Monobloc vs Split Air-Source Heat Pumps: What Changes at Home?

The short answer: where the refrigerant circuit sits

Both configurations are air-to-water heat pumps. Each takes heat from outdoor air and delivers it as warm water to your wet heating system (radiators or underfloor heating) and, usually, a hot water cylinder. The difference is where the refrigerant circuit ends and the water circuit begins.

• Monobloc: the main components, including the refrigerant circuit, sit together in one outdoor unit. Water pipes carry heat from that unit into the house. Indoors you connect to your existing heating and hot water arrangement, typically a cylinder and controls.
• Split: the components are divided between an outdoor unit and an indoor hydronic unit. Refrigerant lines run between the two, and the water circuit stays inside the building.

In practical terms, a monobloc moves the water connection outdoors, so insulated water pipes cross your wall. A split moves the refrigerant connection across your wall, and you give up some indoor space to the hydronic unit.

That single shift explains most of what follows: pipework, freeze protection, who can do the work, indoor space and servicing. Neither is universally better. The right choice depends on your building and where the equipment can sensibly go, and details such as maximum pipe run lengths, specific requirements and product availability must be confirmed with installers and manufacturers.

What goes outside and what goes inside

Here is the physical footprint of each option, side by side.

Outdoors

A monobloc is a single outdoor package, so it tends to be the larger outdoor unit, and its position matters for two reasons: the water pipes to the house should be kept as short and well insulated as practical, and the unit needs clear airflow and sensible noise siting. A split's outdoor unit is typically a different shape and size, and its placement is constrained by how far refrigerant lines can run. Ask the manufacturer for the permitted distance and height difference for the exact model rather than relying on general rules of thumb.

Indoors

This is where the two diverge most visibly. With a monobloc, you do not need an indoor heat pump unit, which can free up space in a small home. You still need somewhere for the cylinder, pumps, valves and controls, so the saving is a module, not the whole plant room. With a split, the indoor hydronic unit takes wall or floor space next to or near the cylinder. Some are designed to sit beside a cylinder, and some are combined with one, but dimensions vary by product, so ask for the actual footprint and service clearances.

If your cylinder cupboard is already tight, a monobloc may be easier to fit. If you have a utility room or garage with space, a split's indoor unit may not be a problem at all.

Pipework: water through the wall or refrigerant through the wall

The pipes between the outdoor unit and the house are the most important practical difference.

Monobloc: insulated water pipes

A monobloc sends heated water outdoors-to-indoors through a flow and return pair. Because those pipes carry water through outside air, three things matter:

• Insulation quality. Poorly insulated outdoor pipes lose heat, which wastes energy, so insulation specification and protection from damage are worth asking about.
• Run length. A longer run between the unit and the house means more pipe to insulate and more heat to lose. Many installers aim to keep it short, but the acceptable distance depends on the product and design. Ask for the manufacturer's limit and the installer's estimate of heat loss on your route.
• Routing. Buried or underground runs, trenching, and the entry point into the house all add work. Ask where the pipes will run and how they will be protected.

Split: refrigerant lines

A split uses sealed refrigerant lines between the outdoor unit and the indoor hydronic unit. These are typically smaller than water pipes, which can make routing through a wall or along a facade neater. But they must be installed to the manufacturer's specification, and maximum length and height difference limits apply. Check those limits against your actual layout, for example if the outdoor unit would need to sit at the far end of a garden or far from where the indoor unit would go.

The practical question

If the best place for the outdoor unit is very close to the house and the indoor space is tight, a monobloc's short water run is usually straightforward. If the outdoor unit must sit further away, the split's refrigerant lines may be easier to extend, but only within the manufacturer's stated limits. Neither option removes the need to confirm the run with the installer.

Freeze risk: glycol, drain-down and refrigerant exposure

Freeze risk is where the two designs behave most differently, and it is a topic to raise early with any installer.

Monobloc

Because water circulates through pipes and a heat exchanger located outdoors, there is a freezing risk if the system loses power, a pump stops or the water stands still in cold weather. Installers generally handle this in one of two ways:

• Antifreeze (glycol) in the circuit. This protects against freezing, but it changes the fluid in the system, so ask how it affects pumping and heat transfer, and whether it will be checked or topped up over time. Online discussions from owners in colder climates suggest stronger glycol mixes can be a concern for efficiency. Treat that as an anecdote and ask the installer or manufacturer how it applies to your product and climate.
• Other frost protection measures. Some designs rely on controls, trace heating, pipe routing or the ability to drain down. Which is used depends on the product and installer, so ask exactly what protects the system during a power cut in cold weather.

Split

In a split, the outdoor connection carries refrigerant rather than water, so the water circuit stays inside the heated building. That moves the freezing concern away from the outdoor pipes. It does not remove cold-weather considerations altogether: the indoor water circuit still needs to be properly designed, and the outdoor unit still has to cope with winter operation, as with any air-source heat pump.

Ask every installer: What happens to my system if there is a power cut on a freezing night, and what do I need to do or check each year?

The answer matters more than the label. A well-designed monobloc with proper freeze protection can be perfectly sensible, and a split still needs a competent design.

Installation, competence and servicing

Who can do the work

Split systems involve connecting refrigerant lines between two units, so the installation typically includes refrigerant handling, which needs appropriate competence. Monobloc units usually arrive with the refrigerant circuit sealed in the outdoor unit, so the installation is mainly plumbing and electrical work. This is one reason monobloc installation is often described as simpler and sometimes less expensive, though cost depends on the property and the quote.

Rules on refrigerant work and qualifications vary by country and change over time. Rather than assume, ask each installer what qualifications and registrations cover the work on the specific product, and confirm requirements with the relevant authority or the manufacturer.

Disruption

• Monobloc: disruption is mostly outdoors (the unit base, the pipe route, the wall entry) plus indoor connection work on the cylinder and heating system.
• Split: work is spread across both the outdoor unit and the indoor hydronic unit, with lines routed between them, and refrigerant commissioning on top of the usual hydraulic work.

If you have decorated, fitted or finished spaces near the planned route, ask where cores, trenches and surface-mounted runs would go.

Commissioning

Both need proper commissioning: flow rates, controls and settings should be checked, and you should receive documentation and a handover. For a split, ask also how the refrigerant circuit was tested. For a monobloc, ask about the freeze protection setup and the water quality or glycol concentration.

Servicing

In a monobloc, most of the machinery is outdoors, so access to the unit matters, and servicing the refrigerant side may mean specialist work on the outdoor unit. In a split, components are spread across two locations, and some can be reached indoors, which some owners find convenient. Ask each installer who will service the system, whether they can service both the outdoor and indoor parts, and what the annual check includes. Avoid assuming either option is cheaper to maintain without a quote.

Choosing for your property: a decision framework

Use this as a guide, not a rulebook. Each row describes a tendency; a good installer may find a sensible solution that goes against it.

Supporting factors apply to both: a well-insulated home helps any heat pump perform well, and the choice between the two depends on building conditions, installation requirements and heating needs rather than on one configuration being universally superior. Efficiency differences between specific models and the effect of pipework heat loss or refrigerant line length should come from product data and the installer's design, not general rules.

If you want to see how the choice interacts with your home's heating demand, running costs and payback, the calculator on this site can help you compare options using your own property details. It does not replace installer surveys or quotes.

Questions to take to installers

Ask for the same answers from every installer so you can compare like for like, whether they propose a monobloc or a split.

1. Why did you recommend this configuration for my property rather than the other?
2. Where exactly will the outdoor unit go, and how will noise and airflow be handled?
3. What is the pipe or line run, what is the manufacturer's limit, and how is it insulated and protected?
4. What indoor equipment is needed, and what are its dimensions and service clearances?
5. Can my existing cylinder be reused, or does it need replacing?
6. How is the system protected against freezing, including during a power cut? If glycol is used, what concentration and what ongoing checks?
7. Who carries out the refrigerant work, and what qualifications or registrations do they hold?
8. What will be disrupted indoors and outdoors, and how will surfaces be reinstated?
9. Who will service it, what does the annual check include, and what parts of the system are accessible?
10. What does the commissioning handover include?

Fast answers to common questions

Does a monobloc need an indoor unit? Not an indoor heat pump unit, but it still connects to a cylinder, controls and heating circuit indoors.

Which suits a small home? Often a monobloc if indoor space is the main constraint, but check the outdoor siting and water run first.

Can any heating engineer install a split? Not necessarily. Refrigerant work typically needs appropriate competence, so confirm qualifications with the installer and the relevant authority.

How far can the outdoor unit be from the house? It depends on the product. Ask for the manufacturer's limits for your exact model.