Air conditioners: How to buy air conditioners

Which type should I buy?

Single unit

The single or mono block unit is a fairly cheap fix that’s easy to install. It works by venting hot air out through a hose - you either poke this through an open window or drill a hole in your outside wall.

Venting it through an open window means you can move it from room to room. But consider whether you want to leave your window unsecured.

Single units are not as effective at cooling as the split unit models, as they get hot and leak heat back into the room.

Split unit

Generic split fixed

A split unit is a good choice if you have one room that gets very hot.

Split units tend to be more expensive than single units, but are quieter, more efficient – as they don’t heat the room in any way - and are usually more powerful. They need to be permanently installed, so are best if you have one room that always gets unbearably hot.  As you don’t need a window permanently open, they are a secure way of air conditioning your home.

Their main disadvantage is that they have to be mounted on an outside wall. If you’re using one to cool a bedroom, this probably means fitting it on the first floor or higher. This may be difficult for all but an experienced DIYer, as you'll need to bore a hole through the wall to take power supplies and refrigerant from one side of the device to the other.

To take it on, you'll need the right equipment – an SDS drill, diamond bit corer and extension bar. These can be rented from many hire shops for around £30 to £40 an hour. Alternatively, a lump hammer and brick bolster can be used to remove a square section of brick with the resulting gaps being filled in with pieces of brick and mortar.

If personally bashing holes in your house doesn't sound ideal, get an expert in.

What size do I need?

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

In our tests we found that few air conditioners met the BTU claimed on the packaging, so we wouldn’t recommend you rely on those claims to make your purchase. 

Don’t worry about using an appliance with a high BTU. If a machine has a higher cooling capacity, like our Best Buys, it should cool quickly. But, as a general rule, 5,000 to 8,000 BTUs is adequate for most living rooms or bedrooms. Avoid using an air conditioner to cool a microscopically small room as it could affect the machine.

Complex calculations can be used to work out what BTU is right for you. As a rough guide, multiply the dimensions (in feet) of the room to be cooled by 5. So for a 20 foot by 15 foot by 12 foot room: 20 x 15 x 12 x 5 = an air conditioner of 18,000 BTUs. 

Installation info

Drilling wall

Get a professional in if you don't fancy installing a fixed unit yourself.

Balancing on ladders, two floors up, while drilling holes into a brick wall is not for the faint hearted.

If you decide to go for a built-in split model, it would be wise to arrange for installation at the same time. Specialist retailers often include installation. Certain models can’t be bought without paying for installation.

Look out for stores that offer installation for either a ground or first floor. If you pay for installation at the same time as the air conditioner, Robert Dyas guarantees a refund if its engineer later discovers that there isn’t a suitable wall or space to install the unit.

Will you be able to maintain it?

This is most relevant with mobile air conditioners. Bungs must be removed to drain water from the unit 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.

Dust filters will need to be cleaned regularly on both mobile and split models. Before buying, ask yourself whether you’ll be able to easily access these parts for cleaning.

How portable is portable?

If you're buying a portable model, check you can physically move it around. This is especially important if you are planning to use it, say, downstairs in a study by day and in a bedroom at night. Make sure it isn’t too heavy for you to lift comfortably and that the hand-holds are easy-to-grip.

More on cooling your home...

Which? works for you