The short answer
The difference is where the refrigerant circuit sits. In a monobloc heat pump, all the refrigeration components are sealed inside the outdoor unit, and only water pipes run into the house. In a split system, the refrigerant circuit is divided between the outdoor unit and an indoor unit, so refrigerant pipes run between them. Monobloc is simpler to install, the refrigerant is factory-sealed, and most installs do not need F-gas handling on site — which is why it is the most common type in the UK. Split systems can be slightly more efficient in some setups and put less heat-loss between unit and house, but they require an F-gas qualified engineer to connect the refrigerant lines and need freeze protection on the indoor pipework. Both qualify for the £7,500 Boiler Upgrade Scheme grant.
When choosing an air source heat pump you will see units described as monobloc or split. The distinction affects installation, maintenance and freeze protection rather than the basic principle. Here is what it means.
Monobloc vs split
- Monobloc refrigerantSealed in the outdoor unit
- Split refrigerantRuns between outdoor and indoor units
- Pipes into house (monobloc)Water only
- Pipes into house (split)Refrigerant lines
- F-gas engineer neededSplit: yes; monobloc: usually no
Where the refrigerant sits — the core difference
A monobloc heat pump packages the entire refrigeration cycle — compressor, heat exchangers and refrigerant — inside the single outdoor box. It produces hot water there and pumps that water indoors to the cylinder and heating circuit. Only water pipes cross into the house.
A split heat pump divides the refrigeration circuit. The outdoor unit holds the compressor and outdoor coil; a separate indoor unit (a hydrobox) contains the indoor heat exchanger. Refrigerant pipes connect the two. The water is heated indoors rather than outdoors.
This single design choice drives most of the practical differences below — installation method, who can do it, and how the system behaves in freezing weather.
| Factor | Monobloc | Split |
|---|---|---|
| Refrigerant location | Sealed in outdoor unit | Outdoor + indoor units |
| Pipes into the house | Water | Refrigerant |
| Install complexity | Simpler | More involved |
| F-gas engineer | Usually not needed | Required |
| Indoor space | Minimal (cylinder only) | Hydrobox plus cylinder |
| Freeze risk | Outdoor water pipes need protection | Indoor water side protected by location |
| BUS grant | £7,500 | £7,500 |
Indicative comparison for guidance. Sources: MCS; manufacturer technical guidance.
Installation, F-gas and maintenance
Monobloc is the simpler install. Because the refrigerant is factory-sealed in the outdoor unit, the installer only connects water pipes and electrics — no on-site refrigerant work is normally required, so a wider pool of installers can fit it. This simplicity is the main reason monobloc dominates the UK domestic market.
A split system needs an F-gas qualified engineer to connect, evacuate and charge the refrigerant lines between the outdoor and indoor units. That narrows who can install it and adds steps, but it gives a continuous refrigerant path into the house, which some manufacturers use to reduce heat losses between the unit and the cylinder.
Indoor space, noise and pipe runs
The two designs place equipment differently inside the home. A monobloc keeps all the refrigeration kit outdoors, so indoors you only need space for the hot water cylinder and the associated valves and controls. That makes monobloc tidy for homes with limited internal space, since the only sizeable indoor item is the cylinder.
A split system adds an indoor hydrobox containing the indoor heat exchanger, so you need wall or cupboard space for that unit in addition to the cylinder. The benefit is that water is heated indoors, which can reduce heat loss on the run between the unit and the cylinder, particularly where the outdoor unit is some distance from the plant room.
Pipe runs differ too. A monobloc runs insulated water pipes from the outdoor unit into the house, and the length of that run affects heat loss and freeze exposure, so it is kept as short and well-insulated as practical. A split runs refrigerant lines, which have their own length and height limits set by the manufacturer. Either way, a good installer plans the route to minimise losses and protect the pipework.
Efficiency, space and which to choose
On efficiency the two are broadly comparable, and a well-designed system of either type performs well. Split systems can have a small edge in some configurations because the water is heated indoors, reducing pipe heat loss; monobloc keeps everything outdoors, which simplifies the indoor footprint to just the cylinder. Real-world performance depends far more on correct sizing, low flow temperatures and good radiators than on the monobloc-versus-split choice.
For most UK homes, a monobloc is the default — simpler to install, fewer trades involved, and no on-site refrigerant handling. A split system can suit cases where indoor heat-loss reduction matters, where the run between outdoor unit and cylinder is long, or where a particular manufacturer's split range fits the project. Both need a heat loss survey and an MCS-certified design, and both qualify for the £7,500 grant.
Frequently asked questions
Which is more common in the UK, monobloc or split?
Monobloc is by far the more common type for UK homes. Because the refrigerant is sealed in the outdoor unit and only water pipes enter the house, it is simpler to install, needs no on-site refrigerant handling in most cases, and can be fitted by a wider range of installers.
Does a split heat pump need an F-gas engineer?
Yes. A split system has refrigerant pipes running between the outdoor and indoor units, and connecting, evacuating and charging those lines must be done by an F-gas qualified engineer. A monobloc, with its factory-sealed refrigerant, usually does not require on-site F-gas work.
Is a monobloc heat pump at risk of freezing?
A monobloc sends water through outdoor pipes, so those pipes must be protected against freezing — normally with glycol antifreeze or a control strategy that keeps water moving. This is a standard part of a correct monobloc installation. A competent MCS installer designs the freeze protection in, so it is not a reason to avoid monobloc.
Sources & further reading
- MCS — find an installer and heat pump guidance
- Energy Saving Trust — air source heat pumps
- Heat Geek — monobloc vs split explained
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.