Heat Pump Winter Troubleshooting Q&A

When winter arrives, air source heat pumps operate under much more demanding conditions. Low outdoor temperatures, high heating loads, evaporator frosting, changes in water flow and freezing risks can expose problems that may not appear during warmer months.

For distributors, installers and service technicians, the challenge is that the same symptom can have several different causes.

For example:

• A high-pressure alarm does not always mean the refrigerant circuit is faulty.
• Frost on refrigerant pipes does not always mean the heat pump is short of refrigerant.
• A unit that does not immediately trigger a high-pressure alarm may still have insufficient water flow.
• Antifreeze concentration should not be selected simply according to a fixed percentage.

In this Winter Heat Pump Troubleshooting FAQ, we answer seven practical questions commonly encountered in air-to-water heat pump projects.

Q1. Why Does the Heat Pump Display a High-Pressure Fault but Continue Running?

Normally, a genuine high-pressure protection signal should cause the heat pump to stop or enter a protection state.

If the display shows a high-pressure fault while the compressor continues operating, the first thing to investigate is the relationship between:

Pressure protection → Main controller → Display/controller

There are several possibilities.

Possible Cause 1: The Main PCB Detects the Alarm but Does Not Execute the Shutdown

The controller may receive the high-pressure protection signal, but the corresponding shutdown logic may not operate correctly.

In this case, the display can show the fault while the compressor continues running.

Possible Cause 2: The Display Shows the Alarm but the Main PCB Does Not Correctly Process the Protection Signal

Another possibility is that the display/controller receives or displays the alarm information, but the main PCB does not correctly execute the associated protection command.

This may indicate a problem with:

• Main PCB
• High-pressure protection input circuit
• Communication between controller and display
• Protection logic

What Should Technicians Do?

Do not assume that the machine is safe simply because it continues running.

Verify:

1. Actual refrigerant high-side pressure
2. High-pressure switch/sensor status
3. Main PCB protection input
4. Controller alarm history
5. Whether the compressor shutdown logic is functioning

If actual refrigerant pressure is dangerously high, the unit should not continue operating until the root cause is identified.

Key diagnostic principle: An alarm shown on the display and the actual protection action of the heat pump are two different things that must be verified separately.

Q2. Why Does the High-Pressure Gauge Rise Rapidly Immediately After Startup?

A common commissioning scenario is:

• Circulation pump appears to be operating
• Heat pump starts
• High-side pressure rises rapidly
• Pressure gauge moves sharply upward
• High-pressure protection follows shortly afterward

One important cause is air trapped in the hydronic circuit.

Why Can Air in the Water Circuit Cause High Pressure?

The condenser/plate heat exchanger needs sufficient water flow to remove heat from the refrigerant.

If a large amount of air remains in the system, the circulation pump may appear to be running, but actual water circulation can be significantly reduced.

The sequence becomes:

Air trapped in water circuit
↓
Insufficient effective water flow
↓
Poor heat removal from condenser
↓
Condensing temperature rises rapidly
↓
Discharge/high-side pressure increases
↓
High-pressure protection

This is why simply checking whether the circulation pump is powered ON is not enough.

Pump running ≠ Correct water flow.

What Should Be Checked?

Before commissioning:

• Fill the system correctly
• Vent trapped air
• Check automatic/manual air vents
• Confirm circulation pump operation
• Check valves are fully open
• Clean strainers
• Confirm adequate flow through the heat exchanger

After filling a new hydronic system, air removal should be considered an essential commissioning procedure.

Q3. Can Mains Water Pressure Be Used to Fill a Closed-Loop Air-to-Water Heat Pump System?

Yes, mains water can normally be used to fill and pressurize a closed hydronic heating system, provided the filling arrangement and final system pressure comply with the system design and manufacturer requirements.

However, one point from the original field material needs an important clarification.

A pressure of 2 MPa would equal approximately 20 bar, which would be far too high for many conventional residential hydronic heating systems.

For many closed heating systems, the cold filling pressure is typically much lower and must be determined according to:

• Building height
• Static head
• Expansion vessel pre-charge
• Safety valve rating
• Heat pump manufacturer's requirements

Therefore, technicians should never blindly fill a heat pump heating circuit to 2 MPa.

The correct principle is:

Fill the closed system to the pressure specified in the heat pump and hydronic system design, then fully vent the circuit and verify stable pressure.

Also inspect:

• Expansion vessel
• Pressure gauge
• Safety relief valve
• Automatic air vent
• Filling valve
• System leakage

For multi-storey buildings, static head must also be considered when determining the correct filling pressure.

Q4. Why Is There Frost Between the Refrigerant Distributor and the Evaporator?

During heating operation, technicians may notice frost forming:

• Near the refrigerant distributor
• On distributor branches
• Close to the evaporator inlet
• On several refrigerant circuits

Does this automatically mean refrigerant shortage?

No.

A small and relatively uniform amount of frost can occur under normal low-temperature heating conditions.

The important issue is the frost pattern.

When Can It Be Normal?

If frosting is:

• Light
• Relatively uniform
• Consistent with cold and humid ambient conditions
• Removed normally during defrost
• Not associated with abnormal operating pressure

it may simply be part of normal heat pump operation.

When Should You Investigate?

If frost becomes:

• Severe
• Highly uneven
• Concentrated on only one branch
• Persistent after defrost
• Associated with low suction pressure
• Associated with reduced heating capacity

then further investigation is required.

Possible causes include:

Uneven Refrigerant Distribution

The refrigerant distributor may not be feeding each evaporator circuit evenly.

Refrigerant Shortage

A low refrigerant charge can reduce evaporation pressure and cause abnormal coil temperature distribution.

Refrigerant Leakage

If the charge has gradually decreased, a leak may be present.

Refrigerant Circuit Restriction

A capillary tube, electronic expansion valve, distributor branch or another refrigerant passage may be partially restricted.

Do not diagnose refrigerant shortage from frost alone.

Pressure, temperature, superheat/subcooling where applicable, ambient conditions and the complete evaporator frost pattern should be evaluated together.

Q5. Does an Air Source Heat Pump Need Antifreeze in Winter?

This is one of the most important questions for heat pump installations in cold climates.

The answer is:

It depends on the system design and the local minimum temperature.

For a monobloc air-to-water heat pump, part of the hydronic circuit is located outdoors.

If the unit stops because of:

• Power failure
• Pump failure
• Control fault
• Long shutdown
• Extreme weather

water inside the outdoor heat exchanger and piping can freeze.

Freezing water expands and can damage:

• Plate heat exchanger
• Water pipes
• Valves
• Pumps
• Fittings

Therefore, freeze protection must be considered during system design.

How Much Antifreeze Should Be Added?

There is no universal percentage that works for every project.

Antifreeze concentration should be selected according to:

Local design minimum temperature + required freeze protection + antifreeze manufacturer's concentration chart

For example, the required concentration for a project designed for -10°C conditions may be very different from a project expected to experience -25°C or -30°C.

Always refer to the antifreeze supplier's concentration/freezing-point data.

Important: Antifreeze Also Affects Heat Pump Performance

Increasing glycol concentration can change:

• Fluid viscosity
• Pumping resistance
• Water flow
• Heat-transfer performance
• Pump power consumption

Therefore:

More antifreeze is not automatically better.

The goal is to provide sufficient freeze protection while maintaining acceptable hydronic performance.

Q6. The Heat Pump Had a High-Pressure Alarm. After Descaling the Heat Exchanger, It Works Normally. Should the Main PCB Still Be Replaced?

Normally, no—not if the root cause has been confirmed as a water-side heat-transfer problem and the protection/control functions test correctly afterward.

Suppose the original situation was:

Heat exchanger scaling
↓
Poor heat transfer / reduced effective flow
↓
Condensing temperature increases
↓
High pressure increases
↓
High-pressure protection

After proper descaling, if:

• Water flow returns to normal
• Heat transfer returns to normal
• Refrigerant pressure is normal
• No high-pressure alarm returns
• Controller operation is normal

then there is no technical reason to replace a functioning PCB simply because a high-pressure alarm occurred previously.

The alarm was a symptom.

Scaling was the root cause.

This distinction is critical in professional after-sales troubleshooting.

A Note About Descaling Chemicals

The source material specifically warns against casually using strong acids such as hydrochloric acid or sulfuric acid.

That is good advice.

Strong or incompatible acids may:

• Corrode heat exchanger materials
• Damage brazed joints
• Attack piping
• Create leakage
• Reduce heat exchanger life

Use a descaling product compatible with the specific heat exchanger material and follow its required:

• Concentration
• Circulation time
• Temperature
• Neutralization procedure
• Flushing procedure

After chemical cleaning, thoroughly flush the circuit with clean water.

Q7. What Happens If the Circulation Pump Is Not Properly Vented?

A hydronic circulation pump can be electrically running while still failing to provide sufficient water flow.

Air trapped in the pump or piping can cause:

• Reduced circulation
• Unstable water flow
• Pump cavitation/noise
• Poor condenser heat transfer
• Higher condensing pressure
• Reduced heating efficiency

One dangerous aspect is that the heat pump may not immediately trigger a high-pressure alarm.

The actual operating pressure may simply remain higher than normal for an extended period.

The sequence can be:

Air in system
↓
Reduced water flow
↓
Higher condensing temperature
↓
Higher operating pressure
↓
Long-term heat exchanger stress / scaling risk

Therefore, the absence of a high-pressure alarm does not prove that water flow is correct.

After Filling or Servicing the System, Check:

• Pump venting
• System air vents
• Water flow
• Pump noise
• Inlet/outlet water temperature difference
• System pressure
• Strainer cleanliness

For units equipped with a flow sensor or flow switch, verify that the reading is consistent with actual system operation.

Winter Heat Pump Troubleshooting: The Most Important Principle

Many heat pump winter problems are interconnected.

For example:

Air in water circuit

can lead to:

Low water flow

which leads to:

Poor heat transfer

which causes:

High condensing temperature

which produces:

High refrigerant pressure

and eventually:

High-pressure protection

This means technicians should avoid diagnosing a heat pump only from the final fault code.

A better troubleshooting philosophy is:

Alarm → Operating data → System condition → Root cause

rather than:

Alarm → Replace component

This is particularly important in hydronic heat pump systems because the heat pump itself is only one part of the complete installation.

Quick Winter Troubleshooting Table

FAQ: Additional Questions for Heat Pump Owners and Installers

Can trapped air cause a heat pump high-pressure alarm?

Yes. Air trapped in the hydronic circuit can reduce effective water flow through the condenser. Poor heat removal raises condensing temperature and pressure and may eventually trigger high-pressure protection.

Can a circulation pump run without actually circulating enough water?

Yes. A pump can be powered and rotating while airlocks, closed valves, blocked strainers or other hydraulic restrictions significantly reduce actual flow.

Does frost on refrigerant piping mean the heat pump is low on refrigerant?

Not necessarily. Light frost can occur under normal winter operation. Severe or uneven frosting should be evaluated together with refrigerant pressures, temperatures, airflow and evaporator performance.

Can a scaled heat exchanger cause high-pressure protection?

Yes. Scaling reduces heat transfer and can restrict water-side performance. This may increase condensing temperature and high-side refrigerant pressure.

Should I add more glycol for better freeze protection?

Not blindly. Higher glycol concentration improves freeze protection but also increases viscosity and can reduce heat-transfer and hydraulic performance. Select the concentration according to the required freeze point and manufacturer recommendations.

Does every monobloc heat pump need glycol?

Not necessarily. Freeze-protection strategy depends on climate, installation configuration, backup power, controls, piping design and manufacturer requirements. However, systems exposed to freezing conditions must have an effective freeze-protection strategy.

A Better Way to Diagnose Winter Heat Pump Problems

For installers and service engineers, we recommend checking the system in this order:

1. Confirm the fault code
↓
2. Check actual operating pressure and temperature
↓
3. Check water flow and pump operation
↓
4. Remove trapped air
↓
5. Check strainers, valves and heat exchanger condition
↓
6. Check outdoor airflow and frosting
↓
7. Check refrigerant-side operation
↓
8. Check sensors and electrical controls

This prevents one of the most common mistakes in heat pump servicing:

Replacing parts before identifying the root cause.

Conclusion

Winter is the real test of an air source heat pump system.

Some of the most common problems are not caused by the compressor or refrigerant circuit itself.

They may originate from:

• Insufficient water flow
• Air trapped in the hydronic system
• Incorrect system pressure
• Heat exchanger scaling
• Poor refrigerant distribution
• Incorrect freeze protection
• Sensor or controller problems

For this reason, successful heat pump troubleshooting requires technicians to look at the complete system rather than only the heat pump unit.

A fault code tells us what the heat pump detected.

It does not necessarily tell us what caused the problem.

That difference is at the heart of professional heat pump troubleshooting.

Heat Pump Winter Troubleshooting Guide Series

This Q&A is part of our practical winter heat pump technical series:

Part 1 — High Pressure Alarm: Causes & Troubleshooting

Part 2 — Low Pressure Alarm: Causes & Troubleshooting

Part 3 — Water Tank Not Refilling: Causes & Troubleshooting

Part 4 — Water Tank Overflow: Causes & Troubleshooting

Part 5 — Evaporator Frosting & Defrost Problems

Part 6 — Winter Heat Pump Troubleshooting FAQ

For more air source heat pump technical knowledge, OEM solutions and troubleshooting guides:

www.ecoheat-pump.com