A typical operating condition where poor return of refrigerant oil at low evaporator temperatures in a heat pump indirectly leads to an abnormal rise in evaporator temperature

Core logic: Refrigeration oil accumulates in the evaporator/heat exchanger and piping → Increased thermal resistance and reduced heat transfer capacity → Poor heat dissipation on the condenser side and increased system high-pressure → Forced reduction in the heat pump’s heating capacity / system imbalance → Insufficient heating of the evaporation chamber, forcing the evaporation temperature to rise, and failing to achieve low-temperature performance. The following are the most commonly overlooked triggering conditions:

1. Continuous operation at low load (Most common)

Reduced equipment capacity and low throughput result in insufficient compressor discharge velocity and excessively low oil-vapour flow velocity. Consequently, the refrigeration oil cannot be carried back by the refrigerant and accumulates in large quantities within the heat pump evaporator coils.

Heat exchange efficiency on the heat pump side declines, leading to insufficient heating output. To maintain evaporation rates, the system is forced to increase the heating temperature to compensate, ultimately causing the material evaporation temperature to rise abnormally and rendering the nominal low-temperature performance ineffective.

2. Large diurnal temperature variation and sudden drops in ambient temperature at night

Low ambient temperature → Low condensing pressure → Slower refrigerant circulation;

Oil return rate decreases further, causing significant oil accumulation in the heat exchanger and liquid separator.

When the load increases again after temperatures rise during the day, the accumulated oil leads to severe heat exchange deficiency, causing the evaporation temperature to surge and the vacuum to deteriorate simultaneously.

3. Frequent start-stop cycles and intermittent operation

During start-stop cycles, refrigerant flow fluctuates wildly, causing unstable oil return; each shutdown leaves a portion of oil trapped in the heat exchange piping;

After multiple start-stop cycles, oil accumulation increases, heat exchange performance in the heat pump continues to deteriorate, and the evaporation temperature gradually rises without triggering a direct alarm.

4. Low winter temperatures + defrost cycles

During defrost mode, the refrigerant flow direction reverses, making it easy for refrigeration oil to become trapped in dead corners of the heat exchanger;

At low temperatures, oil viscosity increases and fluidity decreases, making oil return even more difficult;

Heating efficiency declines, and output can only be maintained by raising the evaporation temperature, causing the evaporation temperature to deviate significantly from the set low-temperature value.

5. Scale build-up on the heat pump evaporator, insufficient airflow, and reduced heat exchange leading to obstructed oil return

Dust accumulation and blockages on the air-cooled fins result in insufficient airflow, excessive refrigerant superheat and uneven flow rates;

Locally low refrigerant flow rates cause oil to settle and accumulate;

Heat pump heating capacity declines, forcing the system to passively raise the evaporation temperature.

6. High concentration and sudden, significant increase in evaporation load

The material suddenly becomes viscous or forms scale, requiring greater heating capacity from the system;

However, prior oil accumulation has reduced the heat pump’s heat exchange capacity, meaning heating cannot keep pace. The system is forced to raise the evaporation temperature and lower the vacuum to maintain evaporation rates, resulting in an abnormally high evaporation temperature.

7. Insufficient refrigerant charge, excessively long piping with numerous bends

High piping resistance and insufficient refrigerant circulation prevent oil from being efficiently returned to the compressor;

Long-term oil accumulation forms a thermal resistance due to an oil film, reducing the heat pump’s output and passively raising the evaporation temperature.