How Does an Air Conditioner Push Heat Outside? The Refrigeration Cycle Explained
An air conditioner does not create cold. It moves heat from inside your room to the outdoors, against the natural direction of heat flow, and it needs electricity to do that.
The animation below follows a 12,000 BTU (3.5 kW) inverter split unit cooling a sunny bedroom. You can change the outdoor temperature and the set point, scrub through time, and watch the refrigerant go around its loop.
The short answer
The refrigerant in the loop does four things in a circle. A compressor squeezes the gas, which makes it hot (about 70 °C in the animation). In the outdoor coil (the condenser) that hot gas is warmer than the outside air, so it gives up heat and turns to liquid. The liquid then passes through a small expansion valve, where the pressure drops, and it becomes very cold and starts to boil. In the indoor coil (the evaporator) the boiling refrigerant absorbs heat from the room air, and the cycle starts again.
In the model at 35 °C outside, the unit moves 3.5 kW of heat for about 1.1 kW of electricity, a coefficient of performance (COP) of about 3.4.
What the animation shows, stage by stage
A hot room. Sun and warm outdoor air keep leaking heat in. The room starts at 30 °C.
Squeeze. The compressor takes the refrigerant gas (R-32 in the model) to about 30 bar. Compressing a gas heats it, to around 70 °C.
Dump the heat. Outside, the hot gas flows through the condenser coil. Even hot outdoor air is cooler than the gas, so heat flows out and the gas becomes liquid at about 50 °C (48 °C in the model's numbers).
Boil it cold. Through a tiny valve the pressure falls to about 10 bar, where the refrigerant boils at about 7 °C. Boiling in the indoor coil soaks up room heat.
The answer. The unit moves heat uphill and the electricity pays for the climb.
The physics: why it takes work to move heat uphill
Heat flows by itself from hot to cold. To move it the other way you need work. The ratio of heat moved to electricity used is the coefficient of performance:
COP = Q_cold / P_electric
The best a machine can do between a cold side at T_c and a hot side at T_h (in kelvin) is the Carnot limit, COP_max = T_c / (T_h − T_c). The smaller the temperature lift, the better the COP. The animation's model does not use the ideal limit directly, but it shows the same trend: a hotter day means a harder lift.
What the numbers say: hotter outside, worse COP
With the unit delivering 3.5 kW of cooling, the model gives:
Outdoor temperature | Electric power | COP |
28 °C | 0.90 kW | 4.1 |
35 °C | 1.08 kW | 3.4 |
42 °C | 1.25 kW | 2.9 |
The condenser has to run hotter to push heat into hotter air, so the compressor must squeeze harder, and that costs more electricity for the same amount of cooling. That is why an AC works harder, and costs more to run, on the hottest days.
In the model the room set point changes how long the unit runs, not its COP, so the three numbers above do not depend on it.
Why the room does not cool instantly
The unit only moves heat at its rated rate. The sun and the outdoor air keep pushing heat in, so the room temperature settles when the heat removed matches the heat leaking in. An inverter unit varies its compressor speed to hold the set point without switching on and off at full power. The animation is a time-lapse of minutes, not seconds.
Honest limits of the model
The model uses fixed refrigerant states (about 7 °C evaporation and about 48 to 50 °C condensation) and a simplified relation between the outdoor temperature and the power. Real units have part-load behaviour, fan power and defrost cycles. Use the animation to understand the cycle and the trend, not to size or compare real air conditioners.
Frequently asked questions
Does an air conditioner create cold air? No. It moves heat from the indoor air to the outdoor air. The indoor air gets cooler because heat is taken out of it.
What does the compressor do? It raises the pressure and temperature of the refrigerant gas so that it is hotter than the outdoor air and can release heat there.
Why does the refrigerant boil at 7 °C? Boiling temperature depends on pressure. The expansion valve drops the pressure to about 10 bar in the model, where this refrigerant boils at about 7 °C, cold enough to absorb heat from room air.
What is COP? The coefficient of performance: heat moved divided by electrical energy used. A COP of 3.4 means 3.4 kW of heat moved for every 1 kW of electricity.
Why does an AC use more power on very hot days? The heat has to be pushed into hotter air, so the refrigerant must be compressed to a higher pressure and temperature. In the model COP falls from about 4.1 at 28 °C to about 2.9 at 42 °C.
Is a 12,000 BTU unit 3.5 kW? Yes, 12,000 BTU per hour is about 3.5 kW of cooling capacity.
Related
Figures in this article come from the animation's simplified model: a 12,000 BTU (3.5 kW) inverter split unit, R-32 refrigerant evaporating near 7 °C, and condensing near 50 °C. They illustrate the mechanism and are not data for any real unit.
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