Heat Pump Not Defrosting With Coil Sensor Fault
A 3-ton residential heat pump with a demand defrost control is icing the outdoor coil in 35°F, 90% RH weather. The outdoor fan runs continuously in heating, and the coil is nearly blocked after 45 minutes. The defrost board has a 5 VDC supply and an NTC outdoor coil sensor. At 35°F, the service manual lists an expected sensor resistance of about 30 kΩ. The technician measures 4.98 VDC at the board’s sensor input when the sensor is disconnected and 4.9 VDC with the sensor connected. The sensor harness reads open circuit (OL) when checked at the board plug. The board’s diagnostic LED blinks 3 times, which the manual lists as coil sensor fault. The reversing valve is energized only in cooling. The auxiliary heat is 10 kW and is energized through a sequencer. The homeowner reports cold air during defrost cycles and occasional breaker trips.
1. Interpret the voltage and resistance readings. What has failed, and why is the coil icing?
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The sensor circuit is open. With the sensor disconnected, the board input is pulled to about 5 VDC. With the sensor connected it remains at 4.9 VDC instead of dropping to a divided value, which means the board sees essentially infinite sensor resistance. The harness reading OL confirms an open sensor or broken wire. The 3-blink code agrees with a coil sensor fault.
Because the board cannot measure coil temperature, it cannot initiate demand defrost reliably. It may fall back to a timed defrost or lock out defrost entirely. The result is a coil that stays below freezing and accumulates ice until airflow is blocked. The outdoor fan running continuously during heating is normal; during defrost the board should stop the fan. The iced coil after 45 minutes is the symptom of missing defrost, not a fan failure.
2. Explain the normal demand-defrost sequence and the risks of operating with this fault.
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In normal demand defrost, the board monitors the outdoor coil sensor and usually an ambient sensor. When the coil is cold enough and the coil-to-ambient differential indicates frost, the board initiates defrost: it energizes the reversing valve, stops the outdoor fan, and energizes auxiliary heat to temper supply air. It terminates defrost when the coil sensor reaches a set temperature or a time limit is reached.
With an open coil sensor, the board may run a backup timed defrost, or it may refuse to defrost. Risks include a blocked coil, low heating capacity, liquid floodback to the compressor, high discharge pressure, and cold supply air during defrost. The reported breaker trips may be related to the 10 kW auxiliary heat operating during defrost, or to a shorted sensor harness; both circuits need inspection. Do not bypass the sensor or force continuous defrost as a permanent repair.
3. What repair and verification steps would you perform, including defrost termination and auxiliary heat checks?
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Replace the coil sensor or repair the open harness, then measure resistance at ambient and compare it with the manual table. Reconnect it and clear the fault. Force a defrost using the board test pins and observe the sequence: reversing valve energizes, outdoor fan stops, auxiliary heat energizes, and defrost terminates at the correct coil temperature or time limit. Confirm the sensor is mounted on the correct coil circuit and not in direct sunlight.
Measure supply temperature during defrost to confirm the auxiliary heat is tempering the air. Check auxiliary heat amp draw and breaker sizing; if the breaker trips only during defrost, investigate the sequencer, element, and wiring. After a full heating cycle, inspect the coil for re-icing. If the board still will not terminate defrost, check the board’s termination settings and the sensor calibration before replacing the board.