With the thermostat blower switch set to ON, which measurement indicates a call for residential indoor blower operation?

When the thermostat blower switch is set to ON, the expected operation of the residential indoor blower is indicated by the presence of voltage at the load side of the relay. Specifically, a measurement of 24 volts at the load side of the relay confirms that the control circuit is calling for the blower operation.

In a typical HVAC system, when the thermostat is set to ON, it closes a switch that sends a signal (24 volts) to energize the relay, permitting power to flow to the blower motor. Thus, when there is 24 volts present at the load side of the relay, it indicates that the relay is energized and the blower motor should be operating as intended.

Measurements such as 0 volts at the load side of the relay would indicate that the circuit is open, and the blower would not be operational. Similarly, a measurement of 0 volts at the relay coil would suggest that the relay is not being energized to close the circuit for the blower operation. Therefore, identifying 24 volts at the load side signifies that the indoor blower should be actively running, fulfilling the thermostat's call for blower operation.