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How to buy an air conditioner

By Aaron West

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How to buy an air conditioner

Air conditioning is now common in offices and shops, but how often do you wish you had air-con in your home?

In the depths of winter, it can be hard to imagine needing an air conditioner. But at the height of summer, a hot, stuffy house can be very uncomfortable. Cooling down your home should help you get a better night's sleep when the outside temperature is high.

Yet buying air con can be bewildering, with words like 'standalone', 'split-unit', 'BTU' and 'refrigerant' being thrown around. Luckily, our guide will help you to choose the best air conditioner for your home.

But before buying, find out the reasons for and against buying an air conditioner, and the alternative ways you can cool down your home. See should I buy an air conditioner?

Standalone air conditioners: pros and cons

Typical prices: start from around £200


  • An effective way to cool your home without needing to install anything
  • You vent hot air out through a hose, so you can move the unit from room to room
  • More expensive models can double as dehumidifiers, air purifiers or electric heaters.


  • Venting the hose through an open window could pose a security risk
  • Portable air conditioners are not as effective at cooling as split-unit models – they get hot and leak heat back into the room
  • Some models can be heavy. Make sure your chosen unit isn’t too heavy for you to lift comfortably and that the hand-holds are easy to grip.

Split-unit air conditioners: pros and cons

Typical prices: start from around £500, not including possible installation fees


  • Ideal if you have one room that regularly gets very hot
  • A secure way of air conditioning your home – no need to leave windows open
  • Quieter and usually more efficient than standalone units


  • Tend to be more expensive than standalone units
  • Needs to be permanently mounted on an outside wall – installation can be tricky, and you may need to hire a professional.

What size air conditioner do I need?

Air conditioners come in various shapes and sizes, but are often described in terms of their BTU (British thermal unit) output. In theory, the higher the BTU claimed, the more efficiently it can cool a room.

As a general rule, 5,000 to 8,000 BTUs is adequate for most living rooms or bedrooms. There's a calculation you can use to work out what BTU is right for you. As a very rough guide, multiply the dimensions (in feet) of the room by five. 

So for a room measuring 15x10x8 feet: 15 x 10 x 8 x 5 = an air conditioner of 6,000 BTUs.

If you're concerned about how much energy an air conditioner will use, look out for one with a good energy efficiency ratio (EER). This is the ratio between an air conditioner's BTU and its the power input (in watts). 

In general, the higher the EER rating the more efficient the air conditioner.

What extra features should I look out for?

Sleep or night modes reduce noise by running the compressor and fan more slowly.

Dehumidifier mode will give you even greater control, and will allow your air conditioner to be useful all throughout the year, not just in summer. Buying an expensive air conditioner just for dehumidifier mode could work out to be more expensive, and less effective, than buying two separate appliances. 

Read our dehumidifier reviews to find out which ones do the best job.

Timer options allow you to set the machine to automatically switch on and off - useful if you want to come home to a cool house or save energy by having the unit switch off once you’ve fallen asleep. Pick a model with a clock and 24-hour setting. Some come with only countdown or delay timers; these need to be reset daily.

Remote controls let you adjust settings without moving from your seat, but are easy to lose. Look for a machine with a dock to store the remote if you're prone to gremlins hiding your things.

Heat pump mode will allow you to use your air conditioner as an electric heater - particularly useful in winter. Although, just like with a dehumidifier, it could work out better value to buy them separately.

Read our electric heater reviews to find the best.

Air filters will be useful if you live in a built-up area, as it will allow you to purify the air and get rid of pollutants like pollen, smoke and dust. 

See our air purifier reviews.

Will I be able to maintain it?

These important tasks will keep your air conditioner in good shape, so it’s worth considering whether you’ll be about to do this easily:

  • Remove bungs to drain water from portable units at the end of summer, before they are packed away for winter. If the summer has been particularly humid, you may need to drain the unit every few weeks. Water pipes are normally located at the bottom of the machine. Before you buy, check in store that you can bend down and unplug the water pipes easily, as sometimes bungs can be fiddly and difficult to grip.
  • Clean dust filters regularly on both portable and split models. Before buying, ask yourself whether you’ll be able to easily access these parts for cleaning.

Does Which? test air conditioners?

As of 2011, we don’t currently test air conditioners. This is because our relatively mild UK climate means there is less demand for air conditioner reviews compared with other appliances.

Due to the increased demand for them, we regularly test electric heaters, air purifiers and dehumidifiers - click on each product to see our reviews.

When we tested air conditioners, we assessed not only how quickly they could cool down a room, but also whether they could keep it at a stable temperature. We also looked at how well-built and easy-to-use each air conditioner was, and how much electricity they took to run.