Illustrated Explanation of Heat Pump Frosting Phenomenon

Why Do Air Source Heat Pumps Frost?

In cold winters, air source heat pumps, as highly efficient and energy-saving heating devices, are increasingly adopted by households and various facilities. However, during operation, frosting sometimes occurs on these units, which not only affects normal functioning but also reduces heating efficiency.

I. Working Principle of Air Source Heat Pumps

Before discussing frosting, it’s essential to understand the basic working principle. Air source heat pumps absorb heat from the air via refrigerant circulation, then transfer this heat to indoor water or air for heating or cooling. The process involves four stages: compression, condensation, expansion (throttling), and evaporation.

In heating mode:

1. Low-temperature, low-pressure refrigerant gas absorbs heat from the air in the evaporator, evaporating into high-temperature, low-pressure gas.
2. The compressor compresses it into high-temperature, high-pressure gas.
3. In the condenser, the refrigerant releases heat to indoor water/air, condensing into low-temperature, high-pressure liquid.
4. The throttle valve reduces its pressure, turning it back into low-temperature, low-pressure liquid, restarting the cycle.

II. Causes of Frosting
(1) Normal Frosting
When the evaporator surface temperature drops below both the air’s dew point and 0°C during heating, airborne moisture freezes into frost on its surface. This is a natural physical phenomenon (like frost on winter windows). Normal frosting is uniform and slow—common in northern winters below -5°C with high humidity.

(2) Abnormal Frosting

1. Poor System Design:
   • Large-circulation installations mixing hot/cold water force prolonged low-temperature operation.
   • Incorrect defrost parameters (start/end temperature, duration) for regional climate conditions.
2. Equipment Malfunctions:
   • Clogged Evaporator: Dirt/debris on fins disrupts airflow, causing uneven surface temperatures and localized frosting.
   • Faulty Outdoor Fan: Low speed or damaged blades reduce heat exchange, lowering evaporator temperature.
   • Refrigerant Issues:
     • Insufficient refrigerant: Heavy frost starts at the evaporator inlet (capillary tube outlet) and spreads upward.
     • Excessive refrigerant: Thick frost begins at the evaporator outlet/receiver and expands downward.
       Both states impair heating and trigger frequent defrost cycles.

III. Hazards of Frosting

1. Reduced Heating Efficiency:
   Frost acts as insulation, increasing thermal resistance and hindering heat transfer. As frost thickens, efficiency drops—indoor temperatures may fall below setpoints (e.g., 15°C instead of 20°C).
2. Increased Energy Consumption:
   Compensating for heat loss requires extended compressor operation and extra defrost energy, doubling electricity costs (e.g., from ¥200 to ¥400/month).
3. Shortened Equipment Lifespan:
   Frequent frost/defrost cycles stress components: compressors wear from repeated starts/stops; reversing valves fail due to constant switching. Lifespan may halve (e.g., 10 years → 5–6 years).
4. Poor Comfort:
   Erratic airflow (alternating hot/cold bursts) disrupts thermal stability, disturbing sleep and reducing quality of life.

IV. Conclusion
Frosting severely impacts heat pump performance, longevity, and user comfort/economy. Understanding its causes and risks enables timely intervention:

• Regularly clean evaporators.
• Optimize defrost settings for local climate.
• Promptly repair malfunctions.
  These measures ensure stable, efficient winter operation and maintain a warm indoor environment.