Defrost That Will Not Terminate
A 2.5-ton R-410A heat pump is icing the outdoor coil and then staying in defrost too long. Outdoor ambient is 34°F and the indoor thermostat is set to 70°F. The defrost board is set for a 30-minute demand cycle. When defrost starts, the outdoor fan stops, the reversing valve switches to cooling mode, and the auxiliary heat energizes, as expected. However, the unit remains in defrost for more than 10 minutes; the outdoor coil temperature rises to 55°F and water runs off the coil, but the board does not terminate defrost until the board's 10-minute time limit. The defrost termination sensor is a 10-kΩ NTC mounted on the outdoor coil. At 55°F coil temperature, its measured resistance is 30 kΩ. The board's sensor chart shows approximately 10 kΩ at 50°F and 30 kΩ at 32°F. The defrost thermostat is a bi-metal that closes at 30°F and opens at 50°F; it currently reads closed with the coil at 55°F. The board receives 24 VAC and the sensor wiring is intact.
1. Explain the normal defrost sequence and why the outdoor fan stops and auxiliary heat comes on during defrost.
Read the worked answer
On a demand-defrost board, a timer or coil sensor initiates defrost. The board de-energizes the reversing valve so the system runs in cooling mode and hot gas warms the outdoor coil. It stops the outdoor fan so the coil temperature rises faster instead of rejecting heat to outdoor air. It also energizes the auxiliary heat so the indoor supply air does not blow cold during defrost. Normal termination occurs when the coil sensor or the defrost thermostat reaches its setpoint, usually around 50°F coil temperature, or when the board's maximum defrost time is reached. After termination, the board returns the reversing valve to heating, starts the outdoor fan (often after a short delay), and turns off auxiliary heat.
2. Using the resistance and thermostat readings, identify the most likely failed component(s) and explain why the board behaves as it does.
Read the worked answer
The sensor is the prime suspect. A 10-kΩ NTC should read about 10 kΩ at 50°F; at 55°F it should read less than that, not 30 kΩ. A 30-kΩ reading tells the board the coil is still near 32°F, so the board keeps defrosting even though the coil is warm. The defrost thermostat is also wrong: it should open near 50°F, but it reads closed at 55°F. Either the sensor has drifted or the thermostat is stuck closed; either one can hold the board in defrost until the time limit. The board is responding correctly to false cold signals, so replacing the board first would be guesswork.
3. What test would you perform to confirm the sensor is out of calibration, and what would you do if the sensor tests good but the thermostat is stuck?
Read the worked answer
Remove the sensor and check its resistance at two known temperatures: ice water at 32°F and room temperature near 70°F. Compare with the manufacturer's chart; if it is more than a few percent out of tolerance, replace it. Also warm the defrost thermostat with a heat gun or warm water while ohming it; it should open near 50°F and close near 30°F. If the sensor tests good but the thermostat is stuck, replace the thermostat. If both test good, check the board's sensor input circuit and wiring for moisture or corrosion. After repair, force a defrost cycle and confirm termination, fan delay, and return to heating.