A defrost termination switch (DTS) is a temperature-sensing device, typically a bimetallic snap-disc thermostat, installed on the outdoor coil of a heat pump system to signal the end of a defrost cycle. It monitors coil temperature and opens its contacts once the coil reaches a predetermined temperature threshold, confirming that accumulated frost has sufficiently melted. This prevents the system from running in defrost mode longer than necessary, protecting both efficiency and equipment longevity.
Technical Details and Specifications
The defrost termination switch operates as a normally closed (NC) switch during standard heating mode. As the heat pump enters a defrost cycle and the outdoor coil warms, the bimetallic disc inside the switch responds to the rising temperature. When the coil surface reaches the calibrated termination point, the disc snaps open, breaking the circuit and signaling the defrost control board to end the cycle and return to normal heating operation.
- Termination Temperature: Most switches are calibrated to open between 50°F (10°C) and 70°F (21°C), though the exact set point varies by manufacturer and system design.
- Amperage Rating: Typically rated for 5 to 10 amps, sized appropriately for the defrost control circuit.
- Mounting Location: Attached directly to the outdoor coil tubing, usually near the liquid line or at a point that accurately represents overall coil temperature.
- Reset Behavior: The switch automatically closes again as the coil temperature drops back below the set point during normal heating operation.
Practical Significance in Heat Pump Systems
Without a properly functioning defrost termination switch, a heat pump system faces two problematic scenarios. If the defrost cycle runs too long, the system operates in cooling mode unnecessarily, blowing cold air into the conditioned space and wasting energy. If the cycle terminates too early, residual ice remains on the coil, restricting airflow and reducing heat transfer. Over time, incomplete defrosting leads to progressive ice buildup, diminished heating capacity, elevated refrigerant pressures, and potential compressor damage.
A failed or miscalibrated DTS is one of the more common causes of defrost-related service calls. Technicians typically test the switch with a multimeter for continuity and verify that it opens and closes at the correct temperatures. Replacement switches must match the original equipment specifications for both temperature rating and amperage capacity.
Applications and Related Components
Defrost termination switches are found in virtually all split-system and packaged heat pumps that use time-temperature defrost control logic. In these systems, the defrost control board initiates a cycle based on a timed interval or accumulated compressor run time, and the DTS provides the temperature-based signal to end it. Some modern systems have moved to thermistor-based defrost sensors that provide continuous temperature readings to the control board, but the snap-disc DTS remains widely used due to its simplicity and reliability.
Many defrost termination switches are combined with a fan delay function in a single component, often labeled as a DTS/fan delay switch. This dual-purpose design keeps the outdoor fan off during defrost and delays fan restart until the coil temperature drops, preventing cold air from being blown across a warm coil immediately after the cycle ends.
Related Terms
- Defrost Cycle
- Defrost Control Board
- Defrost Sensor (Thermistor)
- Bimetallic Thermostat
- Heat Pump
- Reversing Valve