What is commonly used to indicate when the compressor should cycle on and off?

The most appropriate choice is system pressure, as it accurately reflects the operational needs of the refrigeration cycle within the air conditioning or heating system. The compressor's function is to compress refrigerant, transitioning it from a gaseous state to a high-pressure liquid state, which is pivotal in maintaining the desired temperature within the system.

System pressure readings help regulate the compressor's cycling. When the pressure in the refrigerant lines drops below a set threshold, the compressor will activate to increase that pressure, thereby allowing the refrigerant to circulate effectively. Conversely, when the pressure reaches a certain high level, the compressor cycles off to prevent excessive pressure buildup, which could lead to system failure or damage.

While ambient temperature, refrigerant type, and compressor speed all play roles in the overall functioning of an HVAC system, they do not directly control the on-and-off cycling of the compressor in the same way that system pressure does. Ambient temperature might affect system performance, but it isn't the activating parameter for the compressor cycle. Refrigerant type is significant for efficiency and compatibility but does not directly influence the operation cycles. Similarly, compressor speed can vary with load conditions, but it isn't the fundamental signal for determining when to cycle the compressor on or off.