A heat pump does not work like a boiler. This may sound obvious, but it is precisely this lack of understanding that leads to most problems, disappointments and unnecessary costs. Many users judge how a heat pump works based on their experience with gas or coal. This, in turn, leads to incorrect conclusions right from the start. How should a heat pump work?
A heat pump that is working properly operates quietly, predictably and without fanfare. It does not rely on rapidly heating the house ‘on demand’, but rather on maintaining the building’s energy balance. If the system has been well designed and correctly configured, the pump does its job in the background, without constantly starting up, without sudden temperature fluctuations and without the need for constant adjustments by the user.
How should a heat pump work?
One of the most common misconceptions is the belief that a good heat pump should switch off frequently because it has ‘already heated the space’. In reality, the opposite is true. The most efficient operating mode for a heat pump is characterised by long, stable cycles, during which the compressor runs at partial capacity and adapts smoothly to the building’s heating requirements.
Every time the compressor starts up, it experiences peak mechanical and electrical loads. Frequent switching on and off – known as cycling – shortens the unit’s service life and reduces its seasonal efficiency. A heat pump is designed for continuous operation, not for short bursts. If the unit runs for hours on end and the temperature in the house remains stable, this is not a sign of a problem, but a sign that the system is working as intended.
In practice, the user should not be concerned with whether the pump is ‘idling’, but rather with whether the house maintains a comfortable temperature without sudden fluctuations. Quiet operation, no erratic behaviour from the controller and a consistent operating pattern are all positive signs, not cause for concern.
A low supply temperature is key to efficiency
The whole concept of a heat pump is based on operating the heating system at the lowest possible temperature. The smaller the temperature difference between the lower and upper sources, the less electricity is needed to transfer the heat. This isn’t just theory from a manufacturer’s brochure; it’s basic physics.
In a well-functioning system, the supply temperature does not ‘jump’ in response to temporary drops in comfort. Instead, the controller gradually raises or lowers it depending on external conditions. For underfloor heating, this often means operating within a range of 25–35°C for most of the season. With radiators, the values are higher, but should still remain as low as possible in relation to the building’s demand.
If thermal comfort can only be achieved by manually increasing the flow rate to high levels, the problem usually does not lie with the pump itself. The cause may be an incorrectly selected heating curve, incorrect flow rates, hydraulic errors, or unrealistic expectations regarding a system that was never designed for low operating temperatures.
The heating curve controls the system, not the thermostat
In heat pumps, it is the heating curve that acts as the main control mechanism, rather than a room thermostat. The controller analyses the outside temperature and uses this to determine the system’s flow temperature. This ensures that the building receives exactly the amount of energy it is losing at any given moment, without sudden overheating or cooling.
Problems arise when the user tries to control the pump in the same way as a boiler. Closed underfloor heating loops, aggressive thermostats and frequent adjustments to the set temperature disrupt operational stability. The pump loses power, reaches its limit temperature more quickly and switches off, only to start up again a moment later. This is a classic scenario leading to cycling. In a well-configured system, the thermostat plays a supporting or protective role, rather than acting as the main controller. Comfort is regulated by the heating curve and the system’s hydraulics, rather than by constantly ‘cutting off’ the heat source.
Defrosting in air pumps is not a fault, but simply part of the operation
For many users, their first winter with an air-source heat pump can come as a shock. The outdoor unit starts to defrost, frost forms, and after a while the unit stops heating – and sometimes even briefly sends cold air into the system. It all seems counterintuitive, especially to those used to traditional boilers.
In fact, the defrost cycle is an integral part of how an air-source heat pump works. When moist air at low temperatures condenses on the heat exchanger, a layer of ice begins to block the air flow and reduce efficiency. The pump must then briefly reverse the cycle to melt the frost and restore its ability to extract heat from the air.
The problem isn’t the defrost cycle itself, but its frequency and duration. Occasional, short cycles in damp, freezing conditions are normal. However, if the pump switches to defrost mode almost continuously and is unable to return to stable heating for a long time afterwards, it is worth investigating the cause. Most often, this is due to incorrect installation of the outdoor unit, restricted airflow, a dirty heat exchanger, or faults in the system’s hydraulics.
Operating on domestic hot water changes the nature of the entire system
Heating a home and heating domestic hot water are two completely different tasks from a heat pump’s perspective. A heating system, particularly an underfloor heating system, operates at low temperatures. A domestic hot water tank requires much higher temperatures, often above 50°C, and periodically even higher for hygiene reasons.
When the system switches to DHW mode, the pump enters high-performance mode. The compressor increases the temperature, energy consumption rises temporarily, and space heating takes a back seat. To the user, this may appear as a sudden ‘overload’ of the system or a drop in thermal comfort. This is particularly true if the storage tank heats up frequently and without a set schedule.
A properly configured system resolves this issue through priorities and schedules. DHW is heated during specific time slots, and the central heating system has sufficient thermal inertia to cope with a brief interruption. If the pump keeps switching back and forth between DHW and heating, this is usually not due to the pump being ‘underpowered’, but rather to ill-considered settings.
The pump’s behaviour changes depending on the weather, and this is normal
One common source of frustration is the belief that a heat pump should operate in exactly the same way regardless of external conditions. In practice, its performance varies depending on temperature, humidity and the building’s load. These variations are inherent in the way the system works.
During transitional periods, the pump often runs for short periods, as the building’s demand is low. During sustained cold spells, it switches to long, steady operation. At very low temperatures, it may increase the compressor speed, extend the running time or use the auxiliary heater more frequently, if the system has been designed to do so.
These changes do not mean that performance has deteriorated. They simply mean that the vehicle is responding to real-world conditions. The problem only arises when these responses are erratic and disproportionate. For example, frequent stalling in steady weather, or an inability to maintain a comfortable ride despite having, in theory, sufficient power.
When does the operation of a heat pump actually indicate a problem?
How should a heat pump operate? Not every instance of ‘unusual’ behaviour indicates a fault, but there are situations in which the system sends clear warning signals. The most characteristic symptom is cycling, i.e. the compressor frequently switching on and off at short intervals. If the pump starts up every few to several minutes for a significant part of the day, this almost always indicates a systemic problem rather than a temporary anomaly.
The second sign is a lack of thermal comfort despite the unit appearing to be working correctly. If the temperature in the house fluctuates and the user is constantly adjusting the settings, it means that the control system is not keeping up with the building’s actual demand. In such cases, the cause often lies in the system’s hydraulics, poor flow distribution or an incorrectly selected heating curve, rather than in the pump itself.
The third warning sign is when the pump requires increasingly higher supply temperatures to maintain the same level of comfort. If you find yourself having to ‘turn up’ the settings from one season to the next, this is a sign that something in the system is no longer functioning as intended. This is the point at which simple adjustments are no longer sufficient, and you need to reassess the entire system.
Why does adjustment often solve more problems than servicing?
In practice, the vast majority of problems with heat pumps are not caused by mechanical faults. The system is running, the compressor is compressing, the refrigerant is circulating, but the system as a whole is unable to reach a stable operating point. This is because a heat pump is a control device, not merely a source of power.
An incorrectly set heating curve, poorly configured thermostats, a lack of hydraulic balance or incorrect DHW priorities can effectively undermine the efficiency of even a very good system. In such situations, calling out a service engineer without first analysing the settings usually ends in frustration, as the equipment technically appears to be working correctly.
Only when adjusting the settings fails to produce the desired result should you check the technical aspects: components, sensors, valves and flow rates. Heat pumps very rarely ‘break down suddenly’. More often than not, they operate under sub-optimal conditions for a long time until the effects become apparent.
The proper functioning of a heat pump is a state of equilibrium, not perfect graphs
Many users try to assess a heat pump’s performance based on individual parameters: the number of starts, the flow temperature or instantaneous energy consumption. However, the most important criterion is the stability of the entire system over time. A well-functioning heat pump maintains comfort without constant user intervention and without erratic controller behaviour.
The system may seem ‘boring’: the compressor runs for long periods, temperatures change slowly, and energy consumption is spread evenly. It is precisely this lack of dramatic changes that indicates the system has found its equilibrium. The pump isn’t fighting against the building, but calmly compensating for its heat loss. If the user stops checking the app several times a day, this is often the best proof that the heat pump is working as it should.
Summary – How should a heat pump work?
A heat pump does not operate correctly by reacting quickly to every drop in temperature or by switching off frequently once the set temperature is reached. It is a continuous process based on low operating parameters, power modulation and adaptation to the building’s actual heat loss. When the system has been well designed, correctly set up and used sensibly, the heat pump operates in the background, quietly and without the need for constant adjustments.
Most problems attributed to the heat pump itself stem from a misunderstanding of how it works, or from errors in its adjustment or installation. Only by looking at the whole picture – the heat source, the installation, the controls and how it is used – can one reliably assess whether the unit is functioning correctly. A heat pump does not reward haste, but responds very well to calmness, consistency and patient adjustment.
