In the previous article on heating costs, we compared the costs of different heat sources, and heat pumps performed very well in this comparison, especially ground-source models. If you have already made your decision and are choosing an air-to-water heat pump, you face a second question, just as important as the heat source itself: split or monobloc? This is not a cosmetic question about where the unit is located. It is a question of what the entire installation will require from you before it even starts heating.
Split or monobloc: a more important difference than the unit's location
The most common oversimplification is this: a split system has something inside the house, while a monobloc has everything outside. That is true, but it is not the most important difference. The key question is different: where is the refrigerant, the substance circulating in the system that actually transfers heat from the air to the water? In a split system, the refrigerant flows through pipes between the outdoor and indoor units, so some of it is located inside the house. In a monobloc, the entire refrigerant circuit is factory-sealed inside a single outdoor housing, and only water enters the house. This seemingly minor design difference can have significant legal consequences that few people are aware of.
Split: the standard that still dominates the market
In a split system, the outdoor unit contains the compressor and the heat exchanger that extracts heat from the air, while the entire hydraulic system, including the circulation pump, any buffer tank and backup heater, is located in a small indoor unit in the house. A refrigerant pipe therefore enters the house, while all the system water remains indoors and never goes outside. Contrary to the intuition that the easier-to-install monobloc would be displacing the older split system, industry data shows the opposite: according to the Polish Organization for the Development of Heat Pump Technology, in the first half of 2026, split systems sold in Poland approximately three times more often than monoblocs, with a market share approaching eighty percent.
The real advantage of a split system comes precisely from the fact that the system water never leaves the heated house: no section is exposed to freezing temperatures, so unlike a monobloc, a split system does not require additional frost protection. A split system can also be a more practical choice at higher heating capacities, where the indoor unit can additionally serve as an integrated buffer tank or support domestic hot water production, without adding separate components. The price of this flexibility is the bureaucracy described below: since the refrigerant actually flows through the house, the law treats the system more cautiously.
Monobloc: fewer formalities, more work outdoors
A monobloc reverses this arrangement. The entire refrigerant circuit, including the compressor and heat exchanger, is factory-sealed inside a single unit located outside the building. Only hot water enters the house, just as it does from a traditional boiler. This means that the most formally demanding part of the installation, namely work with the refrigerant, takes place once, at the factory, while the installer on site only has to carry out the simpler hydraulic connection. This shortens installation time and can be cheaper, but it comes at a cost: since all the system water passes through equipment standing in freezing temperatures, this section of the installation requires additional protection against freezing.
Bureaucracy tips the scales in favor of the monobloc
The European Union has been ensuring for years that refrigerants used in air conditioners and heat pumps do not escape into the atmosphere without consequence, because many of them contribute significantly to the greenhouse effect. Rather than counting kilograms directly, their quantity is converted into what is known as the CO2 equivalent, a measure of how harmful a given gas is to the climate in terms of carbon dioxide. The more of this equivalent there is in a single installation, the more formalities apply: the need to hire a certified technician, mandatory periodic leak checks and, after a certain threshold is exceeded, registration of the installation in a central register.
You can think of it like regulations on transporting goods: below a certain small quantity, you do not need any additional permits, but when you transport more, additional requirements, notifications and inspections begin to apply. With split installations, this formal threshold is low because the refrigerant flows through pipes throughout the house, so any leak would occur where people live, and the law therefore treats the system more cautiously. For the popular R32 refrigerant, the limit is approximately 7.4 kilograms. For a monobloc, where the entire circuit is factory-sealed and leak-tight, and any gas released in the event of a leak would escape outside the building, the same threshold is twice as high, at approximately 14.8 kilograms. In practice, this means that some split installations come close to the lower limit and require additional formalities, while a comparable monobloc often remains below it without any complications. Nevertheless, as the market data above shows, ease of compliance alone does not determine the choice of most investors.
2027: when R32 will cease to matter
However, this entire calculation based on thresholds and kilograms will soon change, in favor of a completely different refrigerant. From January 1, 2027, EU regulations will prohibit placing new heat pumps using refrigerants with a high global warming potential on the market, including R32. This applies both to monoblocs up to 50 kW and to air-to-water split systems up to 12 kW, which covers practically the entire segment of interest to owners of single-family homes. In practice, the market is already switching to natural refrigerants, primarily propane designated as R290, which has a negligible impact on the climate and is not subject to the same quantity restrictions. When choosing a heat pump today, it is worth checking which refrigerant it uses, because this is a question about the future of servicing and the availability of spare parts over the next decade, not just about current regulations.
Protecting a monobloc against freezing
A monobloc has one weakness to remember: all the system water, not just the refrigerant, passes through a unit standing in freezing temperatures. If the power happened to go out in the middle of a harsh winter, the water in this part of the installation could freeze and rupture the heat exchanger. That is why such systems are fitted with a mechanical anti-freeze valve, which operates purely thermally, without electricity or a controller, releasing a small amount of water before freezing occurs.
If you want the shortest possible installation time, fewer formalities and are willing to accept an additional anti-freeze valve as part of the installation, a monobloc will be the simpler choice, especially for small and medium capacities. If, on the other hand, you are planning a larger installation, want all the heating water to remain safely inside the house without additional frost protection, or have found an installer who is experienced in working with refrigerants, a split system remains the choice made by the overwhelming majority of the Polish market, despite the additional formalities involved.
What a heat pump requires from the installation
Manufacturers are happy to present the unit itself as a ready-made solution, but that is only part of the story. For a heat pump to operate stably and efficiently, the rest of the installation must be properly prepared. Three elements are particularly important: the minimum water flow through the heat exchanger, a sufficiently large heat buffer that stabilizes the unit's operation, and hydraulic separation when the house has more than one heating circuit with different characteristics, such as radiators and underfloor heating operating simultaneously.
Flow errors and compressor short cycling
When these requirements are not met, two problems most commonly arise. The first is flow errors, indicating that not enough water is passing through the heat exchanger to safely remove heat from the compressor. The second is short cycling, meaning that the compressor repeatedly switches on and off at short intervals instead of operating smoothly for a longer period. Both problems have the same underlying cause: the factory-installed pump built into the heat pump unit is designed to overcome the resistance of the unit itself, not that of the entire, extensive installation with long underfloor-heating loops. This is where an additional, properly selected pump group and manifolds can help, supporting flow where the heat pump itself is no longer sufficient.
Do old radiators need to be replaced?
Old radiators are one of the most common reasons people give up on installing a heat pump in their home, and in most cases this concern is unnecessary. Radiators release heat more intensively the higher the temperature of the water supplying them, but that does not mean they stop heating at lower temperatures. It simply means they must be larger or operate for longer to release the same amount of heat. A thermally modernized house with new windows and insulation loses considerably less heat than before, so the same old radiators, supplied with water at a lower temperature, are sufficient to maintain comfort.
COP, SCOP and actual bills
Manufacturers are happy to provide the COP, or the ratio of heat output to electricity consumed, measured at one specific operating point, such as an air temperature of plus seven degrees and heating water at thirty-five degrees. The problem is that conditions in Poland are different for most of the heating season, so COP alone says little about actual bills. A better indicator is SCOP, the average seasonal efficiency that takes changing weather throughout the year into account.
The German research institute Fraunhofer ISE monitored 77 installations operating in real conditions for several years and obtained results worth knowing before making a purchase: air-to-water heat pumps achieved an average SCOP of 3.4, ranging from 2.6 to 5.4 depending on the specific installation and house, while ground-source heat pumps averaged 4.3. The spread of these results clearly shows that the brand and model of the unit are not everything. The same heat pump in a well-prepared installation, with appropriate flow and a supply temperature matched to the building's actual heat losses, can achieve a result close to the upper limit rather than the lower one.
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Frequently asked questions
What is a split-type heat pump?
It is a system in which refrigerant circulates through pipes between the outdoor and indoor units, so part of it is located inside the house. In a monobloc, the entire refrigerant circuit is factory-sealed inside a unit located outdoors, and only water enters the house.
Why does a monobloc require fewer formalities than a split system?
EU regulations on refrigerants set a lower quantity threshold without additional formalities for split installations because any leak would occur inside the house. For a monobloc, where a leak would occur outside the building, the same threshold is twice as high, which in practice means less bureaucracy for typical installations.
Will R32 heat pumps disappear from the market from 2027?
From January 1, 2027, new heat pumps using refrigerants with a high global warming potential, including R32, may no longer be placed on the market in the power range typical of single-family homes. The market is already switching to natural refrigerants, primarily propane R290.
Are old radiators suitable for use with a heat pump?
Usually, yes, especially after the building has been thermally modernized. At a lower supply temperature, radiators release less heat per unit of time, but since the house loses less heat than before, the same radiator surface is usually sufficient to maintain comfort.
What is the difference between COP and SCOP?
COP is the efficiency measured at one specific operating point, such as a particular air and water temperature. SCOP averages efficiency over the entire heating season, taking changing weather into account, so it better reflects actual annual energy consumption.
Why does a heat pump short-cycle, switching on and off at short intervals?
The most common cause is insufficient water flow through the installation for the unit's capacity, often resulting from a lack of an additional circulation pump supporting the factory-installed pump in the unit, or from a heat buffer that is too small to stabilize the system's operation.
