Most people start by asking how much air conditioning costs. The more useful question is which system suits the property, because that decides both the price and whether you are happy with it in five years. This guide covers the four system types found in UK homes, how to size a unit properly, and which ratings actually matter.
The Four System Types
Wall-Mounted Split Systems
One indoor unit paired with one outdoor condenser. This is the default domestic installation and suits a bedroom, living room or home office. It is the cheapest fixed system to install, the quietest in normal use, and the easiest to service.
The trade-off is that each room needs its own indoor unit and its own outdoor unit, which becomes untidy and expensive once you want more than two rooms cooled.
Multi-Split Systems
Several indoor units run from a single outdoor condenser. This is usually the right answer for cooling three or more rooms in an existing house: one outdoor unit instead of four, individual temperature control per room, and a much better look on the exterior wall.
Installation is more involved because pipework has to reach every indoor unit from one location, so labour is higher than a set of single splits — but the equipment cost per room is lower.
Ducted Systems
Cooling is distributed through concealed ductwork, with only discreet grilles visible in the ceiling. Visually this is the best result by some distance.
It is also the least practical retrofit. Ducting needs void space above ceilings or in a loft, which most existing UK houses do not have without significant building work. Ducted is realistic during a renovation, a loft conversion or a new build — rarely otherwise.
Portable Units
Worth being straight about these. A portable unit needs its exhaust hose vented through a window, which means the window stays open and warm air comes back in. They are noisy because the compressor sits in the room with you, and they are inefficient for the cooling they deliver.
They have a place — rented property where you cannot fit a fixed system, or a short-term problem during a heatwave. As a permanent solution for a room you use daily, a fixed split system will cost less to run and perform far better.
Sizing: Why BTU Ratings Matter
Cooling capacity is measured in BTU per hour. Getting this wrong is the most common reason people are disappointed with a system that was otherwise installed correctly.
| Capacity | Typically suits |
|---|---|
| 9,000 BTU/h | Bedroom or small office, roughly up to 20 m² |
| 12,000 BTU/h | Living room or larger bedroom, roughly 20–35 m² |
| 18,000 BTU/h | Open-plan space or large room, roughly 35–50 m² |
Floor area is where sizing starts. A proper calculation also accounts for glazing area and orientation, ceiling height, insulation, how many people normally use the room, and heat from equipment. A south-facing room with large windows needs noticeably more capacity than the same floor area facing north.
Both errors cost you:
- Undersized — the system runs continuously, never reaches the set temperature, and wears out sooner.
- Oversized — the system cools too fast, switches off, switches back on, and repeats. This is called short-cycling. It wastes energy, dehumidifies poorly, and puts avoidable stress on the compressor.
Efficiency Ratings Worth Understanding
- SEER — Seasonal Energy Efficiency Ratio. Cooling output relative to energy consumed, measured across a season rather than at one test point. Higher is better.
- SCOP — Seasonal Coefficient of Performance. The same idea for heating, which matters if the unit is a heat pump and you intend to use it in winter.
- A+++ to G — the energy label you see on the box. Useful for quick comparison, but SEER and SCOP give you the detail behind it.
Inverter technology is the single biggest factor here. A non-inverter compressor is either fully on or fully off. An inverter varies its speed to match demand, so it draws less power holding a room at temperature and avoids the constant on-off cycling. Effectively every system worth installing today is an inverter model.
Features That Actually Matter
- Noise level in dB — check the indoor unit figure at its lowest fan speed, which is what you will hear at night in a bedroom.
- Filtration — useful if anyone in the house has allergies or asthma. Check what the filters cost and how often they need replacing.
- Heating as well as cooling — most modern split systems are heat pumps and will heat the room efficiently in shoulder seasons.
- Controls — app control is genuinely useful; the rest of the feature list usually is not.
- Parts and service support — an established brand means parts are available in five years. This matters more than a slightly better spec sheet.
We install and service equipment from Daikin, Mitsubishi Electric, Samsung and Fujitsu, all of which have solid UK parts and support networks.
Not Sure Which Is Right?
Tell us about the rooms you want to cool and how you use them. We will tell you honestly what suits the property — including when a smaller or simpler system is the better answer.
