Mention the words "air conditioner" to most people and they think of a stream of cool air pouring from a box on the wall to make the summer days and night more bearable. After all, this is the main thing that an air conditioner does. And where would we be without air conditioning when the temperature gets up into the 90s and beyond.

And, even worse, when the humidity rises too. Without air conditioning we feel fatigued, unable to concentrate, and unable to sleep. Which makes us more tired the next day, and even more unable to cope with the heat.

But an air conditioner does more than just cool the air, although that is an air conditioners main function. An air conditioner also cleans the air, and in today's polluted environment that's as important as the air cooling capability.

What this means is that when it comes to choosing an air conditioner, it's best not to just buy the cheapest model, or the one which first catches your eye. There are more things in the equation than just a stream of cool air.

Before buying an air conditioner, think of what it is you want it for. This means think of the size of the room, and what the room will be used for. Buying an air conditioner that is too small for the intended room will leave you feeling hot and flustered! And buying an air conditioner that is too big for the room can have its problems too.

If the conditioner is too big, it might well work erratically and fail to cool the room adequately.

When it comes to what the room will be used for, you also need to think of noise. An air conditioner for a busy room such as a childrens' play room or a living room can be a little noisy without much problem.

But what about the bedroom? Today's air conditioners are much quieter than the ones of years gone by, but a noisy, humming mechanism in a bedroom is the last thing you want. Especially since the whole point of installing an air conditioner in the first place was to enable you to get a good night's sleep.

Think too about running costs, and how much replacement parts might cost you over the life of the unit. The best models can cool the air without costing you the earth in electricity and replacement parts and engineer bills.

Before you buy, make sure you get a good idea from the salesman of how much the unit will cost you in total.

A good air conditioning unit can make the home a real haven against the heat. Take your time to choose wisely and you'll enjoy the summer heat without it wearing you out.