RIBs and Thermostats

Thermostats are cool. Well, thermostats cool…and heat. Do they do other things? The standard thermostat somehow has more colors of wire than your average rainbow so certainly there are a few features left to uncover. Let’s decode those colors and figure out how to turn on a 120V fan with those teeny tiny low voltage wires.

Thermostat Wire Colors

  • Red (R, Rh, Rc) – 24VAC Power
  • Yellow (Y) – Cool Signal
  • White (W) – Heat Signal
  • Green (G) – Fan Signal
  • Black or Blue (C) – 24VAC Common
  • Orange (O) – Reversing Valve Signal for Cooling
  • Dark Blue (B) – Reversing Valve Signal for Heating
  • Light Blue/Other (Y2) – Stage 2 Cool Signal
  • Brown/Other (W2) – Stage 2 Heat Signal
  • No Universal Color (E) – Emergency Heat Signal
  • No Universal Color (X/AUX) – Auxiliary Heat Signal


The control transformer provides 24Vac to the Thermostat. Older systems may not have a common wire, but that common wire is necessary for powering smart thermostats. All the other thermostat wires are applying or removing a 24V signal. For example, if the thermostat is reading the room temperature as higher than desired, it will close the circuit between the Red and Yellow wires. The Yellow wire is sent to the air conditioner’s control board, where it completes the circuit of a relay. There are dozens of configurations of wires that may or may not be available at the thermostat, but hopefully this overview is enough to get an idea of the role of a thermostat.

Using RIB for High Voltage and High Current

If we have access to the common wire, putting a volt meter from (G) to (C) we read 24V when there’s a call for the fan and 0V when there is no call. Let’s say we want to use that signal to turn on and off a 1HP @ 120V make-up air unit. We can use an RIB2401B.


From the RIB2401B datasheet, we can see the RIB will only draw 83mA @ 24Vac from the thermostat green wire. Like magic, now we’ve turned on a 1HP motor! The same concept can be used for turning on gas valves, boilers, pumps, or just about any HVAC equipment that needs high voltage or high current. For more magic or details on any of our products, please go to our website or contact us directly.

Noah Smith
Noah Smith

Noah Smith, or as his friends call him "Noah", is a design engineer at Functional Devices. He graduated from Indiana University in 2018. Just kidding, he graduated from the superior Purdue University with a Bachelors degree in Electrical Engineering in 2018.

Outside of work, Noah spends his time fantasizing about going back to work. He is a seasoned traveler, having visited almost two states. His weekends are spent with his wife and what has been described as "too many cats."

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