The ultimate UK heatwave beater is a portable air conditioner. Here's what you need to know…

Mistakes everyone makes with air conditioners

(Image credit: Getty/iStock)

published 15 June 2022

Summertime, and the living is not easy. That's due to a heatwave raging across multiple continents, making things unpleasantly hot indoors. It's quite nice outdoors admittedly, but do slap on some sunblock. Air conditioning is one excellent solution to indoor heat, but you need to avoid the air conditioner mistakes that all too many people make. I'm talking here about portable air conditioners as there's a lot more that can go wrong with them, and also in T3's native UK, very few homes have permanently installed air conditioning. The weather just doesn't justify it.

We have a guide to the best portable air conditioners, and it is unlikely you'll come across a less sexy-looking batch of gadgets anywhere on the site. However, when your home is a hot mess, you won't worry about that. The best fans – even the very sexiest Dyson fans – are not up to the task of cooling a room when it gets really hot. If you peruse our list of mistakes that everyone makes with fans, you'll find the very first one is 'expecting a fan to cool your home when it's hot'.

Very high temperatures are expected to continue for a week in the UK, and are also likely to become more commonplace and frequent due to climate change – unfortunately more widespread use of air conditioning is likely to contribute to that, but more of that later. So now may seem like a good time to buy a portable air conditioner. But only if you are aware of the mistakes people make with them, and what you need to know before buying one.

1. Not getting a powerful enough unit

Portable air conditioner vs evaporative cooling

This is pretty fundamental. If your room is 20 meters square, you won't be able to cool it with an air conditioner built to cool a room that's 10 meters square at most.

Air conditioners are rated in BTUs. This actually stands for 'British Thermal Unit', believe it or not, but it's a unit that's used around the world. Long story short, you need to have an appliance with enough BTUs for the space you want to cool.

Nothing is ever that simple, however, and there are also other considerations to be taken into account, including what direction the windows face in said room, what the walls are made of, what type of lighting it uses and so on and so on. That said, BTU ratings do put you in at least the right ball park 90% of the time.

Appliances Direct has this simple guide to the BTU requirements of various room sizes. You can also find more advanced calculators online which take into account all those other factors that can make a room naturally hotter or cooler.

2. Not positioning the unit correctly

Air conditioner with duct going out of window — It's all about the ducting

Positioning a portable air conditioner is pretty simple but comes with an in-built paradox. Ideally you don't want it placed by a wall or window, but they usually come with lengths of ducting that make it totally impossible to place them anywhere other than right by a wall or window.

So basically your options are, a) put it near a window and push the far end of the duct out of that window, b) drill a big hole in your wall with a chimney on the outside, and push it through that, or c) get a longer length of ducting.

I suspect only a tiny number of people ever opt for options b) or c) here, but the window option can be tricky. Many portable air conditioners come with adaptors that fill your the gap in your partially opened window, with a hole that the end of the duct fits through. However, these only ever seem to be suitable for sliding windows, not ones with a hinge. Well, they certainly never work with the windows in my flat, anyway.

If you also find you have this problem, your only option is to push the duct as far out of the window as possible, and direct it away from the window itself, and any other open windows in the vicinity. Otherwise, the hot air you have sucked out of your room will be blown straight back in. In the past, short ducts have left me having to stand air con units on large boxes, in order to get them close enough to the window. I don't know if this affected their performance, but it sure does look odd. On the other hand, it meant my room was cool while everyone else I knew was sweltering, so I was not that fussed.

I once went to an event held in a basement, where there was a portable air con unit but nowhere at all to run its short length of ducting. So they just turned it on and left it. The area immediately in front of it was very cold, the area behind it was extremely hot, and the net effect on the room overall was zero.

3. Not being aware of the costs involved

When it comes to cooling your home during a heatwave, you may feel that money is no object. However, the amount of energy used by portable air conditioners is quite significant and, with electricity prices rocketing, is going to be worse than ever this summer. My flat is very energy efficient and doesn't need a lot of heating. Because I use air con in summer, this means I am one of the few people whose bills go UP during the hotter months and down in winter.

Using some rough guesstimation powers and an internet search, the cost of a 12,000BTU unit appears to average out at about 30p per hour in the UK. This may not sound like a lot but if you run it for 12 hours per day you'll end up paying over £100 per month – that's $120 or AU$175 although of course energy prices vary a lot by country.

In the UK this may sound like an extreme example because it is almost unimaginable that it could be hot all day for an entire month. However, that's pretty standard in many countries in high summer. Not wanting to guilt trip anyone here, but running an air con unit is expensive because it burns a lot of electricity, and a lot of that will ultimately come from fossil fuels, which means that the planet will heat up more, and you'll need to use air con more often, and eventually the world will explode and we will all die, which will be a downer.

So please also be aware of the next mistake people make with air con…

4. Not using air con economically

why it's time to buy portable air conditioner — This lady is relaxed, despite looking at her electricity bill online

A lot of people buy a portable air conditioner for a few hundred quid/bucks, and set it to something completely unrealistic like 17ºC during a heatwave that sees outdoor temperatures upwards of 30ºC. All this means is that the air con will never reach its target and hence it will never turn off. Things naturally get better if you buy more expensive units but you should generally look to chill your room to the low to mid 20s rather than going for a genuinely chilled temperature – delicious though that feels when you've just come in from a day in the blazing heat.

The other classic mistake people make is to use air con when they feel hot, and run it with the fan on full. As with heating a house, you are actually much better picking a temperature and leaving the air con to achieve and maintain it. This means running the air con non-stop which doesn't sound very economical but keep in mind that most of the time it will be working only at a low level, or not at all, once the target temperature has been reached.

However, this may become moot at night as most portable air conditioners are pretty bloody loud, and will need to be turned off until you awake. Oh well.

5. Not cleaning your air con

Best portable air conditioner: Amcor SF8000E — Who wouldn't want to lavish attention on something as beautiful as this?

I am not a big one for cleaning filters on vacuum cleaners, or anything else come to that. However, it is essential for portable air conditioners, if you want them to perform during hot weather. Which, obviously, you do.

You should thoroughly clean all filters and empty any waste water tanks when you bring them out for the summer months and before you put them away when it gets cold. And if the period you're using them for is going to be longer than a month, you should also do that every month while it's in use.

Otherwise, when the going gets hot, your air con will not get going. In some cases they won't work at all, and in most cases they will work but less efficiently, which means they'll be even more expensive to run, and less effective at cooling your room. And you don't want that.

Some fan deals right now

Duncan is the former lifestyle editor of T3 and has been writing about tech for almost 15 years. He has covered everything from smartphones to headphones, TV to AC and air fryers to the movies of James Bond and obscure anime. His current brief is everything to do with the home and kitchen, which is good because he is an excellent cook, if he says so himself. He also covers cycling and ebikes – like over-using italics, this is another passion of his. In his long and varied lifestyle-tech career he is one of the few people to have been a fitness editor despite being unfit and a cars editor for not one but two websites, despite being unable to drive. He also has about 400 vacuum cleaners, and is possibly the UK's leading expert on cordless vacuum cleaners, despite being decidedly messy. A cricket fan for over 30 years, he also recently become T3's cricket editor, writing about how to stream obscure T20 tournaments, and turning out some typically no-nonsense opinions on the world's top teams and players.

Before T3, Duncan was a music and film reviewer, worked for a magazine about gambling that employed a surprisingly large number of convicted criminals, and then a magazine called Bizarre that was essentially like a cross between Reddit and DeviantArt, before the invention of the internet. There was also a lengthy period where he essentially wrote all of T3 magazine every month for about 3 years.

A broadcaster, raconteur and public speaker, Duncan used to be on telly loads, but an unfortunate incident put a stop to that, so he now largely contents himself with telling people, "I used to be on the TV, you know."