Bedroom air conditioning in London: sleeping through a heatwave

Why London bedrooms stay hot at night

The hardest part of a London heatwave is not the afternoon. It is lying awake at midnight with the window open and the room still warm. The Met Office heat in cities article explains why: buildings absorb the sun’s heat during the day, tall buildings and narrow streets cut the breeze, and after sunset urban areas stay warmer than the countryside. The London City Pack puts London’s average July high at 23.5°C, the highest in the UK, and projects summer maximum temperatures rising by 1.2 to 2.8°C by the 2030s.

A study in Buildings and Cities found the main bedroom overheated in 4.4 million homes in England, and that only about 2% of English households used fixed or portable air conditioning in 2017 (Buildings and Cities). Most people are still relying on fans.

The window problem in Hounslow and west London

Opening the window is the usual answer to a hot bedroom. In much of Hounslow, that is not always practical. Heathrow sits on the borough’s western edge and the flight paths cross Hounslow, Isleworth and parts of Chiswick, so many people keep bedroom windows shut. Add the A4, the M4 and the Great West Road, and you have bedrooms where the choice is heat or noise. It is also where London recorded its joint UK record: 40.2°C at Heathrow on 19 July 2022 (Met Office).

Air conditioning lets you keep the window closed and still cool the room. That is the main reason many households in west London look at it.

Night noise is the number that matters

Every indoor unit has several fan speeds. For a bedroom, ignore the full power figure and look for the decibel level at the lowest speed, usually called quiet or night mode. That is what you will hear at 3am.

Type	Where the compressor is	Typical bedroom result
Wall mounted split	outside	quietest indoors
All in one wall unit	inside	louder, noticeable at night
Portable unit	inside, on the floor	loudest, plus hose leaks heat

For most bedrooms, a split system is the only type that is properly quiet. Our post on the quietest air conditioning units explains how to compare published figures.

Placing the unit for sleep

Where the indoor unit goes makes a big difference to comfort.

• Put it on a wall where the air flows across the ceiling and down the far side of the room, not straight onto the pillow.
• Avoid placing it directly above the head of the bed.
• Keep it clear of wardrobes or curtains that block the airflow.
• Make sure the condensate drain has a proper fall, so it never drips in the night.

Most units have adjustable louvres, so the air can be directed up and away while you sleep.

Sizing a bedroom properly

Bedrooms are often small, so it is tempting to buy the smallest unit. But size depends on heat, not just floor area. A rear bedroom facing west with a big window catches the evening sun just before bedtime. A loft bedroom under the roof can be several degrees hotter than the floor below. If the room is also a home office in the day, screens and people add heat. We look at all of this at the survey. For top floor rooms, see our guide to loft conversion air conditioning.

Settings that help you sleep

• Sleep mode: lets the set temperature rise gradually through the night.
• Timer: cool the room before bed, then reduce output after you fall asleep.
• Dry mode: on humid nights, removing moisture can make the room feel more comfortable.
• Clean filters: dusty filters reduce airflow and can smell musty. Rinse them every few weeks in summer.

Setting the unit to a comfortable temperature rather than as cold as it will go is kinder to your sleep and your electricity bill.

Nurseries and children’s rooms

Babies and young children are more affected by heat. A quiet split system in a nursery lets you hold a steady temperature without opening windows to noise or pollen. Keep the airflow off the cot and check the night mode sound level carefully.

Planning and consent

For a house in Hounslow, a reversible bedroom unit that also heats can be permitted development under Class G, as long as it meets the noise calculation of 37 dB(A) at the neighbour’s window, the size limits and the siting rules (Planning Portal). A terraced or semi detached house can only have one outdoor unit, so if you want two bedrooms cooled, a multi split may be the answer. Hounslow has 29 conservation areas, and Article 4 directions in Bedford Park and Gunnersbury, so check before relying on permitted development. A cooling only unit always needs permission, and flats need freeholder consent.

Who can fit it

The refrigerant side of the job must be done by an engineer with an F-gas qualification, working for a company certified by an approved body such as Refcom or the F-Gas Register, as set out on gov.uk. Ask for the company certificate number and check it before you book. The new electrical circuit is notifiable under Part P.

Getting started

We survey the bedroom, check sun, roof and window positions, find a quiet spot for the outdoor unit and recommend a unit by its night noise as well as its output. The bedroom and loft air conditioning page explains the process, and the Hounslow area page covers the local housing and planning context.