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Where to connect C Wire on Argo AR826 relay

litedesign Member Posts: 12
edited November 2022 in Thermostats and Controls
Hi. I am hoping to install a new Wifi thermostat for my boiler. My existing thermostat wire has an extra conductor that's coiled back around the wire, out of use. But I'm not sure if there is a terminal on my relay where I can connect it to get 24VAC common for the thermostat. I attached a few pictures.

EDIT: I metered 26VAC between the red thermostat wire going into "Aquastat" and the empty bottom screw on the "24VAC Zone Valve" terminal, just above it. Would that empty screw work as a common?



  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,684
    edited November 2022
    The thermostat is connected to the AR862 control, therefore the transformer on the AR 862 control must be the transformer that you connect the C wire from the thermostat. Picture it this way. You can not light a flashlight bulb by connecting it to the + side of a battery and the - side of a different battery. You must use the + and - side of the same battery in order to illuminate the light bulb. Now where can you find the C wire terminal on the AR862.

    You will need a multi-meter with a zero to 50 volt (or higher) scale. you want to find what terminal is equal to R on the transformer, which terminal is equal to W on the control and which terminal is equal to C on the transformer. this is a pair of wiring diagrams that show how the AR862 might be internally wired.
    The diagram on the left shows how a 24 VAC zone valve might be wired to this control (the DO NOT USE terminals) internally. The transformer will have 2 wires that make up the 24 volt secondary side. One is usually labeled R and the other C. This is not important when you only use 2 wires from the thermostat.

    The R and C and W on your thermostat require that you know which wire is R and which wire is C in order to make the wiring to the thermostat operate the heating control and the internal thermostat processes properly. On the left diagram I have illustrated the R to be connected to the left thermostat terminal. That would make the right thermostat terminal = W. The next thing that is needed is the C from the transformer to connect the C from the thermostat.

    Since a 24 V zone valve requires a return path the C on the transformer, then one of the zone valve (DO NOT USE) terminals on the AR862 MUST be C. To determine which one is C you will need a multi-meter, set to measure 20 to 30 volts, and the AR862 control powered with the thermostat wires disconnected.

    The second (right) diagram shows an alternate possibility. There may be several more. If you need more info, feel free to send me a message.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics