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I’m replacing a older programmable thermostat with Ecobee 4 which is to control a Ruud R801 HVAC unit installed last year. I have access at the thermostat to a wire (orange) that goes to the C terminal on the control board, but the Ecobee doesnt power up. Ecobee customer support I determined there is no significant voltage (<.01VAC) as measured by a multi-meter between the R and C terminals at the control board. That’s true in idle (status 0 on control board display), or heat (status H)

I’ve attached pictures of the thermostat terminal on the control board but can’t figure out what the problem is, much less fix it! The old thermostat (and humidistat) continues to work fine when Ecobee removed and re-installed. Rudd control board Can post other pics if I figure out how to do it on this site.

  • Can you try substituting the blue wire in the cable for the orange wire? Perhaps the orange wire's broken... – ThreePhaseEel Nov 30 '18 at 23:47
  • Measure between R and C on the furnace board. 24V? I have this weird feeling that isn't C as we know it... – Harper - Reinstate Monica Dec 1 '18 at 2:44
  • ThreePhase. It’s not the cable.l because (Harper) there is only .01-.02v between r and c on the furnace board. Maybe C is not what we know, but there is a cable from C going to the humidifier solenoid- which activates in heat when the humidistat trios. – user93716 Dec 1 '18 at 3:36
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    Are you measuring with the AC volts scale of the multi-meter? If not re-do your measurements on AC scale. I edited your question to show AC voltages. – Michael Karas Dec 1 '18 at 11:51
  • Thank you Michael. Silly me, there was 24VAC between R and C in the control board. However, there was 0VAC at the thermostat, do it appears that there is a break in the C site. – user93716 Dec 2 '18 at 18:18

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