How Does Air Conditioning Work?
Air conditioners don't create cold—they move heat from inside to outside using refrigerant. The refrigerant absorbs indoor heat as it evaporates, then releases that heat outside as it condenses. This cycle of evaporation and compression is why ACs have both an indoor and outdoor unit.
Key Takeaways
- The refrigeration cycle has four main steps: 1) Low-pressure liquid refrigerant enters the indoor evaporator coil, 2) Indoor air blows across the coil; heat is absorbed as refrigerant evaporates into gas, 3) The compressor pressurizes the gas, heating it further, 4) The hot gas reaches the outdoor condenser, releases heat to outside air, and becomes liquid again.
- "Heat pumps" are air conditioners that can reverse the cycle, moving heat indoors during winter - a key factor in the cost to run a space heater comparison.
- Portable ACs are less efficient because all components are inside, and hot exhaust must vent through a window.
Explanation
The refrigeration cycle has four main steps: 1) Low-pressure liquid refrigerant enters the indoor evaporator coil, 2) Indoor air blows across the coil; heat is absorbed as refrigerant evaporates into gas, 3) The compressor pressurizes the gas, heating it further, 4) The hot gas reaches the outdoor condenser, releases heat to outside air, and becomes liquid again.
The key principle is that liquids absorb heat when evaporating (like sweat cooling your skin) and release heat when condensing. The compressor maintains pressure differences that force this phase change cycle to continue—it's doing the heavy lifting, which is why AC uses significant electricity. Understanding how electricity is measured in kWh helps make sense of cooling costs.
Besides cooling, ACs also dehumidify. When warm, humid indoor air passes over the cold evaporator coil, moisture condenses (like water on a cold glass). This condensation drains outside, lowering indoor humidity and making the space feel more comfortable.
AC efficiency is measured by SEER (Seasonal Energy Efficiency Ratio) ratings. The minimum required SEER for new central AC units in the US is 14-15 depending on region, while high-efficiency models reach 20-25 SEER. A 20-SEER unit uses roughly 30% less electricity than a 14-SEER unit to produce the same cooling. For a typical household spending $200-300 per month on summer cooling, upgrading from a 10-SEER older unit to a 20-SEER model can cut cooling costs nearly in half. Explore more ways to save electricity at home.
Proper sizing is critical for AC performance. An oversized unit cools the air quickly but cycles off before adequately dehumidifying, leaving the home feeling clammy. An undersized unit runs constantly without reaching the set temperature. HVAC contractors use Manual J load calculations, which factor in home square footage, insulation quality, window area, climate zone, and occupancy to determine the correct tonnage. Most homes need 1 ton of cooling capacity per 400-600 square feet, though this varies significantly by region and construction.
Things to Know
- "Heat pumps" are air conditioners that can reverse the cycle, moving heat indoors during winter - a key factor in the cost to run a space heater comparison.
- Portable ACs are less efficient because all components are inside, and hot exhaust must vent through a window.
- Window units and central AC work on the same principle—just different configurations.
- Refrigerant never "runs out"—if levels are low, there's a leak that needs repair.
- Mini-split (ductless) systems avoid the 20-30% energy loss that occurs in traditional ducted systems from leaks and heat transfer through duct walls.