Most Common Heat Pump Faults

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A heat pump is a machine that runs practically all year round, including the coldest winter nights, when the outdoor unit operates at temperatures well below freezing and with a large difference between the inlet and outlet temperature of the working fluid. It is therefore no surprise that after a few years of operation almost every owner encounters at least one warning message, an unusual sound, or a drop in output. Most of these situations have a clear cause and can either be completely avoided with regular maintenance, or quickly recognised before a small deviation turns into an expensive repair. In this article we go through the most common faults in air-water and ground-water heat pumps, how to recognise them by their symptoms, when it is safe to simply keep an eye on the problem yourself, and when you need to call a service technician immediately.

Why heat pumps develop faults at all

A heat pump is not just "a box outside and a box inside" - it is a closed refrigeration circuit (compressor, condenser, expansion valve, evaporator), a circulation pump, control electronics with dozens of sensors, and, in the air-water version, also a mechanism for defrosting the evaporator. Each of these parts has its own typical failure modes and its own typical cause. In most cases it comes down to a combination of three factors:

Operating load - the compressor runs in cycles, sometimes as much as 2000-3000 hours a year, comparable to the service interval of a car that has covered 60,000-80,000 kilometres.
Outdoor environment - moisture, dust, pollen, leaves, and in winter months also frost and snow, place a direct load on the evaporator and fan of the outdoor unit.
Missing or delayed maintenance - according to service technicians' experience, this is the most common common denominator in nearly half of the call-outs that would otherwise never have needed to happen.

The good news is that the vast majority of faults fall into just a handful of categories that repeat across brands and pump types. Let's go through them in detail.

Overview of the most common types of faults

The following indicative breakdown is based on the experience of service technicians with typical heat pump operation in Slovak family houses, and roughly corresponds to how often each type of problem shows up during service calls:

Faults in control electronics and sensors - approximately 30% of call-outs
Problems with defrosting and icing of the outdoor unit - approximately 25% of call-outs
Low pressure or refrigerant leaks - approximately 20% of call-outs
Compressor faults - approximately 10% of call-outs
Problems with the circulation pump and heating water flow - approximately 10% of call-outs
Mechanical soiling and blocked condensate drainage - approximately 5% of call-outs

1. Icing of the outdoor unit and defrost cycle faults

This is the most visible and most frequently reported "fault" during winter months - and in most cases it isn't a fault at all. An air-water heat pump draws heat from the outdoor air, and when the air contains moisture at temperatures close to freezing (typically -5 to +5 °C), frost naturally forms on the evaporator fins. That's why every pump has an automatic defrost cycle - for a short time (usually 3 to 12 minutes, depending on frost thickness) it switches to reverse operation, the evaporator briefly warms up, and the frost melts and drains away through the condensate drain.

This situation becomes a real fault when:

The unit defrosts too often (for example every 15-20 minutes instead of the usual 1-3 hours in freezing weather) - typically signalling low refrigerant charge or a faulty evaporator temperature sensor.
Conversely, it doesn't defrost at all and the frost turns into a thick layer of ice blocking airflow through the fins - most often due to a faulty 4-way valve, which is supposed to switch the cycle, or a faulty sensor.
The condensate drain from defrosting is frozen or blocked, so the water produced accumulates around the unit and freezes again - this is usually just a mechanical installation issue, not a fault in the pump itself.

A practical example from everyday operation: if during a frosty morning you hear the outdoor unit's fan "starting" and immediately stopping again at intervals shorter than 15-20 minutes, or you notice the bottom of the unit covered in a thick layer of ice even several hours after the frost has eased, this is a clear signal to have it checked by a service technician.

2. Low refrigerant pressure and refrigerant leaks

The refrigeration circuit is sealed from the system, and ideally the amount of refrigerant in it never changes over the years. In practice it's different - the most common leak points are brazed joints on the pipework between the outdoor and indoor units (in split systems), service valves, and, in older installations, also micro-cracks caused by vibration or poorly secured pipework. A refrigerant leak shows up gradually, not all at once, which makes it deceptive - the pump's output falls slowly over weeks or months, so the owner often only notices it once the house stops holding the desired temperature.

Typical symptoms of low refrigerant pressure:

The compressor runs almost continuously, but the indoor unit delivers noticeably colder water than it should.
Frost appears on the pipework or on the compressor even outside the normal defrost cycle (typically on the suction line).
An error code on the display linked to low pressure in the circuit (for example codes such as "low pressure" or "pressure switch fault" - the exact label varies by manufacturer).
The COP (coefficient of performance) noticeably drops - instead of the usual 3.5-4.5 in mild weather, the actual electricity consumption relative to heat produced approaches a ratio of 1:2 or worse.

Important note: topping up refrigerant in a heat pump is professional work that in Slovakia may only be carried out by a person holding an F-gas certificate (under the EU regulation on fluorinated greenhouse gases). At most you can visually check the condition of the pipework and its insulation yourself - the pressure itself and any top-up are strictly the job of a service technician.

3. Compressor faults

The compressor is the "heart" of the heat pump, and replacing it is among the most costly repairs, which is why it deserves separate attention. The most common causes of compressor failure:

Overheating - caused by prolonged low refrigerant pressure, a clogged evaporator, or a faulty fan that isn't moving enough air.
Liquid slugging - liquid refrigerant enters the compressor instead of gas, mechanically damaging the pistons or the scroll - usually the result of a faulty expansion valve.
Wear after years of operation - with quality installation and regular maintenance, typical compressor lifespan is around 12-18 years, roughly 60,000-100,000 operating hours.
Frequent short cycling - if the pump is oversized relative to the house's needs, the compressor keeps switching on and off instead of running smoothly, which accelerates mechanical wear.

Signs of an impending compressor fault: an unusually loud or metallic sound on startup, vibration transmitted into the pipework and walls, a longer ramp-up to reach the required output, or repeated shutdowns with a "compressor fault" or "compressor overheating" error code.

4. Problems with the circulation pump and heating water flow

The indoor unit contains a circulation pump that pushes the heated water into the heating system (radiators or underfloor heating). If this pump fails or flow is insufficient, the condenser overheats and the system generally shuts down protectively with an insufficient-flow error. Common causes:

Air in the system - an air bubble in the circulation pump or at the highest point of the system.
A clogged filter or debris in the heating circuit, especially in older radiator systems.
An incorrectly set circulation pump (output too low relative to circuit length, especially with underfloor heating with long loops).
A dirty filter or fouled heat exchanger creating increased hydraulic resistance.

The solution is often simple - venting the system and checking the pressure (typically 1.5-2 bar in a cold system, the exact value depending on building height and design) resolves a large share of these cases without needing a service call.

5. Faults in control electronics and error codes

A modern heat pump has dozens of sensors (intake air temperature, outlet water temperature, circuit pressure, compressor temperature, evaporator temperature) and a control unit that evaluates this data many times per second. This is exactly why this category is the most common - it only takes one sensor giving inaccurate readings, or moisture causing oxidation on a connector, for the system to either lock into protective mode or start running inefficiently without it being outwardly obvious.

Typical situations:

The display shows a specific error code (for example an "E" or "F" code with a number) - it's always a good idea to photograph or write down the exact code before calling the service, as it speeds up diagnosis.
The system restarts on its own several times a day for no apparent reason.
The WiFi/app reports "connection lost" or doesn't update data - often just a home network issue, not a fault in the unit itself.
Incorrectly functioning weather compensation - the pump heats too little or too much relative to the outdoor temperature, even though everything is mechanically fine.

Practical advice: before calling the service, try a simple restart as described in the manual (switching off at the main breaker for 2-3 minutes) - this surprisingly resolves a large percentage of minor control board "hangs" caused by a brief power dip or a software glitch. If the error returns within a few hours, it's a genuine fault requiring a service technician.

6. Noise and vibration

An increase in noise is one of the earliest noticeable symptoms of most mechanical faults. The typical noise level of the outdoor unit for most residential heat pumps runs around 35-55 dB depending on the output stage - roughly comparable to a quiet conversation up to ordinary background music in a shop. A change in the character of the sound matters more than its loudness:

Metallic scraping or knocking - most often a loose or damaged fan, or a foreign object (a leaf, a small twig) in the fan wheel.
Buzzing or humming from the indoor unit - often a worn circulation pump bearing.
High-pitched whistling - can signal a refrigerant leak at high pressure or a problem with the expansion valve.
Banging in the pipework (water hammer) - most often air in the system or a poorly set three-way valve.

7. Soiling of the evaporator, condenser and blocked condensate drainage

The cheapest and most easily prevented group of faults. The outdoor unit draws in a large volume of air (typically several thousand cubic metres per hour at full output), and along with it pollen, dust, leaves, and in urban environments also fine soot. Clogged evaporator fins can reduce output by tens of percent while also increasing the load on the compressor and fan. Similarly, the indoor heat exchanger and filter on the heating water side also gradually accumulate sediment over time, especially in older systems with steel radiators.

The condensate drain (the hose or pipe carrying away water produced during defrosting and normal operation) can become blocked with leaves, algae, or simply freeze in winter if it doesn't have enough fall or isn't heated. The result is water and ice accumulating around the unit, which in extreme cases can even damage the base itself.

How to recognise a fault from measured values

The table below compares normal operating values with situations that already signal a fault. These are indicative ranges for a typical residential air-water heat pump with a medium heating water temperature - exact values vary by manufacturer and design, so they serve only as a first rough filter, not a substitute for professional diagnostics.

Prevention: regular maintenance that prevents most faults

As the fault breakdown above shows, more than half of common call-outs (defrosting, soiling, part of the electronics problems) can be significantly reduced through simple, regular care. The following schedule is based on recommendations common among most manufacturers for residential installations:

Most manufacturers recommend one professional service call a year, ideally before the start of the heating season (September-October). On this occasion the technician checks the circuit for leaks, refrigerant pressure, the condition of electrical connections, the function of the defrost cycle, and cleans the evaporator more thoroughly than routine home maintenance allows. In practice, this single annual visit prevents most of the more expensive faults described above - especially compressor overheating and gradual refrigerant leaks, which without inspection are only discovered once output has already noticeably dropped.

When to handle a problem yourself and when to call a service technician

Not every deviation from normal means calling the service the same day. A sensible rule is as follows:

Handle yourself: a simple restart after a software glitch, venting the system as described in the manual, cleaning visible debris from around the outdoor unit, checking and topping up the heating circuit pressure if you have a fill valve and know how.
Call the service within a few days: a repeated error code after a restart, a mild drop in output with no obvious cause, an unusual but not loud sound, needing to top up system pressure repeatedly over several weeks.
Call the service immediately / switch off the pump: a strong smell of refrigerant, visible fluid leaking from the indoor unit, loud metallic sounds or a bang followed by an immediate shutdown, an electrical smell or smoke, the circuit breaker tripping repeatedly.

It is exactly this last category that explains why intervention in the refrigeration circuit and electrical parts of a heat pump must never be done yourself - besides the legal requirements (F-gas certificate for refrigerant), there is also a safety risk, and in many cases it is also a condition for keeping the manufacturer's warranty.

A practical example from everyday operation

A typical scenario service technicians encounter is an owner calling in January reporting "the pump isn't heating like it should, the house is getting cold, and it's completely covered in ice outside". After phone-based diagnostics, a large share of such cases turn out to have one of two causes: either the condensate drain is blocked or frozen (the unit can't defrost properly because water from the previous cycle freezes again before it can drain away), or it's a genuine drop in refrigerant pressure, which outwardly looks very similar - a thick layer of ice that the normal short cycle can't keep up with removing. Only a technician on site can tell the difference, based on whether the situation recurs within a few days after manually removing the ice and clearing the drain, or not. That's why it's important, when reporting to the service, to describe exactly how often and for how long the unit defrosts - this one piece of information can significantly shorten diagnosis and sometimes even avoid an unnecessary call-out.

Another common example is a drop in heating water temperature with no error code at all - here it makes sense to start by checking system pressure and venting, since this simple task resolves a surprisingly large share of similar reports without needing a technician's visit. Only once both pressure and venting are fine and output keeps dropping is it more likely a slow refrigerant leak or a clogged evaporator, which does require professional intervention.

Frequently Asked Questions

Is it normal for the outdoor unit to be covered in ice in winter?

A thin layer of frost on the fins is normal and is removed by the automatic defrost cycle. The problem is a thick layer of ice that persists even hours after the frost has eased, or ice that builds up under the unit from condensate that isn't draining - that is a reason for inspection.

How long does a heat pump last before a major fault?

With quality installation, correct sizing and regular annual maintenance, typical compressor lifespan is around 12-18 years and the whole unit's lifespan is 15-20 years. Neglected maintenance can significantly shorten this, especially for faults caused by prolonged low refrigerant pressure or a dirty evaporator.

Can I top up the refrigerant myself if output has dropped?

No. Handling refrigerant in heat pumps is subject to the EU regulation on fluorinated greenhouse gases (F-gas) and may only be done by a certified technician. You can only check the visual condition of the pipework, its insulation, and any traces of oil around joints that might indicate a leak.

Why does the pump restart itself frequently?

Most often this is a brief power dip, a software glitch in the control board, or, in a worse case, a faulty sensor causing false protective shutdowns. If the restarts continue even after switching off and on at the main breaker, it's a reason for a service call.

Why has my heating output dropped even though no error code is showing?

A drop in output with no error message is typical of a slowly progressing refrigerant leak or a gradually clogging evaporator - both phenomena first show up only as higher electricity consumption and lower comfort, not as an error code. We recommend monitoring monthly electricity consumption and contacting the service if there's an unusual increase.

Does the type of heating system (underfloor vs. radiators) affect susceptibility to faults?

Not the type of system directly, but incorrectly set flow and pressure do - underfloor heating with long loops is more prone to insufficient-flow problems if the circulation pump isn't sized for the circuit length, while radiator systems tend to be more sensitive to sediment and filter clogging in older pipework.

Can a fault be prevented without a paid regular service, just through your own care?

Some faults, yes - especially those caused by soiling and a neglected area around the unit. You can handle checking pressure, cleaning visible debris, and keeping free space around the outdoor unit yourself. However, checking the refrigeration circuit for leaks, electrical connections, and precisely measuring refrigerant pressure cannot be done on your own - that requires professional measurement and certification.

Have a question on this topic?

Not sure what to decide, or dealing with a specific situation in your home involving a heat pump? Write to us - we're happy to help.