Core Root Causes

The low-temperature operation of a heat pump relies on the precise coordination of three factors: heat exchange on the air side, thermal equilibrium on the condenser side, and phase change under negative pressure on the evaporator side. When the diurnal temperature variation is significant, the heat pump’s operating equilibrium is disrupted, causing the compressor, fan and throttling components to constantly oscillate between rated and extreme operating conditions. This manifests directly as frequent start-stop cycles and fluctuating evaporation rates.

1. Dramatic diurnal fluctuations in ambient temperature cause significant drift in the heat pump’s heating capacity

The heat output of a heat pump is highly dependent on the outdoor/ambient intake air temperature:

High daytime temperatures: The evaporator absorbs sufficient heat, resulting in strong heating capacity, rapid temperature rise in the evaporator and high evaporation rates;

Sudden drops in night-time temperatures: The heat exchange capacity of the air source plummets, the heat pump cannot keep up with the heating demand, and the temperature inside the vessel drops rapidly.

As soon as the temperature sensor detects the lower limit, the compressor restarts; when it reaches the upper limit, it shuts down, resulting in repeated start-stop cycles.

2. Significant diurnal variations in condenser heat dissipation conditions cause severe fluctuations in system high-pressure levels

Due to large diurnal temperature differences, the heat dissipation conditions for air-cooled condensers change completely:

High daytime temperatures → elevated condensing pressure → high compressor load → prone to high-pressure protection triggering frequency reduction or shutdown;

Low night-time temperatures → Condensing pressure drops sharply → Evaporator boiling point and heat exchange temperature difference fluctuate erratically, causing evaporation intensity to vary wildly.

The heat pump unit’s high and low-pressure protection mechanisms are triggered frequently; the main controller can only protect itself by cycling the compressor, reducing frequency, or limiting load, resulting in erratic fluctuations in evaporation capacity.

3. Mismatch between throttling/expansion components and operating conditions leads to unstable refrigerant circulation

The heat pump’s capillary tubes / electronic expansion valves are matched to the standard ambient temperature range. When the diurnal temperature range widens:

As the ambient temperature changes, superheat and subcooling deviate from design values, causing refrigerant flow to fluctuate erratically.

Heat exchange fluctuates, the evaporation temperature inside the reactor becomes unstable, directly leading to fluctuating evaporation rates.

4. Vacuum level drifts in tandem with diurnal temperature, further amplifying fluctuations

At night, the ambient temperature is low, and the temperatures of the vacuum piping and condenser tube walls are low, causing excessive condensation of secondary vapour:

The system vacuum is passively raised → the boiling point decreases → evaporation intensifies momentarily;

Once the operating conditions become unbalanced, pressure builds up → the vacuum drops → evaporation weakens again.

With the vacuum and heat pump temperature control resonating in both directions, the machine will continuously start and stop to correct the imbalance.

5. Fixed-speed/variable-speed fans lack adaptive control, resulting in mismatched heat exchange airflow between day and night

Many heat pump evaporator fans operate at fixed speeds:

Insufficient heat dissipation during the day leads to high-pressure levels;

Excessive heat dissipation at night causes the system operating conditions to deviate significantly from the optimal range.

As the fans cannot adapt to temperature changes, the system relies on frequent compressor cycling to balance heat, making it impossible to stabilise evaporation rates.

6. Automatic control PID parameters are set for moderate temperatures and cannot adapt to significant diurnal temperature fluctuations

The equipment’s temperature control and vacuum PID are calibrated for moderate spring and autumn temperatures.

When the diurnal temperature difference is large, system lag and heat exchange delays all increase,

leading to overshoot and oscillation in the control loop, manifested as:

Temperature fluctuating wildly, frequent compressor start-stops, and evaporative capacity fluctuating accordingly.

7. Low ambient temperatures at night trigger defrost cycles, forcibly interrupting the evaporation process

With large diurnal temperature variations and low night-time temperatures, the heat pump’s air-side heat exchanger is prone to icing:

The unit enters defrost mode either by timer or temperature sensor, suspending heating and reversing the circulation.

During defrosting, evaporation capacity is completely reduced or even ceases;

Upon completion of defrosting, the load rises sharply, resulting in a dramatic, cliff-like fluctuation in evaporation capacity that is visible to the naked eye.