Split, portable or monoblock air conditioner: the differences explained

If you want to cool a home office or bedroom, you have three main choices: a split system (indoor and outdoor unit), a portable air conditioner (on wheels, with an exhaust hose) and a monoblock (fixed to the wall, with no outdoor unit). They differ in price, but more importantly in efficiency, noise in the room and what you can legally fit yourself.

The three types at a glance

Split system

The compressor – the noisiest and hottest part – sits outside, so indoors you only hear the fan. Inverter-driven splits modulate their output smoothly, which gives high seasonal efficiency. The energy label for this type runs from A+++ to D and shows cooling (SEER) and heating (SCOP) separately ^1,2. If you want more than a few days of relief a year, or plan to use the unit for heating as well, a split is usually the answer.

The downsides: you need a qualified installer, space for an outdoor unit on a wall, balcony or the ground, and the upfront cost is higher. A multi-split connects several indoor units to one outdoor unit – useful if there is only room for one unit outside.

Portable air conditioner

Wheel it in, plug it in and hang the hose out of the window – which makes it popular with renters and for the odd heatwave. Its label is based on a single test point (EER) rather than a whole season, and the scale is not comparable with split systems ². An “A” on a portable means something different from an “A” on a split.

There is also a physical catch. A single-hose (single duct) unit blows room air outside, and that air is replaced by warm outdoor air leaking in through gaps and the part-open window. Dual-hose (double duct) units suffer less from this. Because the compressor is in the room, portables are also clearly audible – worth considering for a bedroom.

Monoblock with no outdoor unit

A monoblock hangs on the inside of an external wall and moves outdoor air through two round holes. There is no outdoor unit, which helps if a freeholder, landlord or conservation area rules one out. The refrigerant circuit is sealed at the factory, so fitting it involves no refrigerant work. For labelling it counts as a double-duct appliance ²; efficiency and noise typically sit between a portable and a split.

Refrigerant and F-gas rules

Many split systems use R32, a fluorinated greenhouse gas. In Great Britain, anyone installing, servicing or decommissioning equipment containing F-gas needs their own F-gas qualification, even if they work for a company ³. Category 1 covers all refrigeration, air conditioning and heat pump work; Category 2 covers systems with less than 3 kg of F-gas ³.

The EU is phasing out high-GWP refrigerants in new split systems during the 2020s and 2030s; Great Britain has its own F-gas regime, so those EU dates do not automatically apply here. Either way, it is worth asking which refrigerant a model uses – units running on propane (R290) are the more future-proof choice.

Outdoor unit: noise and planning

In England, since 29 May 2025, permitted development rights for air source heat pumps also cover units that can cool, so air-to-air heat pumps – in practice most reversible split air conditioners – can benefit from them ⁴. The installation must comply with the MCS 020(a) noise standard, the 1-metre-from-the-boundary rule has been removed, and the maximum unit size for houses is 1.5 m³ ⁴. Scotland, Wales and Northern Ireland have separate rules ⁴. Listed buildings and flats generally need more checks, so confirm with your local planning authority before you commit.

Which should you choose?

• A few hot nights a year, rented home: a portable, ideally dual-hose. Accept the noise in the room.
• No outdoor unit possible or allowed: a monoblock.
• Regular cooling, heating too, as quiet as possible: a split system. Ask the installer for the product fiche with SEER, SCOP and indoor and outdoor sound power levels.

If you are replacing fossil-fuel or electric heating in England or Wales, an air-to-air heat pump fitted by an MCS-certified installer may qualify for a £2,500 Boiler Upgrade Scheme grant ⁵. See our guide to heating with air conditioning, and compare air-to-water options in our heat pump overview.