Window Air Conditioners in the UK

A window air conditioner is a single box that sits in an open window, with the cooling side facing the room and the hot side outside. They are common in some countries, particularly the United States, but you rarely see them in the UK. This guide explains why, and what works better here.

How a Window Unit Works

Everything is in one box: the compressor, both coils and the fans. The part facing the room blows out cool air, and the part outside the window gets rid of the heat. Because the compressor sits in the window, right next to the room, window units are noisier than split systems.

Why Window Units Rarely Suit UK Homes

• The windows are the wrong type. Most window units are designed for sash windows that slide up and down. Most UK homes have casement windows, which open on hinges, and a window unit cannot sit in these properly.

• Security. A window held open around a unit is hard to secure, especially on the ground floor.

• Draughts and rain. Gaps around the unit let in air, insects and rain unless they are sealed well.

• Light and views. The unit blocks part of the window.

• Noise. You hear the compressor in the room, and your neighbours can hear it outside.

• Permissions. A lease or tenancy agreement may not allow one. In a listed building, check with the council first. See air conditioning for listed buildings.

Window Units vs Portable Units

Portable air conditioners are the more common choice in the UK. They stand on the floor and push hot air out through a hose, and window kits with fabric seals are made for casement windows. They share the main drawback of window units: the compressor is right next to you, so they are noisy. Single-hose units also pull warm air in from outside to replace the air they push out through the hose. See do portable air conditioners work?

Through-Wall Units

A through-wall unit mounts on an inside wall and breathes through two holes in the outside wall, typically around 16 to 20 cm across. The window stays free and the unit is fixed securely, but it is still louder and less powerful than a split system. See air conditioning without an outdoor unit.

The Better Option for Most Homes: a Split System

A split system puts the compressor outside, connected to a slim indoor unit by pipes through one small hole in the wall. It is quieter and more efficient than a window or portable unit, it leaves the window free, and most split systems heat as well as cool. See wall-mounted air conditioning.

As a guide, one room typically starts from around £2,500 installed, and most single-room systems are fitted in a day.

Frequently Asked Questions

Can I fit a window air conditioner in a casement window?

Not properly. Most window units are made for sash windows that slide up and down. A casement window would have to be left open, with the gaps around the unit sealed, which is hard to make secure and weatherproof.

What is the quietest way to cool a bedroom?

A split system, because the noisiest parts are in the outdoor unit. See bedroom air conditioning and the quietest air conditioners.

What can I do to cool a room without air conditioning?

Keep curtains closed on sunny windows during the day and open windows once it is cooler outside. Our guide on how to cool down a room has more tips.

Call 01772 288651 or use the form below to book a free survey. We will measure the room, recommend the right system and give you a clear written quote.

Last updated: November 2026.