Heat Pumps: Monoblock or Split?

A Comparison of Benefits and Drawbacks

When purchasing a heat pump, building owners have the option of choosing between a monoblock system (a single appliance) or a two-part split version. Each system comes with its own set of benefits and drawbacks, particularly regarding installation complexity and space requirements. Deciding between a monoblock or split heat pump can be challenging, as the choice depends on specific circumstances. Here, we outline the key factors to consider when making this decision.

Air-to-Water Heat Pump: Monoblock or Split

Before diving into the specifics of different designs, it's essential to understand how a heat pump works. Regardless of the type, heat pumps are highly efficient heating systems that extract free environmental heat while consuming a relatively small amount of electricity.

A heat pump operates like a reverse refrigerator, extracting thermal energy from the surrounding environment and transferring it to the building's heating system. This energy can be used for domestic hot water (DHW) heating and/or as backup for central heating. In residential applications, common energy sources include ambient air, groundwater, and geothermal heat.

Due to the higher initial costs of water-to-water and brine-to-water heat pumps (geothermal heat pumps), many building owners prefer air-to-water heat pumps. These systems offer several advantages, especially in terms of flexible installation. This flexibility can be particularly important when renovating older buildings.

Advantages of Air-to-Water Monoblock Heat Pumps

Monoblock heat pumps refer to air-to-water heat pumps in which all components are integrated into a single, compact unit. This design ensures that the entire refrigerant cycle occurs within the appliance itself. One of the key benefits of this setup is that it simplifies installation, as fitters do not need a refrigeration certificate, which is typically required for installing systems with external refrigerant piping.

When refrigeration piping is involved, only qualified refrigeration technicians can perform the work to avoid the risk of harmful refrigerant gases escaping into the environment. As a result, the installation and maintenance of monoblock heat pumps tend to be less complex and more cost-effective compared to other types. Additionally, the design of monoblock heat pumps allows for versatile installation, as they can be installed either indoors or outdoors.

Operating Principle of Monoblock Heat Pumps

Like all heat pumps, monoblock heat pumps operate by absorbing thermal energy from the ambient medium, specifically the air. This process begins with the heat exchanger located in the outdoor coil block, where thermal energy is transferred to the refrigerant. The compressor then converts the refrigerant from a liquid to a gaseous state.

As the refrigerant changes state, it is able to absorb additional thermal energy, leading to an increase in both its pressure and temperature. In the next phase, the condenser or indoor coil block transfers the heat from the refrigerant to the heating circuit of the heating system or to a buffer cylinder. This cooling process causes the refrigerant to revert to its liquid state, allowing the cycle to begin anew.

Benefits and Drawbacks of Monoblock Heat Pumps

A monoblock heat pump features the entire refrigerant circuit housed within a single unit, regardless of whether it is installed indoors or outdoors. If you choose an indoor installation, you will need to incorporate air ducts to facilitate the supply and discharge of ambient air. In contrast, an outdoor installation requires the routing of connection lines through the walls to link with the heating system. The most appropriate installation type largely depends on the available space within the building. Even for outdoor setups, you must account for the space required for the associated indoor unit, which is considerably smaller than an indoor monoblock heat pump.

In new constructions, the space needed for indoor installation can be planned from the beginning. Conversely, outdoor installations are particularly advantageous for retrofitting in existing buildings. However, indoor installations remain feasible in older structures if there is adequate space, such as in a cellar, or if an oil tank is removed during a heating system upgrade.

Overall, the installation costs for monoblock air-to-water heat pumps tend to be lower since neither installation nor maintenance requires a refrigeration certificate. Nevertheless, the space and weight requirements of a monoblock heat pump can be relatively high, potentially complicating the search for an ideal installation site.

Benefits & Drawbacks of Monoblock Appliances

Drawbacks:

- Refrigerant circuit is completely contained within the monoblock unit.

- No refrigeration certificate is required for installation and maintenance.

- Suitable for both indoor and outdoor installations.

- Lower installation costs compared to other systems.

Benefits:

- Relatively heavy; requires consideration of load capacity at the installation site.

- Larger than comparable split heat pumps.

- More challenging to place due to higher space requirements.

- Noise emissions from the built-in fan.

What Are the Advantages of Split Air-to-Water Heat Pumps?

The working principle of split heat pumps is identical to that of monoblock appliances. The key difference lies in the refrigerant circuit, which is divided into a compact indoor unit and an outdoor unit. These systems often include a buffer cylinder for domestic hot water (DHW) heating or central heating backup. This configuration significantly reduces the space requirements in the boiler room, as the indoor unit occupies much less space compared to a monoblock version installed indoors.

Operating Principle of a Split Heat Pump

In split designs, the refrigerant circuit of the air-to-water heat pump is divided into two appliance units. The actual heat generation occurs in the outdoor unit, which houses the outdoor coil block and compressor. The indoor unit, by contrast, contains the indoor coil block.  

Both components are connected by a hot gas line system. The refrigerant circulates through an additional second line. After being heated and changing to a gaseous state, the thermal energy is transferred to the heating system. The refrigerant is then routed back to the outdoor unit in liquid form, and the process begins anew.

Benefits and Drawbacks of Split Heat Pumps

There are several advantages to the appliance components being divided into an indoor unit and an outdoor unit. The outdoor unit is lighter and more compact than a monoblock heat pump with comparable output. Additionally, the noise level inside the house is lower when the compressor is located outdoors.  

Inside the building, the space requirements for a split heat pump are less than those for the indoor installation of a complete monoblock appliance. However, split appliances cannot be installed entirely outside or entirely inside. Consequently, these heat pumps are mainly installed where available space necessitates a compromise between indoor and outdoor placement.  

One drawback is that installation and maintenance are more complex and costly, as technicians working on the refrigerant line require a refrigeration certificate. If the amount of refrigerant in the appliance exceeds the applicable reference value for the CO2 equivalent, building owners may be subject to an obligatory refrigerant test, resulting in additional costs each year.

Benefits and Drawbacks of a Split Appliance

Benefits  

- Compact and lightweight outdoor unit  

- No risk of system freezing  

- Low noise levels inside the house  

Drawbacks  

- Installation and maintenance require a refrigerant certificate  

- Annual refrigerant inspections may be necessary  

- Space needed for the indoor unit  

Which Is More Efficient: Monoblock or Split Heat Pumps?

Apart from concerns regarding space requirements, installation work, and the associated costs, the main issue when comparing monoblock and split heat pumps is efficiency. Are there differences in efficiency and energy yields between monoblock and split heat pumps? When examining comparable models from the same manufacturer, there is little to no difference in efficiency.

Monoblock or Split Heat Pumps: Size, Weight, Sound Levels

The efficiency of both types of appliances is similar, but they differ significantly in size—monoblock appliances typically have a larger footprint and weight. While monoblock heat pumps can be positioned more flexibly (indoor versus outdoor installation), their size and weight can slightly complicate planning and increase costs, depending on the available space on the property and inside the building.  

Regarding noise, there is not much difference between monoblock and split heat pumps, provided their performance levels are similar. Noise regulations in densely populated areas may prohibit outdoor installation, which can prevent the use of split appliances altogether. Conversely, a monoblock heat pump could be installed indoors.  

Direct price comparisons between the two heat pump types are not particularly useful, as they do not provide a complete picture. Monoblock and split heat pumps with comparable performance rates and energy efficiency have very different installation costs, which depend on the structural conditions on-site. Maintenance costs and other factors must also be taken into account.