Heat Pump Defrost Cycle
Why an outdoor coil frosts in heating, how the control decides to defrost, and what normal defrost looks like.
What it is
In heating mode the outdoor coil is the evaporator and operates below outdoor air temperature. When that coil surface falls below freezing and the air carries moisture, frost forms. Defrost temporarily reverses the system to melt it off.
Compressor: The compressor raises refrigerant pressure and temperature, so hot discharge gas leaves it in either mode.
How defrost runs
- 1The defrost control decides defrost is needed, using a coil temperature sensor combined with either accumulated run time or a demand calculation.
- 2The reversing valve shifts to the cooling position so hot discharge gas flows to the outdoor coil.
- 3The outdoor fan stops so the coil warms quickly.
- 4Auxiliary electric heat is usually energized so the indoor air does not blow cold.
- 5Defrost ends on coil temperature, or on a maximum time limit if the sensor does not terminate it.
- 6The system returns to heating and the outdoor fan restarts.
Steam rising from the outdoor unit during defrost is normal. A brief whoosh as the reversing valve shifts is also normal.
What can go wrong
| Symptom | Likely cause |
|---|---|
| Coil stays iced, unit keeps running | Defrost never initiating: failed sensor, failed board, or low charge keeping coil temperature odd |
| Defrost runs constantly | Sensor out of calibration, board fault, or restricted outdoor airflow |
| Ice only at the bottom of the coil | Drainage problem, defrost water refreezing, or unit set too low to the ground |
| Cold air indoors during defrost | Auxiliary heat not energizing during defrost |
| Unit never reaches setpoint in cold weather | Below balance point, or a charge or airflow problem |
How to diagnose it
Force a defrost using the manufacturer procedure
Confirms whether the valve shifts, the fan stops and auxiliary heat energizes
Measure the defrost sensor resistance against the manufacturer curve at a known temperature
A drifted sensor is a common cause of poor defrost timing
Verify charge by the manufacturer method for heating mode
Low charge lowers coil temperature and worsens frosting
Check outdoor coil cleanliness and clearance
Restricted airflow keeps the coil colder than it should be
Confirm the unit sits high enough above grade with free drainage
Meltwater that cannot drain refreezes into a block of ice
Tools involved
- Digital manifold with temperature clamps
- Multimeter
- Manufacturer defrost sequence and sensor resistance chart
Symptoms
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