For air conditioning and heat pumps, which control can be temperature initiated and temperature terminated?

• A. High limit switches
• B. Thermostats
• C. Defrost controls
• D. Outdoor reset controls

Answer: (C)

Defrost controls are designed to respond to outdoor coil conditions and manage frost without relying solely on a timer. When the outdoor coil gets cold enough for frost to form, the defrost control senses this temperature condition and initiates a defrost cycle to melt the frost. The cycle ends when the coil warms sufficiently and the frost is cleared, allowing normal operation to resume. In this way, the defrost control is both temperature initiated and temperature terminated, driven by the coil’s temperature rather than by a fixed interval alone.

High limit switches are safety devices that shut equipment off if temperatures get too high, not normal cycle controllers. Thermostats control indoor comfort by sensing indoor temperature and turning equipment on or off accordingly, but they don’t manage the outdoor coil defrost process. Outdoor reset controls adjust leaving-air temperature based on outdoor conditions but aren’t the mechanism that starts and stops a defrost cycle.