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Taco SR503 and Nest Thermostat - Says R wire or C wire disconnected, but not both.

Community Member

I have a boiler in the basement, with two thermostats, one in the basement, and one on the first floor.

The first floor thermostat is working with a Google Power Connector, with it being connected to the COM port on the Taco SR503 board, and the old wiring connected to W and R and everything is working fine.

The old basement thermostat used to only have R and W but the thermostat wiring has a third unused wire. Everything seemed OK with just the R and W connected to the Nest Thermostat, but we don't heat the basement much so I think I need a C wire working.

I connected the C wire to the COM port on the Taco SR503 board (next to the other thermostat). However, now the Nest Thermostat says the R wire isn't connected but the C is. If I short the W and R wires (without turning off the power), I can hear clicking near the boiler. If I put the Nest Thermostat back on now, it says both W and R are connected, but now the C wire isn't connected.

If I shut off the power, the same thing happens.

If I put the W and R in the wrong way, the Thermostat says all wires are connected, but I can hear clicking from the boiler like it is being turned on and off repeatedly (I turned this off quickly).

None of this really makes any sense to me. Any suggestions would be helpful!


Community Specialist
Community Specialist

Hello Bobbretto,

Thank you for taking the time to let us know about your inquiry. I'd be happy to take a look at this for you.

It appears to be a power-related issue, and it is most likely that the C wire you installed is not properly grounded, which causes it to show no power to the R wire. To isolate, gather the power readings on your thermostat's technical information settings or in the equipment settings, depending on the Nest Thermostat version you have with and without the C wire.


Important: Turn off the power from your HVAC system at the breaker before touching the wires.

I'll look forward to your response.