member Posts: 12
    edited April 2013
    Confirm 3 wires present and all spliced to new 18/3 going to relay

    The third wire is there, it's just not visible. And yes, when they converted our system and added the relay, they spliced new wire onto the line (why...dunno). All three are spliced with wire nuts. I followed all three from the stat, through the splice, and to the relay and confirmed that:

    - W goes to left T,

    - R goes to right T

    -  and the third ends up as the red wire which is not connected at the relay.

    Sorry if that wasn't clear.

    So, what's your advice regarding connecting the "C" from the stat to the the relay.? Shouldn't it connect at the "COM" to provide the new stat (a Nest by the way...now I've let the cat out of the bag) with continuous 24V power. Or, should it connect to "24VAC" which is right next (to the right) of "COM" on the relay board?

    Or do you have to reconfigure multiple things?

    Thanks for your help.
  • Paul48
    Paul48 Member Posts: 4,469

    There's only 1 transformer, so the COM has to be the same for all the 24v connections. Beyond that, I'd have to read the thermostat directions, and if it called for it, I'd use that COM connection.
  • unclejohn
    unclejohn Member Posts: 1,771
    If you have a volt meter

    Check between that "com" and the "T" terminals. If you get 24v you should be ok. But for some reason I don't think that top left terminal strip is and output.
  • daniel_31
    daniel_31 Member Posts: 12
    Checked with volt meter

    Thanks for the comments folks.

    Did some testing with the volt meter. Found that my initial description of the two Ts on the taco was reversed. With the stat disconnected, com to left T gives 24+ volts, while com to right T gives 0. Which I believe means that the left and right Ts are "R" and "W" respectively. (And com is the 24 V supply for C.)

    Got the nest connected and working with this all figured out. Thanks again.
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