When is the "Y" terminal energized in an HVAC system?

The "Y" terminal in an HVAC system is associated with the cooling function. It is energized specifically when there is a call for cooling from the thermostat. When a thermostat detects that the indoor temperature has risen above a preset temperature, it sends a signal to the air conditioning system to engage the cooling cycle, which is when the "Y" terminal becomes energized. This actions activates the compressor and often the outdoor condenser unit, allowing the air conditioning system to cool the indoor air.

In contrast, during heating cycles, the "Y" terminal is not energized because that function is managed by separate terminals in the system. Similarly, when the temperature is above the set point, the air conditioning unit will not operate since no cooling is needed, and thus the "Y" terminal remains inactive. System diagnostics may involve various checks but do not specifically require the "Y" terminal to be energized unless a cooling cycle is part of the diagnostic process. Therefore, the correct understanding is that the "Y" terminal is energized only in response to a direct call for cooling.