In the schematic, what voltage would be found from terminal Y to the input side of the coil on line #11 when the thermostat is closed and the fan is set to auto?

In the schematic, what voltage would be found from terminal Y to the input side of the coil on line #11 when the thermostat is closed and the fan is set to auto?

• A. 0V
• B. 120V
• C. 24V
• D. 240V

Answer: (A)

When the thermostat is closed, the Y circuit becomes a single energized node connected to the 24V supply (R). The input side of the coil on line that feeds the cooling circuit is tied directly to that same Y node through the wiring. Because they’re effectively the same point, there’s no voltage difference between them. In other words, Y and the coil’s input are at the same potential, so the voltage from Y to the coil input is 0V. The coil itself would see 24V across it (from Y to C) when energized, but the specific measurement described is the difference between two points on the same node, which is zero. The fan setting on auto doesn’t change this relationship; it only affects when the blower runs after the cooling call is made.

When the thermostat is closed, the Y circuit becomes a single energized node connected to the 24V supply (R). The input side of the coil on line that feeds the cooling circuit is tied directly to that same Y node through the wiring. Because they’re effectively the same point, there’s no voltage difference between them. In other words, Y and the coil’s input are at the same potential, so the voltage from Y to the coil input is 0V. The coil itself would see 24V across it (from Y to C) when energized, but the specific measurement described is the difference between two points on the same node, which is zero. The fan setting on auto doesn’t change this relationship; it only affects when the blower runs after the cooling call is made.