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Simple Low Voltage Thermostat Wiring to Control an Indoor Blower Motor (Quick Tip)

Updated: Nov 28, 2023


(This image is courtesy of our friend JD Kelly @student_of_hvac)


This drawing shows how a low voltage wiring circuit can control a high voltage, high current load, such as a blower motor. Make sure to show this pic to your new apprentices!


Remember that a thermostat acts as a simple switch. When manually turning on the fan, the thermostat connects the 24v R wire to G wire in the thermostat.


  • The R wire is the 24 volt power wire which is typically red.

  • The G wire is to control the fan and is typically green.

  • The C wire is the 24 volt common and is the path back to the 24 volt transformer, which is typically blue.


When the G wire touches the R on a call for fan, the G wire is the 24V hot which can be used to power the coil in a relay such as on the 12amp fan relay pictured above. The blue wire on the 12amp fan relay completes the low voltage circuit back to the transformer.


On the 12amp fan relay, the line voltage contacts are completely separate from the 24v circuit. The 24v relay acts as a switching mechanism to turn on or off the high voltage, high current load by opening or closing the contacts.


Teach your apprentices all about the relays we keep in the service truck, in order to get the customer up and going in a pinch!! Contactors, fan relays and DPDT general purpose relays come in the most handy! Remember to disconnect the high current loads when troubleshooting these relays for safety and just troubleshoot the 24v side along with using electrical resistance to measure the contacts.


Learn more about thermostat functions and teaching your apprentice using this video!

Learn about fan relays in this video!

Learn More By Checking Out These HVAC Educational Resources!


Ebook, Book, Workbook, Quick Cards: https://www.acservicetech.com/ac-book

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