What Is the Difference Between a Split and Monobloc Heat Pump?

Air-to-Water Heat Pump: Monobloc or Split

Before comparing monobloc and split system designs, it's important to understand how a heat pump works. All heat pumps operate on the same basic principle: they extract free thermal energy from the environment and use a small amount of electricity to convert it into usable heat.

Functioning like a reversed refrigerator, a heat pump captures heat from the surrounding air, water, or ground and transfers it to your home's heating system. This energy can then be used for domestic hot water (DHW) production and/or central heating.

In residential settings, the most common sources of environmental heat are ambient air, groundwater, and geothermal energy. While water-to-water and brine-to-water (geothermal) systems offer high efficiency, their higher upfront costs make air-to-water heat pumps a more popular choice—especially for renovations or properties where flexible installation is a priority.

Monobloc Heat Pumps

A monobloc heat pump is similar to a boiler in that it is a 'one block' system, with all of its components (except the hot water cylinder which is inside the home) inside a single unit that sits outside the home.

Monobloc heat pumps differ from split systems in that all components of the refrigeration cycle are housed within the outdoor unit. This means only water connections are required at the rear of the unit. A wiring centre, located either inside the house or within the outdoor unit, is used to connect pumps and other electrical components.

Benefits of Monobloc

- Easy install

- All in one box

- Easy maintenance

Split Heat Pumps

Unlike monobloc systems, split heat pumps consist of two separate units: an outdoor unit containing the heat exchanger and refrigerant, and an indoor unit that includes circulation pumps, heat exchangers, and sometimes a built-in hot water cylinder.

Benefits of Split Systems

- Can be installed further away from the property (up to 75 meters)

- No glycol / anti-freeze required

- Smaller and quieter

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

Air-to-water monobloc heat pumps house all components of the heating system—including the entire refrigerant cycle—within a single, compact outdoor unit. Because the refrigerant system is self-contained, installation does not require a refrigeration certificate, which is typically only held by certified refrigeration technicians.

Unlike systems that involve on-site refrigerant piping, monobloc units reduce the risk of harmful refrigerant leaks into the environment. As a result, installation and maintenance are generally simpler and more cost-effective than with split systems. Monobloc heat pumps can also be installed either outdoors or indoors, depending on the property layout.

How Do Monobloc Heat Pumps Work?

Like all heat pumps, monobloc systems extract thermal energy from the surrounding air. This heat is absorbed by a heat exchanger in the outdoor coil, where it's transferred to the refrigerant.

The refrigerant is then compressed, changing from liquid to gas and increasing in pressure and temperature. The heat is transferred from the refrigerant to the home's heating circuit or buffer tank via a condenser. As the refrigerant cools, it returns to its liquid state, and the cycle repeats.

Benefits and Drawbacks of Monobloc Heat Pumps

Monobloc heat pumps house the entire refrigerant circuit within a single unit, whether installed indoors or outdoors. For indoor installations, air ducts must be added to supply and exhaust ambient air. In contrast, outdoor installations require connection pipes to run through the external walls to link the unit with the heating system. The choice between indoor and outdoor installation typically depends on the available space.

Even with outdoor installation, space must be allocated indoors for a small associated unit. However, this indoor component is significantly more compact than a full indoor monobloc system.

In new builds, indoor space for a monobloc heat pump can be planned from the beginning. For existing homes, outdoor installation is often the more practical choice—especially when retrofitting. Indoor installation is still an option in older buildings, provided there's enough room, such as in a basement or a former oil tank space during heating system upgrades.

One of the key advantages of monobloc air-to-water heat pumps is lower installation and maintenance costs, as no refrigeration certification is required. However, they tend to be larger and heavier than other types, which may make locating a suitable installation site more challenging.

Benefits & Drawbacks of Monoblock Appliances

Benefits

- Refrigerant circuit completely contained in monoblock

- No refrigeration certificate required for installation and maintenance

- Suitable for both indoor and outdoor installation

- Lower installation costs

Drawbacks

- Relatively heavy (consider load capacity at the installation site)

- Larger than comparable split heat pumps

- More difficult to place due to higher space requirements

- Noise emissions from built-in fan

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

Split air-to-water heat pumps operate on the same principle as monobloc systems but divide the refrigerant circuit between two separate units—one installed outdoors and one indoors. Many split systems also come with an integrated buffer cylinder for domestic hot water (DHW) or central heating support. This setup significantly reduces space requirements in the boiler room, as the indoor unit is typically much more compact than an indoor monobloc unit.

How Split Heat Pumps Work

In a split system, heat generation occurs in the outdoor unit, which houses the evaporator and compressor. The indoor unit contains the secondary components, such as the indoor heat exchanger (coil block) and sometimes a second compressor.

The two units are connected by a set of refrigerant lines—one carrying the hot gas and the other returning the cooled refrigerant in liquid form. As the refrigerant is compressed and changes to a gas, it absorbs heat and transfers it to the indoor unit. There, the heat is released into the home's heating system. The refrigerant then returns to the outdoor unit to repeat the cycle.

Benefits and Drawbacks of Split Heat Pumps

Split heat pumps offer several key advantages due to the separation of components between an indoor and an outdoor unit. The outdoor unit is typically lighter and more compact than a monobloc system with similar output, and placing the compressor outside helps reduce indoor noise levels.

These systems also take up less interior space compared to a full monobloc unit installed indoors. However, split systems cannot be installed entirely inside or outside, making them ideal for properties where a balance between indoor and outdoor space is needed.

On the downside, installation and maintenance are more complex and often more expensive. Any work involving the refrigerant circuit must be carried out by certified technicians, as it requires a refrigeration certificate. Additionally, if the refrigerant charge exceeds a specific CO₂ equivalent threshold, the homeowner may be legally required to schedule regular refrigerant inspections, leading to ongoing annual costs.

Benefits and Drawbacks of a Split Appliance

Benefits

- Compact and light outdoor unit

- No danger of system freezing

- Low noise levels inside house

Drawbacks

- Installation and maintenance require a refrigerant certificate

- Annual refrigerant inspections may be necessary

- Space required for indoor unit

Which Is More Efficient: Monoblock or Split Heat Pumps?

When comparing monoblock and split heat pumps, efficiency is often a primary concern. However, if you evaluate comparable models from the same manufacturer, there is typically little to no difference in energy efficiency or performance between the two.

Monoblock vs. Split: Size, Weight, and Noise Levels

While both systems offer similar efficiency, they differ significantly in physical characteristics. Monoblock units generally have a larger footprint and higher weight. Although they offer more flexibility in terms of placement—being suitable for both indoor and outdoor installation—their size can increase installation complexity and costs depending on the available space.

In terms of noise output, both monoblock and split systems are comparable when matched for performance. However, in densely populated areas with strict noise regulations, split systems may not be viable for outdoor installation. In such cases, a monoblock heat pump installed indoors could be a better solution.

It's important to avoid comparing prices alone. While two units might have similar output and efficiency ratings, the total cost can vary widely based on installation complexity, structural conditions, and long-term maintenance requirements.