Common heat pump faults and how to recognize them

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Common Heat Pump Failures and How to Recognize Them

The heat pump belongs to the most reliable heating systems available on the market today. Despite this – and this is something every plumber and owner should know – it is not indestructible. From my own experience, I know that the vast majority of service calls follow the same patterns. The same ten to fifteen faults keep coming back, sometimes in different disguises, but always with the same basic cause. If you know what to look for and what to listen for, you can identify most problems before they turn from minor issues into major expenses.

This article is intended for heat pump owners who want to understand their equipment, as well as for technicians and plumbers who need a systematic overview. We won't just skim the surface – we will examine each common failure in depth, explain the physical principles behind why it occurs, and describe the specific symptoms that tell you where the problem is. Where possible, I will also include numbers and measurable values, because these will move you from guessing to proper diagnosis.

If you are looking for a basic overview before purchasing, also see our article How to Choose a Heat Pump – What to Focus On Before Buying or What Heat Pump Capacity Do I Need for My House. Here, however, we will focus exclusively on what can go wrong after installation and how to recognize it.

Why heat pumps fail at all – basic framework

Before we move on to specific failures, it is important to understand where a failure can occur in the system. A heat pump is not a single component – it is a system of several subsystems that must function simultaneously and in coordination. A failure in one subsystem is immediately reflected in the performance of the entire device, or the protective circuits shut it down completely.

Four main subsystems – the refrigerant circuit, the heat source (evaporator), the hydraulic circuit, and the control electronics – are interconnected. This means that a failure that appears to be a problem with the electronics (the controller reports an error code) may have its origin in the hydraulics (the pressure sensor detects low pressure due to an air bubble in the circuit). Diagnosis therefore never consists of just reading a code from the display – you need to understand what the code stands for.

Compressor – the heart of the pump and its most common problems

The compressor is the most expensive and important component of a heat pump. Its replacement costs range from €800 to €2,500 depending on the power of the heat pump, not including labor and refrigerant refilling. That is why it is very important to catch warning signs early.

Compressor noise – when is it normal and when is it not

A new compressor produces a uniform, low hum during operation. A problem arises when this sound changes. Clattering or metallic knocking at startup indicates that the compressor is starting into liquid (so-called liquid slugging) – this occurs due to insufficient refrigerant superheating or oil leakage into the refrigerant circuit. A squeaking sound during normal operation may indicate bearing wear. A deep, irregular, rumbling sound that changes in rhythm with pressure pulses indicates problems with the dampers or mechanical damage to the rotor.

Practical tip from practice: Place your hand on the high-pressure side piping (hot side, opposite the evaporator) – you should feel regular, gentle pulsations. If the pulsations are strong and irregular, or if the piping vibrates strongly, it is a bad sign. The normal discharge temperature of the compressor during normal operation (air-to-water, outside temperature 5 – 7 °C, temperature difference 35/30 °C) ranges between 70 – 90 °C.

Compressor does not start

Most common cause: the compressor's protective thermostat (overload protection) has tripped due to overheating. This happens when the condensing pressure is too high – for example, when the output water temperature is set too high (above 55 – 60 °C for standard air-to-water pumps), or when the condenser does not transfer heat sufficiently (air bubble, dirty heat exchanger). Another cause may be a phase failure in three-phase power supply – the motor relay immediately disconnects the compressor to prevent operation on two phases.

What to check: supply voltage at the compressor terminals (for three-phase it should be 380 – 400 V between phases, for single-phase 220 – 240 V), resistance of the compressor windings measured at the terminals (on a cold machine it should be symmetrical, a deviation of more than 10 % indicates a problem), and last but not least the error log in the controller.

Refrigerant leak – the silent performance killer

A refrigerant leak is one of those faults that develop slowly. The refrigerant circuit is hermetically sealed and was filled with refrigerant only once during installation. If the refrigerant is leaking, its quantity decreases gradually over months to years until the problem becomes more noticeable.

Signs of a refrigerant leak include: a gradual drop in heating performance under the same external conditions (the house takes longer to reach the desired temperature), longer compressor operation, lower COP (ratio of heating performance to electricity consumed), dropping suction pressure below the norm (for R410A the suction pressure at 0 °C evaporating temperature is about 8 bar, for R32 about 8.3 bar), and in an advanced stage, oil stains around the refrigerant pipe connections or on the evaporator.

Important: A layperson must not add refrigerant. Handling refrigerant requires professional qualification according to EU Regulation 517/2014 (so-called F-gas), as well as special service tools. Simply refilling without finding and repairing the leak is also pointless – the refrigerant will leak again. Every service intervention on the refrigerant circuit must be recorded in the equipment log.

Frozen evaporator – a daily routine for air-to-water heat pumps

Air-to-water heat pumps are equipped with automatic evaporator defrosting. During cold, humid weather, moisture from the air condenses and freezes on the evaporator fins, gradually reducing the airflow through the evaporator and thus the performance of the heat pump. The controller therefore regularly (typically every 30 – 90 minutes at temperatures of 0 – 5 °C and high humidity) starts a defrost cycle – it reverses the refrigerant circuit and uses heat from the condenser (from inside) to melt the ice on the evaporator.

When is defrosting normal and when is it a fault

The defrost cycle usually lasts 3 – 8 minutes. During this time, the outdoor unit "exhales" steam, you may see mist or small water droplets. This is normal. A problem occurs if:

Defrosting happens too often (every 10 – 15 minutes) – indicates a fault in the defrost sensor or incorrect setting of the defrost algorithm
The evaporator remains frozen even after defrosting – may indicate a refrigerant shortage, a fault in the four-way valve or a defrost relay failure
Defrosting does not last long enough, but ice accumulates and the outdoor unit makes cracking sounds from deforming ice
The fan cannot rotate freely because the fins are stuck in ice

Special case: If the outdoor unit is installed in a location where water drips from the roof or from the gutter, defrosting cannot remove the ice as quickly as it grows. In such a case, installing a canopy over the unit or rerouting the gutter helps. Another common problem is installation in a recessed niche where there is not enough airflow – this is an installation error, we write more about it in the article Heat pump installation – procedure, requirements and common errors.

To protect the piping at the outdoor unit in freezing weather, special components are used. For example, self-regulating heating cable for ITEC – 2 m prevents freezing of the condensate drain pipe, which is a common cause of re-freezing of the evaporator when the drain is not functioning.

Hydraulic circuit faults – air, pressure, pumps

The hydraulic circuit – that is, the heating water circuit – is the part of the system that laypersons usually have the most experience with, as it resembles conventional central heating. Despite this, it has its own specifics.

Air in the hydraulic circuit

Air in the circuit is by far the most common cause of "malfunctions" of heat pumps that I encounter during service visits. Paradoxically, it is not a real device failure, but a hydraulic problem. It manifests itself as gurgling in the radiators, reduced water flow, uneven heating of floor heating, and in the worst case, the pump shuts down due to the detection of insufficient flow.

Causes: Insufficient air venting during the first filling, malfunction or blocked automatic air vent, micro-bubbles forming due to reduced pressure in the circuit (pressure below 1 bar absolute for most systems). The minimum operating pressure of the heat pump hydraulic circuit is usually 1.0 – 1.5 bar (at a cold system), maximum 3 bar (safety valve). Check the manometer – if it shows less than 0.8 bar, water needs to be added and the system needs to be vented.

Circulation pump – failure or contamination

The circulation pump in the heat pump hydraulic circuit has two most common causes of problems: rotor blockage (sediment deposits in old systems, welding residues in new ones) and bearing wear after long-term operation (typically after 8 – 12 years). A blocked rotor is manifested by the pump humming without flow – the heat pump controller registers this as a flow error and shuts down the compressor. Worn bearings produce frictional squeaking sounds.

Practical procedure: If the pump hums and no water flows, turn off the system, wait 5 minutes, then gently unscrew the service screw on the side of the pump (usually under the cover) and briefly turn the rotor shaft with a flat screwdriver. Sometimes this is enough to free a seized rotor after the system has been idle during the summer. If it doesn't help, the pump needs to be replaced.

In some configurations, special accessories are used for proper integration of circuits. For example, connecting fittings for TČ series CALIBRA DUO, ATLAS DUO ensure correct and tight connection of the hydraulic circuit and eliminate common sealing problems, which are the source of continuous water leakage and pressure drop in the system.

Controller, sensor and electronics faults

Modern heat pumps have sophisticated control electronics with numerous temperature sensors, pressure sensors and communication interfaces. Faulty diagnostics here leads to technicians sometimes replacing sensors instead of looking for the real cause of the deviation in values.

Incorrect or drifted temperature sensors

Temperature sensors (mostly NTC thermistors or PT1000 resistance sensors) can drift from the actual value due to contact oxidation, moisture in the electrical box or physical cable damage. A typical deviation that causes problems: 2 – 5 °C from the actual value. Practical test: Measure the temperature with a contact thermometer directly on the pipe at the sensor installation location and compare it with the value on the controller display. If the difference is more than 3 °C, the sensor needs to be recalibrated or replaced.

A special problem arises with external temperature sensors that are placed in an unsuitable location – for example, on a facade with solar radiation, near a kitchen exhaust air outlet, or in a niche with stagnant air. The controller with incorrect external temperature then incorrectly controls the setpoint curve – the system either heats too much and wastes energy or does not heat sufficiently.

Error codes – how to read them

Each manufacturer has its own system of error codes. What they have in common is that the codes usually indicate a symptom, not the cause. For example, code "E07 – high condensation pressure" can mean: dirty condenser (limescale in water-water systems), too high output water temperature (incorrect setting), air in the circuit (low flow), pressure sensor failure at high pressure, or too much refrigerant in the circuit (overfilling after service work).

Without measuring the pressures on the service manifolds and without measuring the temperatures, the code remains just a number. Therefore, never reset an error without understanding why it occurred. Every unjustified reset means that the compressor will start again into potentially harmful conditions.

Communication faults in extended installations

In more complex systems with multiple circuits (mixed circuits, hot water, solar addition), communication errors occur due to poor contacts on the bus or interference. If the installation includes an expansion of the control, it is important to have the correct expansion module – expansion module for ITEC adds the inputs and outputs needed for the control of a mixed circuit, and if it is missing or incorrectly connected, the controller reports communication errors or does not control the circuit correctly. More about this accessory can be found in the article Expansion module for heat pump – when and why you need it.

A related problem is the control of the mixed circuit pump. Some heat pump controllers output a 3 – 10 V signal for pump speed control, but older or cheaper pumps cannot process this signal and require a classic on/off signal. In such a case, a relay kit is used to convert the 3 – 10 V signal to on/off. Without it, the pump either runs continuously or remains stopped – both are problems.

Problems with hot water heating

Heat pumps with hot water heating (TUV) function are especially sensitive to settings. Hot water heating is the most energy-intensive mode, as it requires a higher output temperature (55 – 65 °C) than heating (35 – 50 °C). Some heat pumps can achieve this temperature without electrical auxiliary heating (bivalent backup electric heating), others cannot.

Common problem: The hot water tank does not heat to the desired temperature. It can be due to: incorrect placement of the tank temperature sensor (the sensor should be in the middle or lower third of the tank, not on top), limescale deposits on the tank heat exchanger (reduces heat transfer), too short TUV heating cycle (the time program does not allow the heating to complete), or reduced heat pump performance at extremely low outdoor temperatures, when the pump cannot heat both the space and the TUV simultaneously.

In systems where TUV requires circulation, proper integration is necessary. Hot water circulation kit for ITEC-T, ATHENA-T, LEGEND, CALIBRA, ATLAS ensures that the TUV circulation pump is properly controlled by the heat pump logic and does not cause energy losses due to continuous operation. If circulation is not properly integrated, the tank can cool back into the distribution faster than it heats up, and the heat pump runs almost continuously. More on this topic in the article Integration of hot water circulation with a heat pump – how to do it.

Excessive electricity consumption – how to recognize it and why it happens

Increased electricity consumption of a heat pump is one of the clearest signals that something is wrong. The problem is that many owners notice it only when looking at annual bills, not on an ongoing basis. If you have an electricity meter or a smart metering system, you can track daily consumption according to outside temperature – there is a so-called regression relationship, and if your heat pump consumes significantly more than the previous year under the same conditions, something has changed.

Specific causes of increased consumption: refrigerant leak (lower COP), frozen or dirty evaporator (less heat from air, more from electricity), incorrect setting of the thermostat (too high water temperature in the heating radiators), activated bivalent backup heating that runs longer than it should (e.g., due to a set bivalent point that is too low), or a fault in the circulation pump (irregular flow, the system cannot remove heat from the condenser).

Noise from a heat pump – what is normal and what is not

Heat pumps are a source of noise – this is a fact that must be accepted. Modern air-to-water units have a sound pressure level of 45 – 55 dB(A) at 1 meter. The noise varies depending on different operating modes and conditions.

Normal sounds: Low hum of the fan and compressor during normal operation. A click when the compressor starts and stops (expansion valve changes position). The sound of flowing water in the hydraulic circuit when the pump starts. Cracking and clicking during defrosting (thermal expansion).

Abnormal sounds and their causes:

Metallic clattering in the fan – foreign object (leaf, twig) caught in the impeller. Immediately turn off the unit and inspect. Operation with a foreign object can damage the blades or damage the fan motor.
Vibrations transferred to the building – insufficient isolation of the unit's base, loose mounting screws, or a direct rigid connection to concrete without vibration dampers. Heat pumps must be installed on anti-vibration pads.
Squeaking from the hydraulics during regulation – regulating valves (thermostatic heads, mixing valves) operating at partial opening can emit a squeaking sound at high flow speed. The solution is to reduce the performance of the circulation pump or to check the setting of the differential pressure limiter.
Strong vibrations and noise at low fan speeds – wear of the fan motor bearings.

System failures depending on the season

From practice, I know that some failures occur seasonally. The first start after the summer break (September/October) reveals: a seized rotor of the circulation pump (it did not move all summer), air in the circuit (spontaneous air bleed through micro-porous connections during the warm season), faults in cables due to humidity (condensation in the electrical panel), and old refrigerant with degraded oil.

The first frosty nights below -10 °C also reveal: insufficient capacity of the DHW tank heater (the heat pump cannot keep up at low outside temperatures), an active bivalent backup source that the owner did not expect (increased electricity consumption on the bill), and problems with condensate drainage from the outdoor unit (frozen drain).

To prevent freezing of the condensate drain in such frosty situations, the mentioned self-regulating heating cable is used – "self-regulation" means that the cable increases its output when it is cold and reduces it when it is warmer, thus saving energy compared to classic resistive cables with a thermostat.

Diagnostic procedure – how to approach a fault systematically

When a heat pump shows a problem, I recommend the following procedure, which will save time and money:

Step 1 – Read the error log: The controller has stored a history of errors. Note the codes and times of their occurrence. A one-time error is different from a recurring or persistent error.
Step 2 – Check basic values: Pressure in the hydraulic circuit (manometer), temperature of the supply and return water (display or measurement), outside temperature (display).
Step 3 – Visual inspection: Evaporator of the outdoor unit (frozen?), space around the unit (free airflow?), hydraulic connections (drips, moisture, limescale around fittings).
Step 4 – Listen: Sound when the compressor starts, sound of the fan, sound of the circulation pump in the panel or in the technical room.
Step 5 – Reset once: If you have checked everything above and did not find an obvious cause, try a one-time reset. If the error occurs again, do not reset again – call service.

A more detailed guide to regular preventive inspections can be found in the article Maintenance and service of a heat pump – what you can do yourself and what you cannot.

What you should never do when a heat pump fails

From practice, I repeatedly encounter what heat pump owners do before a service visit and what later prolongs the repair or makes it more expensive:

Repeatedly resetting the compressor error – each start of the compressor before the fault further damages the winding or bearings.
Opening the refrigerant circuit – without F-gas certification and without special tools, it is not only a legal offense but also a safety risk.
Adding inhibitors or chemicals to the hydraulic circuit without consulting service – some preparations are incompatible with the materials of the heat pump's heat exchangers (especially aluminum).
Covering the outdoor unit during operation for aesthetic or "protection from wind" reasons – this prevents air intake and the heat pump will suffocate.
Deactivating the defrost function for savings – defrosting is not "wasting energy," it is a necessary operating mode. Its deactivation leads to permanent freezing of the evaporator and failure.

Warranty and post-warranty repairs – what is covered by warranty

The standard manufacturer warranty on heat pumps is 2 years for the whole unit and sometimes 5 years for the compressor (depending on the manufacturer). The warranty applies to manufacturing defects, not to failures caused by: incorrect installation (e.g., insufficient water flow), not meeting minimum power supply parameters (voltage fluctuations exceeding ±10 % of the nominal value), neglected maintenance (dirty filter, dirty evaporator), or unauthorized intervention in the refrigerant circuit or electronics.

Therefore, it is extremely important to keep documentation on installation, handover protocols, and service inspections. Without it, you may find yourself in a situation where the manufacturer refuses a warranty repair, even if it is a genuine manufacturing defect.

Most frequently asked questions (FAQ)

The heat pump is running, but the house is not heating – what to check first?

Start with the hydraulic circuit. Check the water pressure on the manometer (it should be 1 – 2 bar when the system is cold). Check whether the circulation pump is running – can you feel vibrations or hear a quiet hum? Verify that shut-off valves on any of the heating circuits are not closed. The next step is to check the regulator settings – is the desired temperature set too low or is the heat pump in summer (TUV only) mode? If everything is in order and the pump still does not heat, call service and inform them of the measured values.

The outdoor unit is making a loud metallic sound – is it dangerous?

A metallic clattering or booming sound from the outdoor unit is always a signal to stop operation immediately. It is most likely a foreign object in the fan impeller or broken fan blades. Operation in this condition can damage the fan motor, the replacement of which costs 150 – 400 € depending on the model. Turn off the unit and call service for inspection.

The heat pump turns on for 2 minutes, then turns off – repeatedly. Is it a fault?

Yes, this is a phenomenon called "short cycling" and it is a fault that quickly destroys the compressor. Common causes: an oversized heat hydraulic accumulator without sufficient heat draw (the system quickly reaches the temperature and turns off), a malfunction of the thermostatic regulator with incorrect hysteresis, or high-pressure protection that shuts off the compressor due to rapidly increasing condensation pressure (which may indicate low water flow). Every compressor start is a mechanical load, and short cycles dramatically accelerate it. A solution requires a service technician.

An error code is displayed on the screen, but otherwise the heat pump is heating normally – should I call a service?

It depends on the code. Informational codes (e.g., "filter maintenance warning") are non-critical and you can resolve them yourself. Warning codes (e.g., "low water pressure") must be addressed immediately, even if the heat pump is currently functioning – the system is operating in a limited mode and the fault may worsen. Emergency codes (e.g., "high refrigerant pressure", "compressor failure") mean you should call a service today. Identifying the code in the service manual of your device is the first step.

How can I find out if the heat pump has enough refrigerant, without a service technician?

It is not possible to directly determine the refrigerant level without a service manometer and a certified technician. Indirectly, you can monitor: is the performance dropping under the same conditions compared to last year? Is the compressor running longer to achieve the same performance? Is the compressor discharge temperature unusually high (over 95 °C)? These symptoms may indicate a possible refrigerant shortage, but only a pressure measurement using a service manometer on the service valves can provide a definitive diagnosis.

Can I turn off the heat pump for the entire summer and turn it back on in the fall?

Yes, heat pumps used only for heating are shut down during the summer. It is important to: keep the hydraulic circuit under pressure before shutdown (to prevent air from entering), do not completely disconnect the power supply to the electrical cabinet (the controller and thermostats need power for protection), and in August/September before starting, visually inspect the evaporator (dirt, leaves), bleed the circuit and check the pressure. A light preventive inspection in the fall before the season is much cheaper than an emergency service call in January. More on what you can do yourself is in the article Maintenance and service of heat pumps – what you can do yourself and what you cannot.

Conclusion – prevention is always cheaper than repair

Heat pumps belong to reliable technology with an average lifespan of 15 – 20 years with proper operation and maintenance. Most of the faults I described in this article can be avoided with proper care and regular maintenance.

Do you have a question on this topic?

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