Monobloc and Split Heat Pumps: The Difference

A monobloc heat pump keeps every part that touches refrigerant, including the compressor and the heat exchanger, inside one sealed outdoor unit, and only water pipes cross the wall into your home. A split heat pump divides that machinery: a compact outdoor unit and an indoor unit, joined on site by refrigerant lines that have to be charged and connected by an F-Gas qualified installer¹. That single difference drives almost everything else: who can fit the system, what it costs, how much space it takes indoors, and how it is serviced.

Both are air source heat pumps in the ordinary sense: they absorb heat from the outside air and deliver it into your home. Energy Saving Trust describes the split arrangement plainly: "A split system separates the heat pump into indoor and outdoor units."³ The monobloc arrangement instead has, in the words of one independent guide, "all the workings in the outdoor unit, so you[r installer] only needs to make water connections"⁴. In the UK the market has chosen overwhelmingly for the monobloc: industry data shows "the dominance of Air-to-Water Monobloc units in the UK market"⁵, and official modelling looks ahead to a stock of 80 to 87% monobloc against 13 to 20% split by 2050, depending on the scenario⁶.

Monobloc: a single sealed unit outside with water pipework indoors

A monobloc air source heat pump is a single self-contained appliance. Dimplex, a maker that sells both architectures, puts it simply: "the monobloc heat pump has all the necessary components for an air source heat pump contained within one single unit"². The refrigerant circulates inside that unit in what the maker describes as "a closed-circuit system for the refrigerant"², and the only connections through your wall are water pipes carrying heated water to radiators, underfloor heating or a cylinder.

Because the refrigerant circuit never enters the house, installation is "more straightforward than that of a split system air source heat pump as the connections are to the water supply of the heating system"². A real installation bears this out: one Energy Saving Trust case study describes a family's system as "a monobloc system that combines all components in one unit and did not require an indoor unit installed"⁹. Nothing more than the outdoor box, its water connections and the usual indoor heating hardware, such as a hot water cylinder, is needed.

The trade-off is size and weight outdoors. "The footprint of the monobloc unit is larger than that of the outdoor unit of a split system air source heat pump"², because everything has to fit in one cabinet. Where outdoor space is tight, that matters. Where indoor plant space is tight, it works in the monobloc's favour, since the only indoor equipment is the cylinder and any buffer, not heat pump machinery.

One point worth being clear about: a monobloc installed outdoors still needs nothing indoors except the wet-side components. If a monobloc were installed indoors instead, which is unusual in UK homes, it would need "air ducts in the building to supply and discharge ambient air"¹. For the typical British installation, the unit sits outside on a wall or plinth and the house only sees water pipes.

Split: paired indoor and outdoor units linked by refrigerant

A split system splits the refrigerant circuit itself. Wolf, a German maker that publishes detailed guidance on the two architectures, explains: "The notable difference is that the refrigerant circuit is divided into a compact indoor unit and an outdoor unit" connected by a hot gas line¹. The outdoor unit holds the compressor and the fan; the indoor unit holds the heat exchanger that transfers heat into your water system, and the two are joined by refrigerant pipework that is charged on site.

That on-site refrigerant work is the split system's defining practical consequence. "Installation and maintenance are more complex and costly as technicians working on the refrigerant [circuit] require certification"¹. Nesta's work on reducing heat pump costs makes the same point from the independent side: "Splits have higher installation costs due to refrigerant charging which requires an F-Gas Certified installer."⁸

The split layout brings its own advantages. "The outdoor unit is much lighter and more compact than a monoblock heat pump with a comparable output"¹, which helps where the outdoor position is constrained, and the indoor units of split systems "often also already contain a buffer cylinder for DHW heating or central heating backup"¹, combining roles in one indoor cabinet. Wolf also notes that "the noise level in the house is lower if the compressor is situated outdoors"¹, though in most UK split installations the compressor is in the outdoor unit anyway.

A split cannot be placed entirely outside or entirely inside: "Split appliances cannot be installed entirely outside or entirely inside"¹. The architecture demands both halves, one outdoors and one in, with refrigerant lines between them. That is a genuine constraint on siting, and it is not negotiable by buying a different model.

How each setup affects your property

The two architectures ask different things of your house. Indoors, the split's indoor unit is compact: "Inside the building, space requirements are lower for a split heat pump than for the indoor installation of a complete m[onoblock]"¹. But the split still needs somewhere for that indoor unit, plus a route for refrigerant lines between it and the outdoor position, which can mean a longer pipe run through the building fabric. The monobloc needs no indoor machinery space at all, only the cylinder and any buffer tank, but its single outdoor cabinet is larger and heavier than a split's outdoor unit of comparable output¹.

Outdoors, the monobloc's bigger footprint and the split's need for a suitable indoor position both shape where the unit can go. Noise rules can be the deciding factor in tight urban sites: "Noise regulations in densely built-up areas can preclude outdoor installation, preventing split appliances from being used"¹, since a split cannot work without its outdoor half. A monobloc faces the same outdoor noise rules, covered by the MCS 020 noise assessment, but it has no indoor half to fall back on either; the assessment of a single heat pump includes its "heat pump installation location" as a project detail¹⁰.

For hot water, both architectures normally work with a cylinder, and the split's indoor unit may already include a buffer for hot water or heating backup¹. Neither architecture removes the need to think about heat pump sizing and emitters; those follow the house, not the configuration. Official modelling of new homes assumes heat pump installations from 2025 of "which all are monoblocs"⁶, which tells you where the UK housebuilding market has settled, but retrofit homes with awkward outdoor access or generous plant rooms are exactly where splits keep a place.

Fitting and the required qualifications

The certification question has a clean answer. For a monobloc, "all refrigerant is sealed within the outdoor unit, so installers do not require F-gas qualifications"⁷. Wolf's maker guidance agrees: "fitters do not need a refrigeration certificate (generally only held by qualified refrigeration technicians)"¹. A competent heating engineer can make the water connections, commission the unit and maintain it without refrigeration training.

For a split, the position reverses. Refrigerant charging requires an F-Gas Certified installer⁸, and the same rule applies to air-to-air heat pumps and air conditioning generally: "They must be installed by an F-Gas certified installer."¹¹ Any engineer breaking into the refrigerant lines, at installation or at any later service, must hold that certification.

Two qualifications sit above both architectures. First, for grant purposes, the installation must be by an MCS-certified installer; independent guidance is blunt: "Choose an MCS-certified heat pump installer"¹². Second, the noise assessment that planning rules in England can require is part of the standard installation process for a single heat pump¹⁰. Neither of these differs between monobloc and split: they are about the installation being competent and lawful, not about the refrigerant architecture.

Siting, sound levels and frost risk

On noise, the honest answer is that the two architectures are much of a muchness. Dimplex states: "The overall noise levels of monobloc and split system air source heat pumps are similar"². Wolf agrees that "there is not much difference between monoblock and split heat pumps as long as their performance is s[imilar]"¹. The noise you hear outdoors comes from the fan and compressor, and a monobloc's "noise emissions from built-in fan"¹ are matched by the equivalent parts of a split's outdoor unit. What differs is indoors: a split's indoor unit can add some indoor noise, while Wolf notes the in-house noise level is lower when the compressor sits outdoors¹, which is where it normally sits in a UK split.

Freezing is where the two genuinely diverge. A split's indoor circuit runs refrigerant, which does not freeze in UK conditions: Wolf lists "No danger of system freezing" among the split's advantages¹. A monobloc's outdoor unit contains water-side components, the heat exchanger and pipework carrying water in and out, and those need freeze protection in cold weather, typically a small electric heater or antifreeze in the water circuit, so that a power cut in a hard frost does not split pipes in the outdoor cabinet. This is a routine engineering detail, not a defect, but it is a real difference in what the hardware must handle through a British winter, and it bears on cold weather performance.

On siting, each architecture has one fixed requirement. A monobloc needs enough outdoor room for a larger, heavier cabinet². A split needs an indoor position for its unit plus a pipe route, and cannot be installed entirely outside or entirely inside¹. In densely built-up areas, noise regulations can preclude outdoor installation altogether, which rules the split out rather than in¹. The right question is not which is better but which constraint your site can absorb; the siting and clearances rules apply to both.

Prices: initial outlay and running costs

The monobloc is generally the cheaper installation. The connections are to the water side of the heating system, which is more straightforward², and no F-Gas certified refrigeration work is needed, which removes the cost that Nesta identifies: "Splits have higher installation costs due to refrigerant charging which requires an F-Gas Certified installer."⁸ Wolf is blunter still: split "installation and maintenance are more complex and costly"¹.

For actual figures, the published cost evidence is strongest on hybrids, and it should be read as such. Official guidance from 2016 put the total additional installed cost of a hybrid heat pump over a gas condensing boiler at "£4,000 to £10,000"¹³. Nesta notes that hybrid upfront costs "will be lower than for a heat pump due to less need for supplementary work such as upgrading radiators, new pipework and a water cylinder"¹⁴, and IGEM cites evidence that "hybrid systems are 30 to 40% cheaper to install than standalone heat pumps"¹⁵. On running costs, Cadent's guidance models a hybrid at "c.10% more compared to a standalone heat pump" at the current spark gap¹⁶. These figures describe hybrid systems, not the monobloc versus split choice, but they frame the honest position on heat pump costs generally: appliance averages from manufacturers are "for the appliance only, not including installation"¹⁷, while official cost modelling includes "one-off installation costs including some home retrofit costs such as replacing radiators"⁶. Published monobloc versus split price differences are not available; prices are installer-quoted, and the full cost picture is set out separately.

Afterwards, the split carries a recurring servicing obligation that the monobloc does not: the refrigerant lines "need to be checked for leaks on a yearly basis"², and where the refrigerant charge exceeds its CO2-equivalent reference value, "an obligatory refrigerant test may apply annually"¹. Annual servicing is wise for both, and hot-water-only heat pumps are "required annually, including for warranties and checking refrigerant"¹¹, but the split's leak check is a specific, recurring cost line the monobloc avoids.

F-gas regulations and refrigerants changing split designs

The refrigerant itself does not differ between the architectures: "The same refrigerants can be used in both monobloc and split system air source heat pumps"². What differs is the regulation that follows the refrigerant, and it is the split systems that the rules are reshaping, because their refrigerant is handled on site.

Under the bans on F-gases in new products, certain single split systems enjoy exceptions that larger systems do not: the exceptions cover, among others, "air to water single split system air-conditioning systems"¹⁸. But the direction of travel is restrictive. From 1 January 2029, only refrigerants with a GWP below 150 may be placed on the market for split systems up to 12kW, and only refrigerants with a GWP below 750 for split systems over 12kW¹⁸. From 1 January 2030, other self-contained heat pumps and split air-to-water heat pumps up to and including 12kW face their own placing-on-the-market restriction¹⁸. From 1 January 2033, F-gases with a GWP of 750 or more are restricted in split systems above 12kW, and from 1 January 2035, split systems up to and including 12kW face a prohibition on any F-gas¹⁸.

The practical effect is that split systems are being pushed towards the same low-GWP refrigerants, like R290 and R32, that monoblocs already use, a shift covered in detail on heat pump refrigerants. A monobloc's sealed factory charge sidesteps the on-site handling rules entirely, which is part of why official modelling expects the monobloc share of the heat pump stock to reach 80 to 87% by 2050⁶.

UK manufacturers offering monobloc and split units

The UK market is dominated by air-to-water monobloc units⁵, and the product ranges reflect that. HPA UK's sales data counts factory-gate sales of air-to-water monobloc units, air-to-water split units, ground and water source units, domestic hot water heat pumps and "other" categories including exhaust air and integrated hybrid heat pumps⁵, which gives a sense of how the industry itself divides the market.

LG sells the Therma V R290 Monobloc range in the UK through LG UK, across one and three phase variants, each described as "a monobloc concept, it is connected by only water piping eliminating the need for refrigerant piping"²¹.

Ideal Heating offers the Logic Air range, described as "Ideal Heating's new range of monobloc heat pumps"²⁷.

InstaGen lists a family of monobloc systems across its output range, all identified as monobloc systems²⁸.

Vaillant sells the aroTHERM split air source heat pump, an MCS accredited split model³³, alongside its wider Vaillant heat pump range.

Daikin offers multi-split air conditioning products with heat pump function, with one outdoor unit listed with a cooling capacity of 5.2 to 9.0 kW, a heating capacity of 6.8 to 10.0 kW, R-32 refrigerant, a maximum piping length of up to 75 m, dimensions of 734 x 974 to 1044 x 401 to 408 mm and a weight of 60 to 73 kg³⁴.

Viessmann has announced a new residential air source heat pump, described as "the German-engineered monobloc heat pump... optimised for UK installations"³⁵.

Household self-sufficiency and your system choice

Both architectures deliver the same core independence: heat drawn from the air outside rather than gas bought from a network, and a home that can be heated by electricity from any source, including your own solar and battery. Neither architecture frees you from the electricity grid; both run on mains power, and both depend on an installer and a manufacturer for servicing and parts. The differences are in the dependencies they add.

A monobloc concentrates everything in one sealed box: one appliance, one warranty, one outdoor unit to maintain, and no refrigerant work ever needed on site. Its dependencies are outdoor space and freeze protection through power cuts. A split spreads the system across two units and a refrigerant circuit that runs through your house's fabric, and it binds you to F-Gas qualified engineers for installation and every refrigerant-side intervention, plus an annual leak check¹. Against that, it offers a lighter outdoor unit, a compact indoor unit that may double as a buffer, and no water-side freezing risk in the outdoor cabinet¹.

The efficiency answer, which is the one most households actually want, is that there is nothing in it: "barely any difference" between comparable models from the same maker¹. The choice is therefore not about performance but about your house: its outdoor space, its indoor plant space, its pipe routes, its noise constraints, and the installers available to you. For most UK homes the market has already answered, with monobloc units dominant in sales⁵ and expected to hold 80 to 87% of the long-term stock⁶. Splits remain the right architecture where their specific advantages fit, and the F-gas timetable will keep reshaping what they contain¹⁸.