For an air-source heat pump the minimum evaporating temperature depends on the compressor design and the manufacturer’s operating envelope. 

Typical minimum evaporator (saturation) temperature for R32 heat pumps

–25 °C to –30 °C which is common for modern domestic R32 ASHPs.

Note, some manufacturers restrict operation to –20 °C for efficiency and reliability reasons (reduced thermal stress).

High-performance “cold-climate” units may reach –35 °C, but these are less common in the UK/EU market.

Why the limit exists

At very low evaporating temperatures:

Suction pressure drops very low (R32 saturation pressure at –25 °C ≈ 1.7 bar abs).

Mass flow and capacity reduce sharply.

Risk of liquid floodback to the compressor  during defrost increases.

Compressor discharge temperature rises significantly.

Most manufacturers provide an operating envelope showing safe combinations of suction (evaporating) and discharge (condensing) temperatures.

Rule of thumb

If the outdoor air temperature is –10 °C, the evaporator surface will usually sit around –15 to –20 °C (because it must be colder than the air to absorb heat).

This matches what owners see during winter: coils often frost at –5 to –15 °C saturation.

Hitachi Yutaki units predict defrost requirements by a combination of:

1. Outdoor coil temperature sensor

2. Ambient outdoor temperature sensor

3. Refrigerant pressure sensor

4. Runtime / algorithmic prediction

Defrost begins when both the coil temp and calculated saturation temperature indicate that the coil is at or below freezing and the system sees an expected reduction in heat-extraction efficiency.

This means the unit will sometimes enter defrost before you see thick ice, because it's detecting the early film of frost that you can’t see.

Posted by: @iaack

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For an air-source heat pump the minimum evaporating temperature depends on the compressor design and the manufacturer’s operating envelope. 

Typical minimum evaporator (saturation) temperature for R32 heat pumps

–25 °C to –30 °C which is common for modern domestic R32 ASHPs.

Note, some manufacturers restrict operation to –20 °C for efficiency and reliability reasons (reduced thermal stress).

High-performance “cold-climate” units may reach –35 °C, but these are less common in the UK/EU market.

Why the limit exists

At very low evaporating temperatures:

Suction pressure drops very low (R32 saturation pressure at –25 °C ≈ 1.7 bar abs).

Mass flow and capacity reduce sharply.

Risk of liquid floodback to the compressor  during defrost increases.

Compressor discharge temperature rises significantly.

Most manufacturers provide an operating envelope showing safe combinations of suction (evaporating) and discharge (condensing) temperatures.

Rule of thumb

If the outdoor air temperature is –10 °C, the evaporator surface will usually sit around –15 to –20 °C (because it must be colder than the air to absorb heat).

This matches what owners see during winter: coils often frost at –5 to –15 °C saturation.

Hitachi Yutaki units predict defrost requirements by a combination of:

1. Outdoor coil temperature sensor

2. Ambient outdoor temperature sensor

3. Refrigerant pressure sensor

4. Runtime / algorithmic prediction

Defrost begins when both the coil temp and calculated saturation temperature indicate that the coil is at or below freezing and the system sees an expected reduction in heat-extraction efficiency.

This means the unit will sometimes enter defrost before you see thick ice, because it's detecting the early film of frost that you can’t see.

Thank you for the reply.  I think you are saying that what I am seeing is normal (Evaporator -23C.  OAT -4C.  Defrost cycle every 46 minutes).

@trebor12345 Yes, expect defrost cycles to initiate at evaporator temperatures between -20 and -35 degC depending on refrigerant gas, plant design and application.

The cycle time is dependent more on environmental conditions, heat extraction rates (how hard the compressor is working) and forced airflow rates to the evaporator.

If you imagine a heat pump where the airflow to the evaporator is restricted in some way (caused by poor clearances between surrounding walls, evaporator fan not attaining the correct speed due to incorrect factory setup or maybe the early failure of the evaporator fan bearings) then the defrost cycle would still occur at the expected temperatures but the period between cycles would definitely decrease.

Referring now to your other post on this forum, the ice buildup on your unit does, in my experience, seem a little heavier than I would expect. Hopefully others will contradict me!