Most people use air conditioning without thinking about what’s going on inside the boxes. Knowing the basics helps you use the system well, spot problems early and understand what an engineer is talking about. This is the plain English version, from the engineer’s side of the job.
The key idea: moving heat
A room feels cooler when heat is taken out of it. Air conditioning does that by picking heat up indoors and dumping it outdoors. It’s the same principle as a fridge, which moves heat from inside the cabinet to the coil at the back.
Moving heat takes much less energy than making it. That’s why a modern system can move more heat than the electrical energy it uses, and why the Energy Saving Trust notes that air-to-air heat pumps are sometimes referred to as air conditioning: reverse the direction and the same machine heats your room.
The four main parts
A split system has an indoor unit and an outdoor unit joined by two copper pipes, a cable and a condensate drain. Refrigerant circulates round a sealed loop, passing through four key parts:
- Evaporator (the coil in the indoor unit when cooling). Cold, low-pressure liquid refrigerant flows through it. The fan blows room air across the coil, the refrigerant absorbs heat and boils into a gas, and cooler air comes out into the room.
- Compressor (in the outdoor unit). It squeezes the gas, raising its pressure and temperature well above the outside air. This is where most of the electricity goes.
- Condenser (the coil in the outdoor unit when cooling). The hot gas gives up its heat to the outside air, helped by the outdoor fan, and condenses back into a liquid. That’s why the air from the outdoor unit feels warm in summer.
- Expansion device. An electronic valve drops the liquid’s pressure suddenly, which makes it very cold, ready to go back to the indoor coil and pick up more heat.
Round and round it goes. The refrigerant isn’t used up. If a system needs topping up, it has a leak, which is why a “regas” without finding the leak is only a short-term fix. See our regas page.
Where the water comes from
When warm, humid room air hits the cold indoor coil, moisture in the air condenses on it, just like a cold glass on a summer day. That water runs into a drain tray and away through the condensate pipe, either by gravity or with a small pump. It’s why air conditioning makes a room feel less sticky, and why a blocked drain leads to water dripping from the unit. Our guide to air conditioning leaking water covers that.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Inverters: why modern systems run smoothly
Older systems had a compressor that was either fully on or off, so the room swung between too warm and too cold. Modern systems use an inverter, which varies the compressor’s speed. It runs hard to bring the room to temperature, then slows right down to hold it there. That’s quieter, steadier and usually more efficient, and it’s why a modern unit may seem to run all day at a low hum rather than starting and stopping.
It’s also why sizing matters. A unit that’s far too big keeps hitting its minimum speed and cycling, which wastes energy and doesn’t dry the air well. Our sizing guide explains more.
Heating mode and defrost
A reversing valve in the outdoor unit swaps the direction the refrigerant flows. Now the outdoor coil becomes the evaporator, picking heat out of the outside air even on a cold day, and the indoor coil becomes the condenser, releasing that heat into your room.
In cold, damp weather the outdoor coil can ice up. The system handles this itself with a defrost cycle: it briefly reverses, warms the outdoor coil, and you may see steam coming off it and notice the indoor fan pause. That’s normal. Our guide to using heat pump mode in winter has more.
Refrigerants and the law
Most new split systems use R32, which has a global warming potential of 675 under Annex I of Regulation 517/2014. Older systems commonly use R410A, with a GWP of 2088. From 2025, new single split systems holding less than 3 kg can’t use F gases with a GWP above 750, under the GOV.UK rules on F gas bans in new equipment. Anyone opening the refrigerant circuit must be qualified, and the company must be F-Gas certified. That’s why installation and refrigerant work isn’t a DIY job.
Quick answers
Does air conditioning bring in fresh air?
Usually not. A split system cools, dries and filters the room air and recirculates it. Fresh air comes from windows or a separate ventilation system.
Why is the outdoor unit blowing hot air?
In cooling mode, that’s the heat taken out of your room being released outside. It means the system is working.
Why does my system steam outside in winter?
That’s the defrost cycle melting ice off the outdoor coil. It’s normal and only lasts a few minutes.
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More advice
- Heat Pump vs Air Conditioning: What’s the Difference?
- R32 vs R410A Refrigerant: What You Need to Know
- Air Conditioning in Winter: Using Heat Pump Mode Effectively
- How to Choose the Right Size Air Conditioning Unit
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.