Will Air Source Heat Pumps Frost at -25°C?
Air source heat pumps can indeed frost at -25°C, but whether or not frost forms depends not only on the temperature but also, more importantly, on the humidity in the air and the unit's operating conditions. In extremely cold and dry regions, despite the extremely low temperatures, the low moisture content in the air may actually reduce frost formation; conversely, in low-temperature, high-humidity environments, frost formation is more severe.
1. The Physical Mechanism of Frost Formation on Air Source Heat Pumps When a heat pump is operating for heating, the surface temperature of the outdoor unit's evaporator is much lower than the ambient temperature. When the evaporator surface temperature is below 0°C and the relative humidity is high, water vapor in the air condenses and freezes into a frost layer, a process similar to the icing of car windows in winter.
Three essential conditions for frosting:
1) Evaporator surface temperature < 0℃
2) Relative humidity > 60%
3) Continuous contact between airflow carrying water molecules and the cold surface
* Research shows that air source heat pumps are most prone to frosting when the ambient temperature is -2~5℃ and the relative humidity is above 65%; however, when the temperature drops below -15℃, the air humidity decreases significantly, and the frosting rate slows down.
2. Analysis of Actual Frosting Conditions of Heat Pumps at -25°C
1) Extremely Cold and Dry (e.g., Northern Winters): Slight or Slow Frosting. Very little moisture in the air; even at extremely low temperatures, it's difficult for a large frost layer to form.
2) Extremely Cold and Humid (e.g., Sunny Days After Snowfall in Northeast China): Obvious Frosting. Evaporation from snow increases air humidity, raising the risk of frost formation.
3) Sleet or Snow Melting: Severe Frosting or Even Ice Formation. Liquid water adheres directly to the fins and quickly freezes into ice, having a greater impact.
* Actual Measurement Data: In Gansu, at -22°C and under dry climate conditions, the air source heat pump only experienced slight frost after 120 minutes of operation, far lower than the frost levels under simulated laboratory conditions.
3. Impact of Frosting on Heat Pump System Performance
1) Decreased Heat Exchange Efficiency: The frost layer acts as a thermal resistance, hindering heat exchange between the air and the refrigerant, resulting in a 20%~40% reduction in heating capacity.
2) Duct blockage and increased fan load: Severe frosting can clog the fin gaps, forcing the fan to increase its speed, increasing power consumption and generating noise.
3) Frequent defrosting leading to heating interruptions: Each defrosting process typically lasts 5-10 minutes, during which the heat pump stops providing heat, affecting indoor thermal comfort.
4) Increased compressor load: After defrosting, the system needs to re-establish a pressure differential. Frequent start-stop cycles can easily overload the compressor, shortening the lifespan of the air source heat pump.
4. Countermeasures: How to reduce the impact of frosting?
1) Intelligent defrosting control: Determines defrosting needs based on multiple parameters such as ambient temperature and humidity, and evaporator temperature change rate, avoiding "defrosting without frost" or "delayed defrosting."
2) Optimized fin design: Flat fins offer superior heat exchange performance at low temperatures compared to louvered fins, and produce less frost, helping to maintain the heat pump's COP.
3) Regular cleaning of filters and fins: Prevents dust and leaf accumulation, improves ventilation, and reduces the risk of localized frost buildup on the heat pump.
4) Maintaining proper drainage: Timely drainage of condensate produced during defrosting prevents ice buildup and blockage of the air source heat pump's bottom at low temperatures.
