A reversible heat pump is a system that can not only heat a building but also cool it. This does not involve two separate units, but a single refrigeration circuit which, when switched, reverses the direction of heat transfer. In winter, energy is fed into the heating system; in summer, heat is extracted from the building and discharged outside or to a ground source. At the core, the same components continue to operate: the compressor, heat exchangers and expansion valve. The difference lies in the reversible valve, which switches the functions of the heat exchangers.
This solution is often described in overly simplistic terms, as if it were merely a ‘heat pump with a cooling function’. Such oversimplification obscures what is most important. The performance of a reversible system is determined by the overall temperature profile, the type of loads, the control system, air humidity and the way the installation is set up. The mere ability to reverse the cycle does not in itself guarantee good performance, either in winter or summer. The efficiency of a heat pump is closely linked to the temperature difference between the source and the system, and reversible systems must be analysed in conjunction with the building’s load profile, rather than solely on the basis of the unit’s parameters.
How does a reversible heat pump work?
From a thermodynamic perspective, a reversible heat pump is essentially a conventional compressor-type heat pump. The refrigerant evaporates in one heat exchanger; the compressor increases its pressure and temperature; the refrigerant then releases heat in the second heat exchanger, after which it passes through the expansion valve and returns to the starting point. When the system is set to heat the building, one heat exchanger acts as the evaporator and the other as the condenser. When the reversing valve is switched, these roles are reversed.
From an installation perspective, this switch is more important than it might appear at first glance. When the unit switches from heating to cooling, the operating conditions of the heat exchangers, the direction of energy flow, the preparation of the load, and the requirements for the control system all change. In air-source heat pumps, the same mechanism is also used to defrost the outdoor unit in winter. During defrosting, the unit temporarily reverses the cycle in order to melt the frost on the heat exchanger that was previously operating as an evaporator.
| Circuit component | Heating mode | Cooling mode | What’s changing |
|---|---|---|---|
| external heat exchanger or source | draws heat from the source | transfers heat to the source or to the outside | sometimes it acts as an evaporator, sometimes as a condenser |
| heat exchanger on the building side | transfers heat to the system | draws heat from the system | its function and operating temperature change |
| check valve | sets the flow rate for heating | sets the flow rate for cooling | switches the direction of the refrigerant flow |
| automation and equipment | controls the heating and defrost | controls cooling and prevents condensation | the importance of sensors and control logic is growing |
Conclusion
The key takeaway from this comparison is simple. Reversibility does not create a new type of refrigeration circuit. It simply reconfigures the functions of the same components so that the unit can operate throughout the entire year. As a result, a single unit can replace two separate systems: one for heating and one for cooling.
However, this does not mean that the entire system behaves in exactly the same way in both modes. On the building installation side, switching from heating to cooling entails different hydraulic conditions, different flow temperatures and different risks. In winter, the main issue is performance at low source temperatures. In summer, there are additional considerations such as dew point, condensation and the ability of the loads to operate safely in cold conditions.
Which circuits can be reversible?
Reversibility is not limited to a single type of heat pump. Such a system can operate as an air-to-air, air-to-water, water-to-water, ground-source, or DX system, as well as in more complex variants incorporating heat recovery. This is important because, in everyday usage, the term ‘reversible heat pump’ is often associated mainly with a split system or an air conditioner that heats in winter. However, it could just as easily be a water-based system providing heating and cooling throughout the entire building.
For residential buildings, there are generally two main types. The first is air-to-air, where the unit directly heats or cools the air inside the room. The second is air-to-water or water-to-water, where a pump supplies the water system. This group of units is most frequently discussed today in relation to homes, small commercial buildings and modern low-temperature systems.
Reversal is not the same as heat recovery
It is worth making this distinction clear, as marketing materials often lump everything together. A standard reversible system usually means that the entire system operates either in heating or cooling mode. A heat recovery system goes a step further, as it can use the energy extracted from one zone to heat another.
This is of great importance in terms of design. In a detached house, a seasonal change of mode is usually sufficient. In an office, hotel or commercial building, the situation is different, as some zones may require cooling whilst others still need heating. Analysing simultaneous heating and cooling loads is one of the fundamental steps in designing such systems. Only then can one assess whether simple reversibility is sufficient, or whether it is worth opting for a system with heat recovery.
| Layout type | What does he do? | Where does it make sense? | The main limitation |
|---|---|---|---|
| reversible | switches between heating and cooling | homes, small offices, simple seasonal setups | The whole system usually operates in one mode at a time |
| reversible with heat recovery | can switch between modes and also recover energy | multi-zone facilities | greater complexity of the system and control |
| heat recovery system | uses heat recovered from one part of the building | buildings with parallel loads | cost-effectiveness depends on the building’s profile |
This comparison shows that reversibility alone is not the ultimate form of flexibility. It works very well in many buildings, but where cooling and heating overlap, a system capable of genuine heat recovery offers greater potential. However, this is no longer a matter of a ‘single unit’, but of the entire HVAC system.
The most important factor affecting efficiency: the temperature of the heat source and the temperature of the system
The key technical point of the whole matter is simple: a heat pump works best when there is a small temperature difference between the source and the load. The closer the temperatures on the evaporator and condenser sides are to each other, the less work the compressor has to do and the less electricity the system consumes. This is precisely why heat pumps work so well with underfloor heating, wall-mounted radiators or other low-temperature heat emitters.
This immediately explains why not every building is suitable for a straightforward switch to a reversible heat pump. If the heating system relies on high flow temperatures, the unit will operate less efficiently. If the same building is also to be cooled, it is necessary to check not only the unit’s capacity, but also whether the loads and control systems are capable of handling two different operating modes. In older buildings, this often means refurbishing part of the system, rather than simply replacing the heat source.
Where does a reversible heat pump make the most sense?
This setup works best where a building actually requires both heating and cooling. In a detached house, this will most often be a new or well-refurbished building with low-temperature heating and reasonable cooling requirements. In small commercial premises, reversibility offers a similar benefit: a single unit serves two seasons and simplifies the layout of the heating and cooling sources.
The situation is even more interesting in multi-zone buildings. A load analysis may reveal both high potential for reversion and moderate or high potential for heat recovery. Where zones have different operating profiles, the system need not be treated as a typical ‘winter-summer’ source. It can become a central element of thermal energy management in the building.
Limitations on the cooling side: dew point and condensation
This is where a topic often overlooked comes into play. In water-cooled systems, the greatest risk is not the pump itself, but rather air humidity and temperatures dropping below the dew point. Condensation can form on cool surfaces within the system if their temperature drops below the dew point in a given area. This applies not only to heat emitters, but also to pipes, fittings, manifolds and other components of the circuit.
For this reason, water-based cooling requires far greater design discipline than heating alone. It is not enough simply to run cold water through the system. You need to monitor the supply temperature, indoor humidity, insulation of components and control logic. In the case of surface-mounted systems, the safe operating range is narrower than with fan coil units, as the cooling surface must not fall below the dew point.
Water cooling and the choice of heat sinks
This leads to another important difference. A reversible heat pump can work with various heat sinks, but not every heat sink offers the same flexibility when it comes to cooling. Fan coils and air handling units are better suited to both sensible and latent cooling, as they not only remove heat but also help to reduce humidity. Surface cooling is more subtle and comfortable, but requires more careful temperature management and good control of the dew point.
This does not mean that surface cooling is a bad idea. It simply means that it requires a better-designed system. If the client expects aggressive cooling in high-humidity conditions, a cooling floor or ceiling alone may not be sufficient. If the aim is to gently lower the temperature whilst maintaining well-controlled indoor air, such a system can work very well. The source of the problems here is not the reversibility of the pump, but unrealistic expectations regarding the load.
| Area | Benefit | Restriction | What needs to be taken into account |
|---|---|---|---|
| heating | one unit covers the winter season | a drop in efficiency over a wide temperature range | low-temperature system |
| air cooling or fan coil | effective control of cooling capacity | a more complex distribution than with heating alone | condensate drainage and control |
| surface cooling | high comfort and quiet operation | risk of condensation | dew point control and surface temperature limitation |
| heat recovery | more efficient use of energy in the building | greater system complexity | zonal load analysis |
This comparison shows that reversibility is just one of the system’s features. The final outcome depends entirely on how the unit is paired with the right type of loads and control logic. A well-designed system can heat and cool very effectively. A poorly designed one will give the impression that ‘the pump can’t cope’, even though the problem lies elsewhere.
Limitations on the source side and on the device itself
With air-source heat pumps, there is also the issue of defrosting. The reversing valve is also used for winter defrosting, and when the temperature drops and humidity is high, frost builds up on the outdoor coil. This requires periodic switching of the circuit, which temporarily affects the unit’s operational balance. In a mild climate, this is of less significance, but in a cold and damp environment, it becomes a crucial aspect of the entire system’s operation.
In air-to-water monoblock systems, the issue of frost protection also arises. If water or an aqueous solution is used on the outdoor side, measures must be taken to protect against low temperatures, system downtime or power failures. One solution is to use a glycol-based circuit, although this option increases flow resistance and may reduce the system’s efficiency.
What should you check before making your choice?
Before choosing a reversible heat pump, you must first determine whether the building actually requires operation in both seasons. If cooling is only to be used occasionally and the plumbing system is not designed for this purpose, the solution may prove to be overly complex in relation to actual needs. If the building has high internal heat gains or extensive glazing, the cooling mode ceases to be an optional extra and must be treated with the same seriousness as heating.
Next, you need to look at the temperature profile and the heat consumers. Does the heating system operate effectively at low temperatures? Can the heat consumers on the cooling side cope without condensation? And does the building have zones with different requirements that justify heat recovery? And finally: is the control system capable of monitoring not only temperature, but also humidity and dew point? Only the answers to these questions will reveal whether a reversible heat pump is a sound solution, or merely a flashy buzzword in the specifications.
Conclusion
A reversible heat pump makes sense when a building genuinely requires both heating and cooling, and the system has been designed to operate in two different modes. Reversing the cycle itself is technically straightforward. The rest is more difficult: selecting the heat source, limiting the temperature difference, choosing the load, controlling humidity and ensuring sensible control. When these elements are well put together, a single system can operate all year round and do so very efficiently. When they are poorly configured, problems arise quickly and usually stem not from the compressor, but from systemic errors.
Sources:
https://www.energy.gov/energysaver/air-source-heat-pumps
https://www.seai.ie/sites/default/files/publications/Heat-Pump-Technology-Guide.pdf
https://www.caleffi.com/sites/default/files/media/external-file/Idronics_27_NA_Air-to-water%20heat%20pump%20systems.pdf
https://iea-ebc.org/Data/publications/EBC_Annex_48_Final_Report_R4.pdf
