Introduction

During winter operation, air source heat pumps work under challenging outdoor conditions, especially in cold climates where low ambient temperatures, frost, and long heating cycles can affect system performance.

Besides high pressure alarms, low pressure alarms are another common fault reported by installers and end users.

When a heat pump displays a low pressure alarm, many users immediately assume that the refrigerant is insufficient. However, a low pressure fault can be caused by several different issues, including:

• Refrigerant leakage
• Evaporator blockage or icing
• Low ambient temperature operation
• Expansion valve problems
• Sensor or control system failures

Correct diagnosis is essential because adding refrigerant without identifying the real cause may temporarily hide the problem while creating new risks.

This article explains the common causes of air source heat pump low pressure alarms and how to troubleshoot them step by step.

1. What Does a Heat Pump Low Pressure Alarm Mean?

An air source heat pump works through a refrigeration cycle:

Outdoor air
↓
Evaporator absorbs heat
↓
Compressor increases refrigerant pressure
↓
Condenser releases heat to water system
↓
Heating is delivered to the building

The low pressure side refers mainly to:

• Evaporator pressure
• Refrigerant suction pressure

When the suction pressure becomes lower than the protection limit, the controller activates the low pressure protection to prevent:

• Compressor damage
• Excessive overheating
• Refrigeration system failure

2. Common Situation One: Low Pressure Alarm After Startup

Symptoms:

• Unit powers on normally
• Low pressure value is normal initially
• After operation, low pressure alarm appears
• System cannot continue heating

Possible causes:

2.1 Refrigerant Leakage

Refrigerant leakage is one of the most common causes.

When refrigerant quantity decreases:

• Refrigerant circulation becomes insufficient
• Evaporator heat absorption decreases
• Suction pressure drops
• Compressor overheating risk increases

Inspection Method:

Check:

✔ Refrigerant pressure
✔ Refrigerant pipe connections
✔ Brazing joints
✔ Service valves
✔ Signs of oil leakage

A small refrigerant leak may not cause an immediate failure but can gradually reduce heating performance.

3. Common Situation Two: Low Pressure Alarm Immediately After Startup

Symptoms:

• Unit starts
• Low pressure alarm appears quickly
• Refrigerant pressure drops below normal range

Possible causes:

3.1 Refrigerant System Blockage

A blocked refrigeration circuit can prevent refrigerant circulation.

Possible locations:

• Filter drier blockage
• Expansion valve blockage
• Capillary restriction (for some systems)

Result:

The evaporator receives insufficient refrigerant.

The low pressure side pressure decreases rapidly.

3.2 Expansion Valve Failure

The electronic expansion valve (EEV) controls refrigerant flow.

If the valve:

• Cannot open normally
• Is blocked by impurities
• Has control failure

The evaporator will receive insufficient refrigerant.

Symptoms:

• Low suction pressure
• Poor heating capacity
• Compressor running abnormally

Inspection:

Check:

• EEV opening signal
• Valve movement
• Controller output
• Refrigerant superheat

4. Evaporator Icing Problems

In winter, another common reason for low pressure alarms is evaporator icing.

The outdoor evaporator absorbs heat from air.

When:

• Outdoor temperature is low
• Humidity is high
• Airflow is insufficient

Ice may accumulate on the coil.

Causes of Excessive Frost:

1) Dirty Evaporator Coil

Dust and debris reduce airflow.

Result:

• Heat absorption decreases
• Evaporating temperature drops
• Low pressure alarm occurs

Solution:

Clean the evaporator surface regularly.

2) Fan Problem

If outdoor airflow is insufficient:

Possible reasons:

• Fan motor failure
• Fan speed abnormal
• Fan blade damage

Reduced airflow causes insufficient heat exchange.

3) Defrost System Failure

During winter, heat pumps must periodically enter defrost mode.

If defrost fails:

• Ice continues accumulating
• Evaporator efficiency decreases
• Low pressure protection activates

Check:

✔ Defrost sensor
✔ Four-way valve operation
✔ Defrost control logic
✔ Outdoor coil temperature sensor

5. Check Whether Low Pressure Is Caused by Refrigerant or Other Problems

Before adding refrigerant, technicians should confirm the actual reason.

A simple diagnostic process:

Step 1: Check Refrigerant Pressure

Measure:

• Suction pressure
• Discharge pressure

Compare with:

• Outdoor temperature
• Water temperature
• Operating conditions

Step 2: Check Evaporator Condition

Observe:

• Is the coil frozen?
• Is airflow normal?
• Is the fan working?

Step 3: Check Refrigerant Leakage

Inspect:

• Pipe joints
• Welding points
• Valves
• Oil traces

Step 4: Check Expansion Valve Operation

Confirm:

• EEV receives signal
• Valve opens correctly
• Refrigerant flow is stable

6. Electrical Control Causes of Low Pressure Alarm

Not all low pressure alarms are refrigeration problems.

Some faults come from:

• Pressure sensor
• PCB control board
• Temperature sensor

Typical Symptoms:

• Low pressure alarm appears randomly
• Refrigerant pressure is normal
• Fault occurs at different operating conditions

Inspection:

Compare:

Displayed pressure value
vs
Actual pressure measured by gauges

If there is a large difference:

Possible causes:

• Pressure sensor failure
• Control board signal error

7. How to Prevent Heat Pump Low Pressure Problems?

Proper maintenance can significantly reduce low pressure faults.

Installation Recommendations:

✔ Perform refrigerant leakage inspection
✔ Ensure correct pipe installation
✔ Keep outdoor coil clean
✔ Ensure sufficient airflow
✔ Check sensors during commissioning

Regular Maintenance:

✔ Clean evaporator coil
✔ Check fan operation
✔ Inspect defrost performance
✔ Monitor refrigerant pressure
✔ Check system operating parameters

Conclusion

A low pressure alarm does not always mean “lack of refrigerant”.

The real cause may be:

• Refrigerant leakage
• Refrigeration blockage
• Expansion valve failure
• Evaporator icing
• Poor airflow
• Defrost system problems
• Sensor or PCB faults

A professional troubleshooting approach should always start with:

Check operating conditions → Confirm refrigeration system → Verify electrical components

Correct diagnosis helps installers reduce maintenance time, avoid unnecessary refrigerant charging, and improve the long-term reliability of air source heat pump systems.