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Air Conditioner Buying Guide

Air Conditioner Image

How does an air conditioning unit work?

Air conditioners cool the room by removing heat and moisture from the air, just like how a fridge cools food. But how does it work? First, warm air is drawn into the system and passed over a cold evaporator coil. Next, the air is cooled, removing some of the moisture too.

A refrigerant (a fluid that transfers heat) absorbs the heat from the warm air and vents through a wall or window via an exhaust hose. Water condenses in the system, which is either drained away or collected in a tank for you to empty. Most air conditioners are self-evaporating, meaning the tank won’t need emptying regularly. Finally, cold air is recirculated into the room, reducing the overall temperature and humidity.

Types of air conditioners



Portable air conditioners are easy to move between rooms and even up and down stairs. They’re powerful enough to cool all room sizes without needing to be professionally installed, and they’re vented through an open window or door via an exhaust hose. Portable air conditioners tend to be the cheapest type, and they’re perfect for homes or offices.

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Wall-mounted air conditioners (or wall-split) take up no floor space but require professional installation. They feature a unit on the wall inside plus an external unit to remove heat and condensation from the room. They’re quieter than portable units and can heat rooms more efficiently than central heating.

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Ceiling cassette

Ceiling cassette

Ceiling cassette air conditioners are another type of split unit; they feature an external unit to remove the heat and need installing professionally, but the internal unit fits into the ceiling. As well as cooling, they can heat rooms more efficiently than central heating.

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How to buy the right size air conditioner

The size of the air conditioner you need depends on the size of the room you want to cool. Larger rooms require a unit with a higher BTU, but these use more energy than ones with a lower BTU. Consider what the space is used for too. Any devices or appliances will contribute to heating the room, so you’ll need a more powerful unit to cool a room with many electricals. Rooms with big windows, like conservatories, require more powerful air conditioning than rooms with small windows.

Did you know? BTU stands for British Thermal Unit, which is the amount of energy it takes to heat one pound of water (about a pint) by 1°F.

You can calculate the BTUs you need using our Calculator.

What air conditioner features should I look for?

Heating mode

Many air conditioners also have a heating mode, perfect for year-round use. It acts alongside your central heating to keep you warm during those colder months.

Sleep mode

There’s nothing worse than not being able to sleep because it’s too hot. An air conditioner with sleep mode reduces noise levels so that you can enjoy an uninterrupted cool night’s sleep.

Energy saving mode

Generally, air conditioners use a lot of energy, but some air conditioners feature an energy-saving mode to reduce power consumption, helping you to save money on your energy bills.


A timer function is handy to ensure electricity isn’t being wasted. You can set the air conditioner to turn on and off when needed, which helps you save money on energy bills.

Smart features

A smart air conditioner connects to an app, allowing you to control it from your smartphone. It’s perfect for those hot days when you’ve been at work, and you can ensure you’re returning home to a nice cool room.

Oscillating vent

Oscillating vents switch between facing up and down or left and right, which spreads the air around to help cool the room evenly.


Many air conditioners also have a dehumidifier function, meaning that as well as cooling the room, they’ll remove some of the moisture in the air, which is handy for drying laundry inside and combatting damp and mould.


A purifier function clears the air of bacteria, allergens, mould and dust, helping asthma or allergy sufferers breathe easier.

Fan speeds

Control the breeze to suit your preferences. Some portable air conditioners have a ‘fan only’ mode, meaning you can use it as a fan if required (it doesn’t need venting when only using the fan).

Remote control

All our air conditioning units come with a remote control, which is handy for changing the settings from a distance.

Adjustable thermostat

Take complete control over the temperature in your room. Set the most comfortable temperature, and the thermostat will maintain it.

How to install an air conditioning unit



  • Slide open the window, adjust the slide bar to fit into the gap, attach it with a screwdriver, and close the window.
  • Attach the exhaust pipe to the back of the air con unit and secure the flat exhaust nozzle to the other end of the pipe. Extend the pipe and insert the nozzle into the slide bar.
  • Plug the unit in, switch it on, and adjust the temperature.
Air con installation guide »
Split units

Split units

Wall-mounted and ceiling cassette air conditioners are a lot more complex and require installation by a licensed professional. We have over 20 years of experience; give us a call on 0330 041 2742 for a quote.

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Energy rating

Portable Image


Most portable air conditioners we offer have an energy rating of A. Each model’s efficiency depends on how long the unit is turned on and its power level or mode.

Wall-mounted image

Split unit

Most of the wall-split and ceiling cassette air conditioners we offer have an energy rating of A++ or A+. They can cool a larger area with less power, making it more efficient.

How to troubleshoot common issues

Cooling troubleshoot

Not cooling

If the unit isn’t cooling the room efficiently, there’re a couple of things you should look at.

First, the room. Is it too big for the unit to cool? How many devices or appliances are there? How many people are in the room? Check the BTUs required with our Calculator. It’s also important to note that conservatories and rooms with many windows require double the stated BTU.

It could also be the venting. If hot air can’t escape, it won’t cool the room effectively. Ensure the vent hose is as straight and flat as possible, with no bends greater than a 30° angle. You can also remove the hose from the unit and check that the air coming out of the front is cold and the air coming out of the back is hot (when in cooling mode).

Leaking troubleshoot


If the unit has started to leak, it’s because the expelled air contains a lot of moisture, so any airflow problems will cause water to collect inside and then leak.

If it’s leaking from the exhaust hose, it’s probably been vented incorrectly. There’ll be condensation build-up in the hose, and the hot air can’t escape.

If it’s leaking from the base, check for visible cracks in the bottom. If you can’t see any damage, ensure the unit is on a level surface; otherwise, the drip tray (tank) will overflow and leak.

Noisy troubleshoot


Air conditioners become noisy due to extra strain on the fan. If you catch it early, it may not cause long-term damage, and correcting the venting may fix the problem. The fan is damaged if the noise persists when the vent pipe isn’t connected.

Compressor overheating troubleshoot

Compressor overheating

Hot air builds within the unit and stops the cooling effect from kicking in. If this happens repeatedly, the compressor will fail. To resolve this, let the unit cool for at least 30 minutes and then test without venting. The compressor is still functional if it produces cold air from the front and hot air from the exhaust. Correct the venting and continue to use the unit. If it does not operate correctly during this test, irreparable damage has been caused.

Compressor failure troubleshoot

Compressor failure

Same as the above, but irreparable damage has been caused. You’ll need to purchase a replacement unit.

Damage to vent fan troubleshoot

Damage to vent fan

Same as the above, but irreparable damage has been caused. You’ll need to purchase a replacement unit.

Lack of power to the unit troubleshoot

Lack of power to the unit

Same as the above, but if the water stays inside the aircon, it can cause an electrical short circuit and corrosion, leading to future faults.

For more advice, give us a call on 03300 412 271 to speak to one of our air con experts.

Frequently asked questions

  • What to look for when buying an air conditioner?

      The first thing you need to decide on is whether you’d like a portable unit or an installed unit. Then, consider the room size you want to cool; use our BTU calculator to determine what’s required.

      Look out for features you like, such as built-in air purifiers, fans, or even ones that connect to an app to control it from your phone..

  • How do I know what size air conditioner I need?

      Use our simple BTU calculator to determine how many BTUs you need to cool the room. A higher BTU means it can cool a larger area.

  • Can I install my own air conditioner
  • Are air conditioning units expensive to run?

      Portable air conditioners are cheaper to buy than installed units, plus you don’t need to pay for installation; however, they’re more expensive to run relative to their size and cooling power. Wall-mounted and ceiling cassettes air conditioners require professional installation, so the initial cost is higher. In the long term, they’re much more efficient than portable units.

  • Can an air conditioner heat my home as well as cool?

      Many portable air conditioners have a heating mode that acts alongside your central heating to heat the room. Split units can heat a room more efficiently than central heating.

Air Conditioner Calculator

Use our handy calculator, which will help you figure out exactly what size of unit you require for your office or home.

What are the room dimensions?

How many people and heat sources are in the room?

What is BTU?

A BTU (British Thermal Unit) is a rating used to determine the heating or in the case of an Air Conditioner the cooling capacity of an appliance.

Typically a minimum of 7000 BTU is needed to cool a small room, whereas commercial units can reach up to 25,000 BTU and beyond. Simply, the greater the BTU, the larger the space that the unit can cool.

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